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- Boron deficiency occurs in soil that are inherently low in boron and soils from which boron reserves have been leached.

- The most conspicuous and characteristic symptoms of the deficiency are expressed in the fruit.

- Affected fruit are small, hard, and misshapen and have a thickened peel. Pockets of brownish gum, similar to those caused by impietratura, are found in the peel and some times also in the core.

- Boron toxicity can easily occur if too much of the mineral is applied in an attempt to correct a deficiency. The foliar symptom of the toxicity is a yellowing of the leaves, beginning at the leaf tips and gradually extending back along the margins.

- Calcium deficiency in field-grown citrus has been reported only rarely. It does not produce distinctive foliar symptoms, but it can significantly limit overall tree growth and reduce fruit yields.

- Typical symptoms of copper deficiency are excessive growth of young shoots, resulting in enlarged leaves on long, willowy branches; pockets of brownish gum may form on the stem and affected twigs commonly dieback from the tips.

- Copper toxicity can occur where excessive a mounts of the mineral accumulate in the soil.

- It can greatly reduce the growth and vigor of citrus without causing visible symptoms, as an indirect consequence of root damage.

- In many citrus regions, iron deficiency is caused by the unavailability of the mineral in the soil, and not by an insufficient supply of total soil iron.

- The most prominent symptom is a characteristic chlorotic leaf pattern, which first appears on young, expanding leaves. The entire leaf appears light green veins, which remain a distinct dark green.

- Magnesium deficiency is common in orchards on sandy soils, because of inherently low magnesium applied in fertilizer.

-The most prominent symptoms of magnesium deficiency is a chlorosis that gradually extends along the leaf margins, until the pattern covers most of the leaf, except for a small, delta – shaped, dark green area at the base.

- Some citrus fruit develop a peel injury known as chilling injury when stored at temperatures below 10.C.This should not be confused with freeze injury, which is caused by temperatures below 0.C. Chilling injury occurs most frequently on grapefruit, lemons, and limes and papers as sunken pits on the peel surface.

- Chilling injury can be avoided by storing and transporting fruit at higher temperatures, but this increases the risk of decay.

- Partial control can be achieved by applying wax or certain film warps to decrease moisture loss from the rind.

- Granulation, some times known as crystallization is a condition in which the juice sacs shrivel because of gel formation rather than because of the desiccation.

- In Valencia orange, tangerines, and tangerine hybrids, granulation occurs mostly at the stem end of the fruit, whereas navel oranges, it generally extends through the center of the fruit.

- Blast is a disease of leaves and twigs that occur under cool, wet conditions. Grapefruit and oranges are most susceptible.

- Blast lesions most commonly appear on the wings or petioles of leaves, as water- soaked or black areas, which quickly move in both directions, up the midvein of the leaves and down to the wigs.

- Once the phloem in the petiole is severely damaged, the leaves whither, curl, turn brown, and eventually drop.

- The necrosis usually progresses for only a short distance into the twigs. It then becomes surrounded by a reddish brown to chestnut – colored scab or crust like callus. Occasionally, entire twigs are girdled and killed.

- Blast may be confused with frost damage when the latter occurs only on fresh, growing tips and leaves.

- The disease can be serious disease where rainfall is frequent during periods of shoot.

-Canker is mostly a leaf –sporting disease when conditions are highly favorable for infection, it also causes defoliation and fruit drop.

- Canker lesions start as pinpoint spots and attain a diameter of 2-10mm. The lesions are initially circular but later may develop irregularly. They are often aggregated at the leaf tend to be about the same size, become of the short period of susceptibility of leaves to infection and the improbability of the occurrence of more than one infection period before the leaf becomes resistant.

- On fruit, the lesions can vary in size, because the rind is susceptible for a longer time and further infection cycles can occur.

-Damping off is world wide problem, which can affect recently germinated seedlings of any citrus cultivars.

-The typical damping- off symptom results from fungal penetrations just above the soil line however, damping –off fungi can also cause a seed rot or pre emergence rot resulting in. sparse stands of seedlings.

- Seedling is rapidly killed by the casual fungi, especially when moisture is abundant. The pathogens can persist in soil for long periods by saprophytic growth or by the production of resistant structures.

Seed should be subjected to hot-water treatment to ensure freedom from phytophthora spp

The disease is also known as Armillaria root rot. The pathogen has a widely distributed in temperate and tropical regions of the world.

- Mushroom root rot sometimes causes a sudden wilt and collapse of a tree, but more often it induces a slow decline, which usually involves the whole canopy. Symptoms of decline do not appear until a major part of the root system has been killed or unless major girdling of the trunk or crown roots has occurred.

- The only aboveground symptoms helpful in the diagnosis of the disease are a rooting of the bark, which sometimes occurs at the trunk.

- Positive identification of mushroom root rot is based on the presence of mushroom like sporophores at the base of an infected tree, and characteristic white, fan-shaped mycelia mats between the rotting bark and the wood.

- Apparently, aerial dispersal of basidiospores formed on sporophores presents relatively little threat to healthy citrus trees.

-Infection in citrus orchards is generally initiated by contact between healthy roots and the infected roots and stumps of trees left behind during land clearing.

- To reduce the risks of attack, land with woody vegetation should be cleared of all stumps and large roots well ahead of planting time.

- Limiting the spread of mushroom root rot is difficult once the disease appears.

- The symptoms on leaves vary from large, necrotic, blighted areas to small, circular spots.

- Symptoms develop on fruit within a few days after infection and first appear as small, slightly depressed black spots. Fruit infected soon after petal fall usually drop immediately.

- Older fruit may not drop; they from periderm, creating corky eruption on the rind surface.

-Infection of new shoots and foliage is seldom severing enough to effect tree growth. The main concern with shoot infection is that it can promote a rapid increase in the inoculums available to infest the fruit. Sporulation is more abundant on disease shoots than on infected fruit.

Black spot is important only as a fruit disease. All commercial cultivars are susceptible to some degrees. Lemons are particularly susceptible, and when the disease appears for the first time in a new area, it usually occurs on lemons

- Infected leaves seldom show symptoms, when present, are small necrotic spots with a gray center surrounded by a dark brown ring and yellow halo.

