Leaf Spot (fungus - Cercospora carotae): This disease may occur at any place on the leaf but is most common on margins. Spots are circular in shape and with age they coalesce to form larger spots. Lesions on the leaves are sometimes surrounded by a lighter circle or halo. Lesions on the petioles are enlongated with a pale center and dark margin. Spots on the petiole may encircle it causing defoliation. Leaf spot is distinguished from leaf blight by the nearly circular, sharply defined lesions with a yellow halo. The disease usually occurs during the latter part of the growing season and can be controlled with the same recommendations as for leaf blight.
Powdery Mildew (fungus - Erysiphe polygoni:) Although the fungus does not appear to cause excessive damage, it may cause serious infection under conditions favorable for disease development. Affected leaves are covered by a white, powdery mass of spores. Symptoms may also be found along petioles. Fungicide applications at 10 to 14 day intervals will control the disease.
Root Knot Nematode (nematode - Meloidogyne spp.): Root-knot nematodes seriously damage carrots and cause multiple tap-root formation. Root production is severely limited rendering the crop unmarketable. Nematodes contained in the roots are not harmful to humans when consumed. It is best to select areas free of root-knot nematodes for carrot production. Certain pre-plant nematicides can be used where the soil is infested.
Damping-Off (fungi- Rhizoctonia, Fusarium, etc.): Like many other vegetables, carrot seedlings are susceptible to attack by several kinds of soil- borne fungi, particularly during the periods of cool, humid weather. Infected seedlings wilt, turn brown and die, resulting in poor stands. Plant on a bed, do not overwater, and control weeds as soon as possible after emergence.
Aster Yellows (Mycoplasma): The first symptom of aster yellows is yellowing of the foliage, followed by excessive growth and bunching of shoots. Older leaves become twisted and may fall off. Roots are misshapen, and of poor quality with many adventitious roots. The organism is introduced in the field by leafhoppers from overwintering infected weeds. The disease usually occurs sporadically with little economic loss.
See Cotton Root Rot:
Late Blight (fungi - Septoria apiicola): Small, yellowish spots develop on the underside of the leaves which later spread to the entire foliage. Spots enlarge and turn brown, similar to lesions caused by Cercospora. Minute, black fruiting bodies can be found scattered in the affected areas. Lesions with fruiting structures can be seen on petioles and stems. High humidity, free water, and cool temperatures favor disease development. Fungicides recommended for Cercospora blight will also control late blight.
Stalk Rot (fungi - Rhizoctonia solani): A serious disease of celery that is favored by high temperatures and moist conditions. Symptoms are first seen as small lesions on the base of the petioles, near the ground. As the disease progresses, the spots are enlarged, appear watersoaked at first, turning later to a brick-red color. When matured, the spots are sunken and turn to a dark brown color that is characteristic of the disease. The lesion can be a few or numerous, making it necessary to trim a lot of the leaves, thus reducing quality and yield. Shallow planting on raised beds and fungicide applications serve to reduce disease losses.
See Root Knot Nematode (nematode - Meloidogyne spp.):
Control is obtained by using high quality seed which have been treated with a protective fungicide. Sweet corn should be planted on raised bed after the soil temperature is above 55 degrees F.
Stalk Rot and Kernel Rot (fungus - Fusarium spp.): Infected plants are stunted and delayed in maturity. During periods of high wind plants often lodge. Ears may hang downward on the stalk. The Gibberella stage of the fungus will infect kernels causing them to be pink in color. Infected ears have a strong odor and should not be used as food or feed.
Control stalk rots by rotating with non-related crops, planting in well drained soils and by using treated seed.
Southern Corn Leaf Blight (fungus - Bipolaris maydis): The disease is easy to recognize under field conditions. Spots on the leaves are tan to brown in color. On the ear the fungus causes oblong, bleached spots which penetrate through the shuck layers and finally into the ear.
The fungus overwinters in crop residue and produces spores which can be carried for long distances by wind.
Northern Corn Leaf Blight (fungus - Exserohilum turcicum): This disease is found in most sweet corn fields, yet is seldom severe enough to cause economic loss. Spots produced are larger than those caused by the southern corn leafspot fungus. Spots are from one to six inches long and one-half to one inch wide. With maturity, the center of the spot has a dark brown color. Infection occurs first on older foliage. High humidity and temperatures between 60 to 80 degrees F favor disease development. Varieties vary in their reaction to the fungus.
Brown Leaf Spot (fungus - Physoderma maydis): The fungus causing this disease occurs in most fields but seldom does economic damage. Infection requires high temperatures and presence of surface moisture. The first symptom of the disease is small circular spots. As they mature they turn dark brown. Rotation and deep burial of stalks will help reduce losses to this fungus.
