Fungal Disease

1- Cotton Root Rot

The causal organism          Phymatotrichum omnivourm

          The fungus is prevalent in calcareous clay loam soils with a pH range of 7.0 to 8.5 and in areas with high summer temperatures.

Symptoms

Symptoms are most likely to occur from June through September when soil temperatures reach 28 ° C. The first symptoms are slight yellowing or bronzing of the leaves. The upper-most leaves wilt within 24 to 48 hours after bronzing, followed by wilting of the lower leaves within 72 hours. Permanent wilt occurs by the third day, followed by death. The leaves remain firmly attached to the plant. Affected plants die suddenly, often after excellent growth. Trees and shrubs may die more slowly.

Roots are usually extensively invaded by the fungus by the time wilting occurs. Affected plants can be pulled from the soil with little effort. Root bark is decayed and brownish, and bronze colored wooly strands of the fungus are frequently apparent on the root surface.

The fungus generally invades new areas by continual slow growth through the soil from plant to plant. It may also be moved about on roots of infected plants moved to new areas. The fungus can survive in the soil for many years and often is found as deep in the soil as roots penetrate. Affected areas often appear as circular patterns of dead plants. These areas gradually enlarge during the season or in subsequent years as the fungus grows through the soil from plant to plant.


2- Mushroom Root Rot

The causal organisms         Armillaria tabescens, Ganoderma lucidum or Armillaria mellea

Symptoms

Dead areas are produced in the bark on the main stem and larger roots just beneath the soil surface. When the dead bark is peeled back, the white growth of the fungus over the surface of the wood constitutes a distinguishing character of this disease. The fungus occurs most frequently in woody areas or in recently cleared land. All stumps and large roots should be removed as thoroughly as possible before orchard or valuable trees are set in infested land. Planting of newly cleared land to an annual crop for several years helps to reduce this fungus

Since the pathogen exists in most forested soils, mushroom root rot is usually related to previous stress in the tree which lowers host vigor. The stress may be environmental such as drought, flood or poor drainage; people-caused, such as construction, fill, or chemical injury; or biotic, such as severe or repeated defoliation by insects or diseases. Helping the tree to avoid stress is therefore an important part of prevention.

Control

The fungus can survive on small dead roots. Therefore, remove from the soil as many roots as possible before replanting. In addition, soil around the old root system should be fumigated or replaced with fresh soil. The objectives of these treatments are to reduce the amount of the fungus in the soil and to remove any substance that the fungus can live on.


Nematode Disease

1- Root Knot Nematode

The causal organism         Meloidogyne ٍٍSٍٍpp.

Symptoms

disease occurs in nearly all parts on most plant species. Certain species are specific to individual plant hosts. Above ground symptoms are similar to many other root diseases or environmental factors limiting water and nutrient uptake. These symptoms consist of wilting during periods of moisture stress, stunted plants, chlorotic or pale green leaves, and reduced yields. Most characteristic symptoms; however, are those occurring on underground plant parts. Infected roots swell at the point of infection and form knots or galls. Several infections may occur along the same area resulting in large fleshy galls. The appearance of galls will depend in part upon the host and the nematode species involved. Generally, fast growing annuals will have a large flesh gall and woody perennials, small hard galls. Infected roots are retarded in growth and lack fine feeder roots. Rotting of roots may develop late in the season. When tubers, corms or other edible root portions are infected, small swellings or pimpling is evident on the surface.

Life Cycle
The Pathogen (Meloidogyne spp.): Although different species of root knot nematodes vary in their host-parasite relationships, all have basically the same life cycle. The infectious stage of this nematode is the second stage larvae which occurs free in the soil (The larvae has already molted once in the egg). The second stage larvae will penetrate the plant at or near the root tip and become sedentary. An enzyme is released which causes the plant cells surrounding the head region to enlarge, forming giant cells which serve as a source of nourishment for the parasite. The female nematodes swell until they become pear-shaped or oval as shown in the accompanying illustration. During this time, the nematode undergoes two more molts. Females begin laying eggs around 20 days after penetration of the host. The average female will lay approximately 30 eggs a day for two weeks. The average life cycle is 25 days. Populations will build up rapidly when environmental conditions are favorable.

Control
Use
- Resistant or tolerant varieties
- Select transplants free of knots
Chemical Control

Nematodes are effective, well accepted and can give good economic returns on high value crops. Treatment is sufficient for a year and re-treatment usually will be required the following year if susceptible plants are to be grown. Application methods are dependent upon the type of material used, along with conditions in the area requiring treatment. Most nematicides are either injected into the soil or washed in by a water drench. Nematicides may be purchased in liquid or granular forms, with application directions dependent upon the type of plants growing or to be grown. When possible, nematode-infested soil should be treated before planting seed or setting transplants.