- Fruit symptoms can be classified under four categories hard spot, freckle spot, virulent or spreading spot, and false melaonse.

- Hard spot lesions are the most typical black spot symptoms. They sometimes appear before the color changes from green to orange. When this occurs, the area around the lesion turns yellow. On colored fruit, green Tissue may remain out side the lesion, which forms as a dark brown to gray-white center.

- Virulent spot develop late in season, particularly when the fruit is fully mature and temperatures rise. It is an important cause of post harvest losses.

- Ascocarps of the fungus are found are found on decomposing fallen leaves. They are mostly aggregated and globosely with a non-papillate, circular ostiole. The asci are clavate cylindrical and eight-spores.

- Pycnidiospores formed on dead leaves on the ground can reach the susceptible fruit only by the splashing of raindrops and they are not considered an important source of inoculums.

-Ascosporous from dead leaves on the orchard floor provide the main source of inoculums.

- The fungus is a widely distributed, with a wide host range. It affects citrus twigs, leaves, bark and fruit, but it is generally a minor pathogen of citrus.

- Lemons are more commonly affected than other citrus. If the fungus attacks young trees in a nursery, losses can be heavy.

- The disease may develop in blossoms or on a twig or limb, appearing first as a yellowish buff spot. Only twigs or branches less than 2cmin diameter are normally attacked.

-The fungus is most frequently found attacking flowers and young fruit of lemons.

- The fungus lives saprophytic ally on decaying organic matter, such as leaves, twigs and fruit.

- Phytophthora spp. Cause the most serious soil borne disease of citrus. These fungi are world wide in distribution. They cause losses in production in arid areas that are irrigated and in areas receiving high rainfall.

- Losses due to phytophthora occur in seedbeds from damping-off; in nurseries from foot rot, gummosis, and root rot; in orchards from foot rot gummosis, feeder root rot, and brown rot; and in packing houses from the further development and spread of brown rot.

Nurseries

Foot rot

Gummosis

Root rot

Seedbeds

Damping – off

Orchards

Foot rot

Gummosis

Feeder root rot

Brown rot

- The most serious disease caused by Phytophthora ssp. Are foot rot and gummosis. Foot rot is an injury of bark on the trunk or roots near ground level.

- Gummosis is a rotting of bark any where on the tree. Infection occurs through wounds or natural cracks in the bark. The fungus grows into the cambium, producing a necrosis, which is commonly accompanied by abundant gum exudation.

- Badly affected tree have pale green leaves with yellow veins, as is typical of a girdling affected area is surrounded by callus tissue.

- The lesions do not extend below the bud union of trees on a resistant rootstock. On susceptible cultivars under favorable conditions for disease development, lesions may extend downward on the trunk.

- Nursery trees and young orchard trees can be rapidly girdled and killed. Large trees may be killed but are usually only partially girdled, and the injury causes a decline of the canopy, with defoliation, twigs dieback, and short growth flushes.

- On susceptible root stocks, lesions may occur below the soil line, and canopy symptoms may develop without obvious damage to the aboveground portion of the trunk.

- Leaves and shoots near the soil may be blighted by Phytophthora, but generally only tender new growth in nurseries under very wet condition is affected

- Phytophthora causes a decay of feeder roots, which slough their cortex, leaving only the stele and giving the root system a stringy appearance. Feeder root rot of highly susceptible root stocks also causes tree decline and yield losses in mature orchards.

- Phytophthora also infects fruit, causing a firm, light brown decay. In the orchard, fruit near the ground become infected when splashed with soil containing the fungus.

- Phytophthora spp is endemic in the soil of citrus orchards in most areas infection usually occurs by means of zoospores, which are released when free moisture is abundant

- One of the most effective and widely recommended control measures is the practice of budding trees well above the soil line and planting them so that the bud union in the orchard is not buried.

- Foot rot and gummosis can be prevented by painting the trunk with copper fungicides or by applying systemic fungicides such as Metalaxyl or fosetyl- Al, Which can be used as soil drenches, foliar sprays, or trunk paints or applied through irrigation systems.

Melanose is present in most citrus-growing countries but is important only where inoculums are abundant and rainfall occurs during the period of early fruit development. All species of citrus are susceptible; grapefruit and lemons tend to be more seriously afflicted than other cultivars. Infection of shoots is severe only if the tree canopy contains much dead wood, providing a substrate for the production of inoculums, as on unthrifty trees or after a severe freeze. The injury to fruit rind caused by melanose remains superficial and is not important if the crop is processed.

Stem-end rots of fruit (see Phomopsis Stem-End Rot, under post harvest Fungal Diseases).

Melanose symptoms appear about I week after infection, as small, brown, discrete or confluent, sunken spots. Epidermal and sub epidermal cells, up to six cells deep, are killed and become impregnated with reddish brown gum. A periderm develops underneath and lifts the dead cells above the host surface.

Pustules on leaves arc at first surrounded by a yellow halo. Later the chlorotic areas re green and corky pustules are the only symptoms .Severe infection of shoot apices while leave s a r c unfolding ca uses leaf distortion and even dieback. When infection occurs at a later stage of shoot development, the effects are less severe; the pustules are smaller and mostly discrete, and little or no distortion of leaves or reduction in leaf size occurs. Leaves become resistant to infection once they actually expanded.

Pustules on fruit vary considerably in size, depending on the age of the fruit at the time of infection. When infection occurs soon after petal fall, the pustules become relatively large and, if numerous enough, coalesce to form extensive diseased areas. Such areas often crack, producing a pattern described as mud cake melanose .Infection at a later stage of fruit development produces relatively small and mostly discrete pustules (Plate 3Y). The distribution of pustules depends on the manner in which water settles or flows over the fruit surface. A tear-streaked melanose pattern results when spore-laden water flows over the fruit in definite paths

Melauose can be distinguished from citrus rust mite injury (see Pest injuries, Resembling Disease Effects) by the raised pustules it produces, in contrast to the smooth blemish cause by rust mites.

