Symptoms: Browning or yellowing of the leaf margins and brownish or discolored irregular areas on leaves are common symptoms of chemical burns. Severe injury may be evidenced by distortion of new leaves, or defoliation, but unless some serious mistake has been made, spray or dust injury is not an important consideration. Pesticides in emulsifiable formulations are more likely to cause foliage burn than wettable powders, particularly when combined with wettable powders.
Copper: Stone fruits are sensitive to copper. Bordeaux mixture causes russeting of apples and may stunt cantaloupe and watermelon leaves. Copper compounds should be used with caution on any cucurbit crop. Injury is most likely to occur when materials are applied to wet leaves and under damp, slow-drying conditions. Copper containing sprays will also defoliate peaches, plums and apricots.
Zinc Sulfate: This causes severe defoliation of fruit trees and should not be included in fruit sprays.
Sulfur: Frequent applications of sulfur may cause light foliage burn on rose and tender foliaged ornamentals when temperatures exceed 85 degrees F. Sulfur should not be used on cucurbit crops such as squash and cucumbers.
Insecticides: Injury to plants may occur when materials are improperly applied or applied under adverse environmental conditions. Foliage burn is most likely to occur when materials are applied to wet foliage or when extremely high temperatures follow application.
Herbicides: Severe injury to foliage may result from spraying plants with equipment contaminated by previous use with hormone type herbicides. Common symptoms are severe growth abnormalities of leaves and leaf roll. Some herbicides are taken up by plant roots and cause a marginal burn. This burn develops inward to the veins of the leaf as the material is concentrated. Defoliation eventually occurs. New leaves are then formed which may also burn and defoliate. This will continue until the plant either dies or the chemical is no longer present in a toxic level in the root zone. Equipment used to apply herbicides should be well marked and should not be used for disease or insect control purposes. Some trees and certain ornamentals are sensitive to herbicides that are blended with fertilizers.
Cotton root rot has been reported in Texas counties from the Red River to the Rio Grande and from Tom Green County to the Neches River.
DISEASE SYMPTOMS - Symptoms are most likely to occur from June through September when soil temperatures reach 28 degrees C (82 degrees F). 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. Infested areas in cotton may increase 5 to 30 feet per year in cotton.
CAUSAL ORGANISM - Phymatotrichum omnivorum exists in the soil in three distinct forms: (1) hyphae and strands (rhizomorph), (2) sclerotia, and (3) sporemats and conidia.
Landscape - A list of ornamental plants with some resistance to cotton root rot is found in Bulletin L-2056.
Rotation. Research shows that rotations of 3 or 4 years with a monocotyledonous crop have reduced disease incidence up to 60 percent on cotton in some instances. Shorter rotations are less effective.
Organic amendments. Significant control of cotton root rot has been achieved by using residues of various crops. A delay in infection of cotton is readily apparent and has resulted in up to 90 percent reduction in root rot. Wheat, oats, and other cereal crops are effective in delaying infection and reducing losses when incorporated in soil in the spring before cotton is planted.
Deep plowing. Use of a moldboard plow to flat break infested areas 6 to 10 inches deep has markedly reduced the incidence of disease. Flat breaking immediately after cotton harvest reduces the strands' ability to form sclerotia. The upper 6 inches of soil should thus have a reduced sclerotia level where 90 percent of the roots of a cotton plant are found.
Plant barriers. This technique consists of planting a resistant grass crop such as sorghum around an infected area in a field. These barriers either exclude or limit the spread of disease within the field.
Fertilizer applications. When nitrogen is applied as ammonia in a manner to fumigate as much soil as possible, root rot may be reduced. Early plant maturity. A successful technique for cotton is to plant early maturing varieties, such as TAMCOT CAMD-E, TAMCOT SP37H and TAMCOT CAB-CS, as early as possible in the season so that the crops reach maturity before the plant is killed by the disease. Disease activity increases from June through August; therefore, complete production as early as possible.