Mushroom root rot refers to root rots caused by certain pathogenic mushroom like fungi. These diseases are also known as Armillaria root rot. The pathogen has a wide host range and is widely distributed in temperate and tropical regions of the world. Mushroom root rot is a problem of major, though normally localized importance on citrus trees in Australia, Italy, California, Mexico, and Florida.

Mushroom root rot sometimes causes a sudden wilt and collapse of a tree, but more often it induces a slow decline, which usually involves the whole canopy. Symptoms of decline do not appear until a major part of the root system has been killed or unless major girdling of the trunk or crown roots has occurred. Larger trees with only partially girdled crown roots or trunks may remain profitable, provided no further extension of the rot occurs.

Unlike some other declines, mushroom root rot tends to affect localized groups of trees, and it usually does not affect all of them at the same time. When replant trees of various ages are in proximity to a declining tree, the disease can be suspected to have caused the loss of the trees that were replaced.

The only aboveground symptom helpful in the diagnosis of the disease is a rotting of the bark, which sometimes occurs at the base of the trunk. The trunk lesions are similar to those caused by foot rot; however, they always arise from infected roots, whereas foot rot lesions seldom extend much below ground level.

Positive identification of mushroom root rot is based on the presence of mushroom like sporophores at the base of an infected tree and characteristic white, fan-shaped mycelia mats between the rotting bark and the wood .Black rhizomorphs (shoestrings) are sometimes found on the bark surface. Infected roots have a pronounced mushroom odor.

In California, Australia, and Italy, there is apparently little difficulty in diagnosing mushroom root rot. In Florida, however, diagnosis is often difficult: sporophores are seldom present, and by the time a tree exhibits an obvious decline, the fungal sheets beneath the bark have usually become unrecognizable. Even after extensive observations of the dead roots of an uprooted tree, no mycelia mats may be found.

Sooty mold is a black, removable fungal growth, most commonly of Capnodium citri Berk, & Desm, which appears on the surface of leaves sterns, and fruit after trees become infested with honeydew-excreting insects. The mold deposits are usually heavier on the upper surface than on the lower surface of leaves. Although sooty mold does no penetrate the tissue; it may affect tree performance and fruit development by interfering with photosynthesis. The mol deposits may delay fruit coloring and can be difficult to remove in the packinghouse from cultivars with a rough rind.

Honeydew-excreting insects include aphids, soft scales mealy bugs, and whiteflies. Where sooty mold is serious enough to potentially affect fruit yield or quality, the insects responsible for the honeydew need to be controlled. Where circumstances permit, spray oil can be applied to loosen the mold deposits thereby facilitating their subsequent removal by wind and rain Spray oil can also control some of the pests that excrete honeydew.

D - Post harvest Fungal Diseases

Alternaria rot is mostly a problem on fruit that is stored for a long time, but it sometimes develops in the orchard, where it may cause premature fruit drop. In orchards, it can be particularly serious on navel oranges, because imperfections at the navel facilitate infection. The disease is also a problem for the processing industry, because only a small amount of rot imparts a bitter flavor, and small black fragments of rotted tissue spoil the appearance of the juice.

Diseased fruit in the orchard color prematurely and may develop a light brown to blackish discoloration of the rind at or near the astylar end. Some fruit, however, show no external evidence of infection and must be cut to reveal internal decay, called black rot or center rot.

Aspergillus rot has been reported in many citrus-producing regions and on all types of citrus fruit. Generally it is a minor disease, causing problems only when fruit is held in storage at high temperatures.

Several species of Aspergillus cause a rot of citrus fruit; the most common one is A. niger. Spores of this fungus are produced in chains from sterigmata on swollen knobs or vesicles at t he tip 01' the conidiophores. The conidia are 2.5-4 Um in diameter and rough-walled.

Initially, the decay is light-colored, very soft, and easily punctured, somewhat like sour rot. On oranges, the lesions arc pale orange, yellow, or primrose yellow, and they eventually become sunken and wrinkled ·Mycelium is produced on the lesion surface, and later the rotted surface is covered with a black, powder y layer of spores.

The fungus survives as a saprophyte on many different plant substrates. Spores are carried by air currents to fruit surfaces. Infection occurs through injuries inflicted during harvesting and handling. The decay can spruce in packed containers from infected to adjacent healthy fruit. The optimum temperature for fungal growth is near 32° C.

Blue mold occurs in all citrus-production regions of the world, like green mold, although it is generally less prevalent than green mold. These two types of mold behave similarly in many respects. All types of citrus fruit arc susceptible to blue mold.

Early symptoms are similar to those of green mold and sour rot. Diseased tissue becomes soft, watery, and slightly discolored and is easily punctured; the lesions do not enlarge as rapidly as those of green mold. A white, powdery growth of mycelium develops on the lesion surface, and soon blue spore mass forms, leaving only a narrow white fringe of mycelium surrounding tile lesion. A pronounced halo of water soaked, faded tissue surrounds the lesion, between the fringes of mycelium and the sound tissue. The blue spores covering the fruit may become brownish olive with age. Healthy fruit in lacked containers are soiled by spores dislodged from diseased fruit.

Blue meld and green mold are similar in disease cycle, infection process, and epidemiology. However, in contrast to green mold, blue mold spreads in packed containers and results in nests or pockets of diseased fruit. Like green mold, it develops most rapidly at about 24° C; however, blue mold grows better than green mold below 10°C and may predominate over green mold in fruit stored at such temperatures. It may also predominate in fruit treated with benzimidazole fungicides, because resistance 10 these materials usually occurs more frequently in isolates of P. italicum than in P.digitatum, the cause of green mold.

In addition to causing losses of fruit in orchards (phytophthora- Induced Diseases), brown rot is a post harvest problem. Occurs in citrus-growing areas where rainfall occurs during the late stages of fruit development. The disease attacks fruit of all cultivars and can be particularly serious on lemons.

Generall,the pathogens attack relatively few fruit, and only those close to or in contact with the soil.
P.citrophthora produces sporangia on infected fruit more rapidly and abundantly than P. parasitica, which may contribute to the predominance of P. citrophthora in brown rot.

The decay is first observed as a light brown discoloration of the rind. The affected area is firm and leathery, and it retains the same degree of firmness and elevation as t he adjacent healthy rind. Delicate white mycelium forms on the rind surface under humid conditions .Infected fruit have a characteristic pungent, rancid odor, which distinguishes this disease from stem-end rots.

Under wet conditions, zoospores are splashed Iron the soil onto low-hanging fruit. Under favorable conditions, spores produced on these fruit are then splashed higher into the canopy.

Fruit that become infected shortly before harvest may not show symptoms until after they have been held in storage fur a few days, lf such fruit are packed, and they may cause other fruit in the same container to become infected.

Gray mold is caused by the widely distributed fungus Botrytis cinerea Pers. ex Fr., which attacks many different hosts, The fungus is a problem mostly on lemons in regions with cool, foggy, drizzly weather during flowering, It also causes reduced fruit-set, fruit drop, and a ridging deformity of the rind (sec Botrytis is Blight).

Rot associated with gray mold on lemon fruit appears as a brown, leathery decay, similar to cottony rot, Trichoderma rot, and brown rot. It tends to be darker than cottony rot and lighter than Trichoderma rot. The odor of Botrytis -infected fruit is not as distinctive as that of fruit with brown rot or Trichoderma rot, at high humidity, distinctive patches of gray-brown to olive spore masses appear on the fruit surface. The pathogen spreads readily by contact with adjacent fruit, giving rise to large nests of diseased fruit in packed containers.

In many countries, green mold is the most common and serious post harvest disease of citrus. Millions or spores or the causal fungus are produced on the surface of infected d fruit, and these spores are present in the field, packing area, storage room, transit containers, ,and marketplace.

Initial symptoms of green mold arc similar to those of sour rot and blue mold. In the early, pinhole stage, the decay appears as a soft. watery, slightly discolored spot, 6-12mm in diameter The spot enlarges to 2-4 cm in diameter within 24-36 In at 24° C. and the rot soon involves the juice vesicles, White mycelium appears on the rind surface, and after it reaches a diameter of approximately 2,5 cm. olive green spores are produced ,The sporulating area is surrounded by a broad zone of white mycelium and an outer zone of softened rind, The entire fruit is soon encompassed by a mass of olive green spores, which are easily dispersed if the fruit is handled, shaken, or exposed to air currents, If the relative humidity is low, the whole fruit shrinks to a wrinkled, dry mummy, If the relative humidity is high, other molds and bacteria become involved, and the fruit collapses into a soft, decomposing mass.

Spores detached from diseased fruit during the opening of packed canons affect the value of the remaining healthy fruit by settling on them and causing soilage.

The fungus survives in the orchard from season to season primarily as conidia. Infection is initiated by airborne spores, which enter the rind through injuries. Even injuries to the oil glands alone can promote some infection. P. digitatum can also invade fruit through certain physiologically induced injuries, such as chilling injury, oleocellosis, and stem-end rind breakdown, In packed containers, the fungus docs not usually spread from decayed fruit to adjacent intact healthy fruit. The infection and sporulation cycle can be repeated many times through the season in a packinghouse, and inoculums pressure increases as the picking season advances, if precautions are not taken,

Green mold develops most rapidly at temperatures near 24°C and much more slowly above 30⁰e and below 10°C. The rot is almost completely inhibited at 1°C.

Phomopsis stem-end rot is a serious decay, similar to Diplodia stern-cud rot. It is mort prevalent in humid sub, tropical and tropical regions than in drier or cooler citrus, growing areas, All types of citrus are susceptible to this decay The fungus that causes Phomopsis stem-end rot also cause; melaonse.

Phornopsis stem-end rot occurs after harvest during transit or storage, The fungus proceeds from the stem end of the fruit through the rind and central axis, and it eventually invades t h. juice Sacs, In the initial stages, the decay cannot be distinguished from Diplodia stem-end rot without isolation and culture of the causal organism, However, with Phomopsis stem-end rot, the infected tissue shrinks, and a clear line of demarcation is formed at the junction between diseased and healthy rind. No streaks or fingerlike projections of discolored rind, which are characteristic of Diplodia stem-end rot, are normally seen in t h rind of fruit affected by Phornopsis stem-end rot. The fungus docs not grow through the center of the fruit as rapidly a Physalospora rhodina, the cause of Diplodia stem-end rot, an unlike P. rliodina, it rarely reaches the stylar end by this rout before reaching it by way of the rind, Surface mycelium occasionally develops on the rind if the fruit is exposed to very moist conditions, but the disease does not spread from decayed to healthy fruit in packed cartons,

D.citri completes its lift cycle as a saprophyte on dead twig and inoculums originates from this substrate, Hence, Phomops stern-end rot is likely to be particularly severe on fruit harvests from unthrifty trees, Conidia dispersed by rain splash establish quiescent infections in necrotic tissue on the button, in manner similar to that of P.rhodino . Whereas D. citri can penetrate intact rind and cause a melanose symptom for only limited time after petal fall, it can infect the button at any Stab, of fruit development. After harvest, the fungus enters the will through natural openings that develop between the buttons at the fruit after the button senesces.

More than 40 nematode species have been associated with citrus worldwide, but not all reduce tree productivity. The most widely occurring species is the citrus nematode, Tylenchulus semipenetrans,which is especially prominent as a replant problem. The burrowing nematode, Radopholus citrophilus, causes a serious disease called spreading decline, which has been reported only in Florida. The effects of the feeding of other nematodes on citrus roots are less well known.

The name slow decline designates unthrift ness of trees due to the feeding of large populations of the citrus nematode, Tylenchulus semiprnetrans Cobb. This nematode was first recognized in association with-citrus roots in the early 1900s and has since been detected in most citrus-growing areas of t he world. It is assumed that the nematode was distributed into new geographic regions unknowingly on nursery stock.

The sedentary, saccate citrus nematode females (0.35-0.40 mm long) are generally found in small groups on the surface of fibrous citrus roots under debris-covered egg masses embedded in a gelatinous matrix. Each female is associated with a permanent feeding site located within the root cortex, which is composed of nurse cells containing dense cytoplasm. The female produces fro 11l 75 to 100 eggs, which average 33 X 67 um and require 12-14 days to hatch under laboratory conditions. Development to the second-stage juvenile occurs within the egg. Hatching requires the presence of free moisture in soil. Adult males (0.3-0.4 mm long) develop within 12 days of hatching, and they do not feed. The females are obligate parasites, whose development is dependent upon successful establishment and maintenance of feeding sites. Reproduction is facultative parthenogenesis

Four biotypes of the citrus nematode have been identified worldwide. These differ in host range and relative virulence, but most rutaceous species are susceptible to some extent to all of them. Additional hosts include olive, lilac, grape, and persimmon. However, the nonrutaceous hosts may not be of great importance in building up nematode populations in areas where citrus is later planted. When nematode-free nursery trees are planted on areas virgin for citrus, the nematode seldom becomes an immediate problem, which suggests that infestations do not commonly come from native plants

The citrus nematode docs not kill trees but may greatly reduce their vigor. The growth of young trees may be impaired by residual nematode populations in old orchard sites. Trees whose roots are attacked by high populations of the nematode may have leaf yellowing, sparse foliage, and small fruit. Affected trees do not respond to fertilization, and they succumb to water stress earlier than healthy trees. The degree to which trees suffer from the feeding of the nematode is greatly influenced by cultural practices, soil type, and the rootstock cultivar.

Roots infected by significant numbers of tile citrus nematode may appear coarse and dirty, because of the adherence of soil particles and debris to egg masses, but this feature is not diagnostically reliable. The identity of the nematode still has to be confirmed microscopically.

Soil samples for citrus nematode evaluation should be collected in locations with abundant feeder roots between the trunk and the drip line of the tree. Larvae may be extracted from soil by the Barman funnel method or from roots by root incubation. For quantification of the female population density, females may be counted at a magnification of 40X on fibrous roots stained with acid fuchsine, or they may be retrieved and counted following root maceration with a blender.

Spreading decline is caused by the burrowing nematode Radopholus citrophilus Huettel, Dickson, & Kaplan, which is an obligate parasite, surviving for no more than 6 months in the absence of living roots, However, it has a wide host range, including ornamentals, cultivated crops, and many weeds, which enables it to persist in the absence of citrus trees,

Although the burrowing nematode has been detected throughout the citrus-growing areas of Florida, spreading decline occurs only in areas or central Florida with deep, well drained, sandy soils, Apparently, infested trees growing in shallow, poorly-drained soils or in soils with high levels or organic mutterer and better water retention arc not seriously affected by the nematode,

When the burrowing nematode feeds on fibrous roots, it causes a cessation of apical meristem activity. An infested tree may have 50% fewer functional feeder roots than a healthy tree. Affected trees have an undernourished appearance, with sparse foliage, dead twigs, and small leaves and fruit. Under drought conditions, fruit-set is reduced and trees wilt readily, Symptoms are generally present on groups of trees, with the number of infested trees increasing with time, and hence the name spreading decline.

Sampling for R. citrophilus is difficult because of the deep Vertical distribution of the nematode. Fibrous roots should be collected between 0.3 and 1.0 m below the soil surface. Nematodes may be extracted by jar incubation.

Populations of R. ciuophilus vary with depth. Between 0 and 30 cm, less than 20% of the destroyed by the nematode; at depths between 25 and 25-30% of the roots may be destroyed; and at depths below, 75 cm, 90% of the feeder roots arc often destroyed. The optimum temperature for nematode growth and reproduction is 24°C; however, reproduction and root invasion may occur at temperatures from 12 to 32°C. In the upper root zone, to depth of 25 cm, soil temperatures can rise above the upper limit for nematode development

Cachexia primarily affects some mandarins, certain tangelos, ale mow, and kumquat. Disease problems occur when infected bud wood is grafted on sensitive rootstocks or when existing old-line infected trees are top worked with a sensitive cultivar the cachexia viroid is widespread in old-line cultivars. The degree of injury can vary from slight stunting to severe stunting, chlorotic. And tree decline Cachexia and xyloporosis have often been considered synonymous, but the disease in sweet lime originally described as xylnporosis was probably not caused by the cachexia viroid.

Most citrus species and cultivars can be infected by the cachcxia viroid, but many are symptom less hosts. Some cultivars, such as Orlando tangelo are highly reactive. Mild symptoms in sweet lime, rough lemon, and Rangpur lime have been associated with cachexia infect ion, but confirmation is still needed with pure sources of inoculate. Transmission of the viroid to cucumber was recently reported.

Symptoms of cachexia in reactive citrus hosts arc discoloration and gum impregnation of the bark, which is revealed by scraping away the outer bark or by making tangential cuts to remove the outer bark tissues. The inner bark surface is bumpy, with numerous rounded bumps or pegs, which fit into corresponding depressions in the wood. These bark projections, unlike those associated with tristeza and cristacorus, are not sharp and dentate. Warm conditions favor symptom expression. Severely affected trees are stunted and chlorotic, and may decline and die. More usually, affected trees linger indefinitely in a debilitated condition

Symptoms similar to those of cachexia have been described as gummy bark in sweet orange, but the relationship between these three diseases remains unknown. Tristeza also causes stunting, chlorosis, and stem pitting of the rootstock in trees budded on ale mow, but the pits are sharper and less gum is produced.

Recently, cachexia has been associated with a viroid species, which contains approximately 300 nucleotides, or about 70 fewer than citrus exocortis viroid Nucleic acid hybridization studies indicate that it is distinct from CEV.

Concave gum is a graft-transmissible virus like disease common in old-line citrus trees. It is often considered pan of the psoriasis complex. Blind pocket is probably a closely related disease, which differs mainly in the sharpness of the concavities in the trunks of affected trees. Concave gum and blind pocket may both reduce tree vigor and productivity.

The causal agent of concave gum and blind locket has not been characterized but is presumed to be a virus.

Mandarin and sweet orange trees affected with concave gum or blind pocket have concavities in the trunk and main branches. Bark scaling or external gumming is uncommon, but gum is deposited in concentric rings in the wood of affected trees and may be present at the base of the concavity or pocket when the bark is removed. In the spring, young leaves of trees affected with concave gum or blind pocket may develop chlorotic necking or "oak leaf" patterns.

Some healthy mandarin and mandarin hybrids may exhibit trunk deformities or depressions similar to those caused, by concave gum or blind pocket, but not leaf symptoms.

In contrast to cristacortis, cachexia, and tristeza-induced stem pitting, concave gum disease does not cause the bark to become abnormally thickened over depressions in the wood. Leaf symptoms similar to those produced by concave gum often appear in trees affected by cristacortis and impetrator. Symptomatology is often confusing in trees affected by several of the pathogens of the psorosis complex.

Exocortis is a bark-scaling and tree-stunting disease induced by citrus exocortis viroid (CEV). The symptoms were first described in 1948 and soon afterward were associated with a bud-transmissible agent. The descriptive name scaly butt has also been a policed to this disease in Australia.

The causal agent occurs in many countries, and its commercial effects are primarily on trees grafted on trifoliate orange, some trifoliate orange hybrids, and Rangpur lime. Affected trees are stunted (Plate 81) to various degrees, but CEY is rarely lethal, and fruit quality is not affected. Extensive losses in production result when bud wood infected with severe isolates is propagated on sensitive rootstocks. Trifoliate orange, cit ranges, and Rangpur lime are desirable tristeza-tolerant rootstocks, but the occurrence of CEV has restricted their use to grafts with bud wood sources that are CEY-free. Recently, the deliberate use of cxocort is-Iike agents to induce tree dwarfing for high-density plantings has been investigated.

CEV can infect most citrus species and cultivars, some citrus relatives, and several no citrus hosts, but it is latent in most of them. Sensitive species include trifoliate orange, Rangpur lime, and some citrons and lemons, which develop stem blotching or bark splitting. Some citrons are affected with leaf epinasty and vein necrosis. CEV-infected sweet orange, grapefruit, and mandarin trees arc symptom less, but if they arc budded on sensitive rootstocks, bark scaling occurs o n the rootstock, and the entire tree is stunted. Longitudinal splits occur in the initially, and eventually patches of dead scales outer bark appear. The inner bark remains alive, and a new layer of scales can form as the outer ones slough off.

Causal Agent

C E V consists of an infect circular single-stranded RNA molecule with 371 nucleotides, which are highly base-pierced, running a stable to like structure. Some regions of the CEY molecule have homologies with some other plant viroids, including potato spindle tuber viroid and chrysanthemum stunt viroid.

Several other viroid species haw recently been purified from Etrog citron plants with mild exocortis-like symptoms. These citron viroid s are all smaller t ha n CEY, and cross-protection arid nucleic acid hybridization tests indicate that they are not highly homologous with it. This suggests that citrus may be infected by several other viroids besides CEY and the cachexia viroid.
The CEV molecule is highly resistant to heat inactivation and to many chemicals used to inactivate viruses. It can remain infectious for long periods in dry tissue or as a contaminant on dry surfaces. It can be inactivated by hydrolysis or by rib nucleuses under appropriate conditions.

Greening
            Greening is a highly destructive disease, which was once considered to be a virus disease but is now known to be caused by a fastidious, phloem-limited bacterium. It apparently originated in China and was common there by 1925. It seriously affects production in the Philippines, Taiwan, and Indonesia and in parts of India, Southeast Asia, and Africa. Trees infected while young fail to reach production. When mature trees are infected, they soon become nonproductive. Other commonly used names for the disease are yellow shoot, or huanglonglbing, in China; likubin, in Taiwan; leaf mottling, in the Philippines; and win-phloem degeneration, in Indonesia.

Host Range and Symptoms
            Two forms or strains of greening have been described. The African form induces symptoms best under relatively cool conditions (2G-24°C). The Asian form induces symptoms well under warm conditions (up to 32°C). The two strains have "similar host ranges and symptomatologies.

The greening agent can infect most citrus cultivars, species, and hybrids and some citrus relatives. Most sweet oranges, mandarins, arid mandarin hybrids are severely affected. Grapefruit, Rangpur lime, lemons, calarnoudin, and some pummelos show somewhat less severe symptoms. Mexican lime, some pummelos, trifoliate orange, and trifoliate orange hybrids arc the most tolerant and may develop only same leaf mottling. Early symptoms are frequently a yellowing of only one limb or sector of the tree canopy. The casual agent moves slowly within the tree in the absence of vectors. Chronically infected trees are sparsely foliated and are affected by extensive twig or limb dieback

When trees are first infected, leaves may develop vein chlorosis or chlorotic mottling of all or part of the leaf blade. On chronically infected trees, leaves are small and frequently have zinc deficiency symptoms, with green veins and chlorotic intervene areas. Somewhat similar symptoms can be produced by stubborn and severe forms of tristeza. Zinc deficiency symptom, are also frequently associated with the early stages of blight; however, greening does 1I0t induce the xylem dysfunction and wilting observed in blighted trees. Greening differs from tristeza in that it does not produce the characteristic stem-pitting symptoms of tristeza on indicator hosts. Mandarins are tolerant to certain severe strains of tristeza but subscribe to greening Fruit on infected trees are frequently small, underdeveloped, and poorly colored, and hence I he origin of t he name greening. The juice affected fruit is low in soluble solids, high in acids, and abnormally bitter.

Causal Agent
            The causal agent of greening is a fastidious, phloem-limited, end cellular bacterium. Electron microscopy of infected tissues indicates that the bacterium is polymorphic and, in contrast to Spiro plasma citric and mycoplasmas, has a three-layer envelope or cell wall, approximately 25 nm thick. Filamentous and spherical bodies helve been observed, and replication by budding has been suggested. Isolation of the causal bacterium has been reported but not fully confirmed

Psorosis

            Psorosis is probably a complex of several diseases, which share some common symptoms. These diseases are present in old-line trees in many citrus-growing areas of the world. Bark scaling forms of psorosis are destructive and cause tree debilitation and decline. Other forms are not as destructive but are also considered detrimental to tree vigor and fruit production. The incidence of psorosis has been reduced in many areas by the use of virus-free bud wood; however, despite this precaution, the n a t u r al spread of a severe form of it remains a major problem in Argentina.

Psorosis B, a severe form of the disease, may be synonymous with ring spot. Concave gum (blind pocket), crustaceous, and impietratura also share some similarities with psorosis, in that all cause young leaf symptoms.

Citrus crinkly leaf and infectious variegation were once considered forms of psorosis, but they are caused by distinct ilarviruses.

Host Range and Symptoms
            The psorosis disease agent can be graft-transmitted to most citrus species and their hybrid, and to son, citrus relatives. Some hosts develop few symptoms, if any, wiles others are highly reactive. Leaf symptoms are generally express vividly in sweet oranges and mandarins, but even cultivars they may be erratic. Seedlings of pineapple Madam Vinous sweet oranges, mandarins, and Sweet tangor are commonly used as indicators. The causal agent of pserosis A is usually widely distributed within infected plants. The causal agent of psorosis B is sometimes reconvened only Iron symptomatic tissue.
Symptoms are induced on young leaves by all pathogens causing psorosis and psorosis-like diseases. These range from chlorotic flecks, which are irregularly distributed areas of vein banding, to more general leaf mottling or distinct chlorotic patterns. The symptoms are best seen in young leaves nearing full expansion on spring or fall growth flushes. Leaf symptoms usually fade as the leaves mature, except in psorosis-B, in which they persist in mature leaves. Young leaf symptoms are often faint or absent in hot weather.
Leaf flecking is usually associated with bark-scaling forms of. psorosis, wireless oak leaf patterns are associated with limb symptoms of concave gum or blind pocket. psorosis -like patterns in young leaves are also observed in trees infected with crisiacortis and impietratura. Those diseases are differentiated from psorosis by wood or fruit symptoms. Psorosis-induced vein flecking in Mexican lime can be mistaken for a unites reaction, hut the latter is a distinct vein clearing symptom rather than vein banding.
Classical psorosis a produces scaling and flaking of the bark . Wood becomes impregnated with gum, which forms an irregular circular pattern in the trunk, viewed in cross section.
When twig bark from a tree affected with psorosis A is grafted to sweet orange seedlings, a necrotic shock reaction occurs in new shoots and leaves. Subsequent growth l1ushes develop the characteristic young leaf necking without necrosis.

Psorosis; B is a severe form of the disease, in which bark lesions are rampant and expand rapidly, with sloughing of large strips of bark. Twigs develop raised , suberized, gum-impregnated lesions. Typical symptoms of type B can be induced by inoculation of sweet orange seedlings with bark taken directly from a trunk lesion of a tree affected with psorosis A.

Causal Agent
The casual Agent or agents of psorosis and psorosis-like diseases (ring spot, impietratura, crislacortis, and concave gum) have not been characterized but arc presumed to be virus like.

Transmission and Epidemiology
            Psorosi:: is readily graft-transmitted. Mechanical transmission of psorosis B has been reponed (see Ring spot). Seed transmission is apparently rare. It was reported in trifoliate orange a!,d Carrizo and Troyer citranges. In those cases, however, concave gum, and not psurosis, may have been involved (see Concave Gum). The method of natural spread of psorosis in Argentina
Tristeza

Citrus tristeza virus (CTV) is the most economically important virus pathogen of citrus and one of the major disease problems affecting citrus production worldwide. Millions of trees on sour orange rootstock have been killed or rendered unproductive by a CTV-induced decline in Argentina, Brazil, the United States, Spain, and Venezuela. Other countries that use sour orange rootstock are being threatened. In addition, some isolates of the virus cause a serious stem-pining disease of susceptible scion cultivars, even when these are propagated on tolerant rootstocks
-The virus apparently originated in Asia and has been disseminated by the movement of infected plant materials and by aphid vectors. Epidemics of tristeza decline began in the Western Hemisphere in the 1930s, following the introduction of CTV-infected plants. The viral etiology of the disease was recognized in I ':146 and was later associated with seem-pining syndromes in other hosts.

Host Range and Symptoms

CTV infects nearly all species, cultivars, and intergeneric hybrids of citrus and some citrus relatives. The only known nonrutaceous host is Passiflora .

Symptom expression in citrus hosts is highly variable and affected by environment, host species, and the severity of the isolate. In general, mandarins are especially tolerant to CTV infection. Sweet orange, sour orange, rough lemon, and Rangpur lime are usually symptom less but may react to some severe isolates. Reactive hosts include limes, grapefruit, some purnmelos, alemow, some sweet oranges, some citrus hybrids, and some citrus relatives. Stunting, stem pitting, learn cupping, vein clearing, chlorotic, and reduced fruit size are common symptoms. Vein clearing and stem pitting in Mexican lime are diagnostic.

One of the most economically significant symptoms of the disease is the decline reaction, which occurs in infections of sweet orange, mandarin, or grapefruit trees on sour orange rootstock. Virus infection in the scion causes necrosis in the phloem of the sour orange rootstock immediately below the bud union. This necrosis has a girdling effect, and trees decline as starch reserves in the rootstock are depleted. In declining trees, inverse pinhole pitting (honeycombing) may occur on the inner face of-the sour-orange bark, and some thickening of the bark may occur in this area. Often trees decline rapidly, and the only macroscopic symptom of this quick-decline react ion may be a yellow-brown discoloration at the bud union. When sour orange seedlings are budded with scions infected with decline-inducing isolates of CTV, the propagations are stunted and often chlorotic, but they normally do not decline. Some isolates of the virus do not induce decline symptoms, even in trees on sour orange.

Stern pitting may sometimes cause a bumpy or ropy appearance or the trunks and limbs of larger trees. Deep pits in the wood are present under depressed areas of the bark. Some severe stem-pitting strains cause more general disruption of the stem tissue. The bark is abnormally thick, the wood has- many fine pits, and the trees soon decline. Some isolates of CTV cause stunting and chlorosis when graft-inoculated to seed lings of sour orange, acid lemon, and grapefruit. This reaction, called seedling yellows, is used experimentally to detect severe isolates of the virus. It is seldom observed in the field.

Casual agent

- CTV is a Closter virus with flexuous rod-shaped particles approximately 12 X 2,. The particles contain a single-stranded RNA with a molecular weight of approximately 6.5 x 10". The primary cased protein has a molecular weight of 26,000. The flexuous rods are susceptible to shearing, and purified preparations usually contain particles of heterogeneous length. Infectivity has been associated only with full-length particles. Numerous isolates of CTY have been described, varying in vector ability and in the symptoms they induce in specific citrus hosts. Distinct serotypes have not been reported. The virus is essentially phloem-limited, but it has been observed in the cortex of young shoots. Large aggregates of CTV particles in Para crystalline inclusions commonly occur in young phloem tissues; they are easily detected by light microscopy with an azure a staining procedure or by immunofluorescent microscopy Transmission
-CTV is readily graft-transmitted if a vascular union is established between donor and receptor tissues. Buds, internodes stem tissue, and leaf tissue has all been used successfully as inoculums. Bud propagation has accounted for long-distance spread of CTV and for a very high incidence of tristeza in new plantings where t he bud wood source trees were infected.
-CTV is transmitted in a semi persistent manner by several aphid species; the efficiency of transmission varies with the aphid species, the isolate, and the donor and receptor hosts. Toxoptera citricida is the most efficient vector, and it is often very abundant on citrus. It is not yet present in North America or in the Mediterranean Basin. Aphis gossiped is an efficient vector of many isolates but is usually not abundant on citrus. A. citricola is a less efficient vector but is frequently abundant on citrus and may be more important in spreading the virus in the field than laboratory transmission tests indicate. 1aurantii has transmitted CTV under experimental conditions but is probably not a significant vector.

G - Insects & Pests on Citrus

Introduction:

The citrus crop can be affected by many pests which not only reduce vigor and yield of the fruit but also act as vectors of potential viral and bacterial diseases, such as (tristeza, greening and canker which lead to citrus decline)

A-Aphids

-Aphids are small (1-3 mm), pear shaped, and soft bodied insects. They can be winged or wingless and are usually slow moving

Damage
- Aphids are most abundant when there is new, flushing growth, usually in September/October and February to April
- The aphids cluster on blossoms and young shoot growth, causing twisting and distortion
- Aphids also excrete honeydew on which sooty mould, a black fungus, often grows. Sooty mould can grow on any part of the tree, including the fruit

Control
            To control aphids on citrus use one of the following

             APHOX                       50%                  50g/100L.W
OR       MOSPILAN                20%                  25g/100L.W
OR       NUDRIN                     90%                 75g/100L.W

B- Mealy bugs

- Mealy bugs are coated with a fluffy layer of wax
- Three species, were recorder under Egypt conditions : Icerya aegyotiaca - Planococcus citri
- Mealy bugs are up to 3 mm long and are found on naval ends and under calyxes of fruit, as well as between touching fruit and leaves
Damage
- Mealy bugs extract the juice from the cells of tender branches and fruits, turn pale, wilt and dry up.
- Simultaneously, they excrete large amount of honeydew on which grows a sooty mould fungus
- In sever infestation the flowers do not from fruits
Control
Use
            ACTELLIC                    50%                  150cm/100 L.W
OR      MOSPILAN                   20%                  25g/100L.W
OR      APPLAUD                      25%                  75-100cm/100L.W
OR      CHALLENGER             36%                  40cm/100.W

C- Whiteflies

have an annulated, wedge-shaped body, lemon-yellow in color and about 1/200 inch in length. They have 2 pairs of legs and piercing-sucking mouthparts. They can just barely be seen with the naked eye and can be seen better with a 10-power magnifying glass

- The life cycle of the citrus rust mite is completed within 5-7 days during the summer. Eggs are spherical (round), transparent, and laid singly on leaves, stems, and fruit of all commercial citrus varieties. The mite has 2 immature stages that are similar to the adult in appearance. Although present throughout the year, in Florida the citrus rust mite is most prevalent during the summer months.

- Injury from extensive citrus rust mite feeding causes surface blemish to the fruit. This can reduce theexternal quality of marketable fruit, and reduce yield by causing premature fruit drop and reduced fruit size. None of these affects the internal fruit quality.

Control
Use       NOMOLT                 35cm/100L.W
or         CHALLENGER        40cm/100L.W
or          ORTUS                      50cm/100L.W

Citrus red mites (purple mites) are only about 1/50 inch long. They are bright red to deep purple in color and infest leaves, fruit, and new growth. Injury results from feeding and appears as a scratching or etching of fruit and the upper surface of leaves. In periods of prolonged dry weather, they can cause a collapse of leaf cells and even leaf drop. Use a 10X magnifying glass to inspect for citrus red mites and eggs on the upper surface of the leaf, looking especially along the midrib, as well as in angular crevices of the leaf stems and the young, tender twigs. Citrus red mites are more numerous from May through July, but can be the most damaging in the fall and winter months during periods of low rainfall or inadequate irrigation

-Texas citrus mites are about the same size as citrus red mites, but are brownish-green in color with dark brown to greenish spots or bars near the lateral margins (edges). Numbers of mites are generally much heavier along the midrib on the upper surface of leaves . They also are most numerous May through July, and most damaging October through February.

- Six-spotted mites are also about the same size as citrus red mites but are white-yellow to sulfur-yellow in color. Adults usually have six dark spots that are barely visible with a 10-power magnifying glass, arranged in two rows on the back or abdomen. These mites live in colonies on the under surface of leaves, especially along the veins and midribs. Injury appears as yellow spots , often cupped toward the top of the leaf. Six-spotted mites prefer grapefruit, but can be found occasionally on other varieties of citrus. They usually appear in early February and have disappeared by mid July.