Downy Mildew (fungus - Peronosclerospora sorghi): Infected plants are chlorotic, stunted and have striped leaves. Infected leaves have a downy growth on the underside, toward the basal part. Potential infection is increased when the crop is grown in soil previously grown to infected sorghum, field corn or sweet corn. Although high populations of spores are produced on the leaf surface, they are short lived and require extended periods of high humidity for infection. Overwintering spores produced between leaf veins exist in the soil for long periods. Practices which hasten the breakdown of crop residue will help reduce the amount of inoculum carried over in the soil. Varieties vary in their reaction to this disease. Growers should consult their county Extension agent for current hybrids and their reaction to this disease.
Crazy Top Downy Mildew (fungus - Sclerophthora macrospora): This disease is a problem when fields become flooded early in the life of the plant. The fungus produces swimming spores which require water for mobility.
Infected plants are sterile and have numerous shoots at the base of the stalk. Leaves are thickened, distorted, of a lighter green color than normal leaves. Tassels and ears develop green leafy shoots.
The fungus is commonly found in grasses along the edge of the field. Spores are washed into the field in flood water. Infection of the young corn plants takes place at this time.
Varieties vary in their reaction to this fungus. Due to the low percentage of occurrence, little has been done to rate varieties for their reaction. Avoid fields that flood regularly and plant on a raised bed which will help reduce the exposure of young seedlings to standing or flowing water.
Common Rust (fungus - Puccinia sorghi): Common rust occurs in most home gardens and commercial fields, but seldom causes economic losses. Infected leaves have raised spots or pustules formed primarily on the upper surface. The pustules are rectangular, brick red and occur in bands on the leaf. Spores are produced in the pustules, which are blown to neighboring leaves where infection can be repeated. Infection is encouraged by high humidity and cool temperatures (60 to 70 degrees F).
Common Smut (fungus - Ustilago maydis): Common smut is often found in fields of sweet corn. Losses from common smut vary based on the amount of infected plants in the field. Galls are formed as the common smut fungus causes cells of the corn plant to increase in size and number. These galls at first are covered with a thin white membrane. As the gall ages, the membranes break open to reveal a black powdery spore mass underneath. The spores are blown to adjoining corn plants where infection is repeated. Common smut of sweet corn is more of a problem during dry weather, which slows down the growth of the corn plant. Plants grown in soils high in nitrogen or plants damaged through cultivation are most susceptible to infection.
To control common smut, use resistant hybrids and plant high quality seed. Sweet corn should be grown on soil that is fertilized according to a current soil test recommendation.
Corn Stunt (spiroplasma): Corn stunt occurs in a small percentage in most sweet corn fields. It seldom reaches levels high enough to cause economic loss. The corn stunt spiroplasma is transmitted by leafhoppers. Infected plants are stunted, young leaves are yellow in color, and with age they take on a reddish-purple color. Internodes are reduced in length and infected stalks are sterile. Control is not required due to the very low percentage of plants that normally show this symptom in the field.
Maize Dwarf Mosaic (virus): Maize Dwarf Mosaic virus is the most common virus disease of sweet corn in Texas. Infected plants have mottled upper leaves that are lighter in color than healthy leaves. The mottled or mosaic pattern consists of alternate yellow and green islands in the leaf tissue. Aphids transmit virus particles from surrounding Johnsongrass. Johnsongrass rhizomes serve as the overwintering host for this virus. Early infected plants may be sterile. Late infection will reduce yields and quality of corn produced. Insect control is not successful due to the feeding pattern of the aphid. Elimination of Johnsongrass and isolation of sweet corn fields from Johnsongrass stands will help reduce the occurrence of this disease. There are a number of hybrids of sweet corn that are resistant.
See Charcoal Rot
Plant Parasitic Nematodes: (See Root Knot and other nematode sections.)
Colletotrichum Fruit Rot (fungus - Colletotrichum melongenae): Lesions on the fruit vary from small spots to one-half inch in diameter. The tissue is sunken, with an area filled with a flesh-colored ooze of fungal spores. Spots vary from one to several on the fruit surface. Severely infected fruit drop to the ground with the pedicel still attached to the plant. The fungus overwinters in plant residue and grows at temperatures of 55 to 95 degrees F with optimum growth at 80 degrees F. Rainfall and overhead irrigation favor disease development. The fungus develops when the humidity is 93 percent or above. Although field sanitation is important, a preventive fungicide spray program is required during periods favorable for disease development.
Wilt (fungus - Verticillium albo-atrum): The pathogen attacks nearly 200 species of plants but eggplant and okra are the two most seriously affected vegetables. Young plants appear normal, but become stunted as they develop. Severely affected plants turn yellow. The lower foliage wilts and defoliation occurs. Symptoms continue to progress until death occurs. When the stem is cut, there is a dark brown, discolored band around the vascular system. Infection occurs directly through the root hairs. The fungus survives for indefinite periods in the soil. Survival is aided by weeds which are susceptible to the fungus. Infection takes place when the temperature ranges from 55 to 86 degrees F. Verticillium is favored in its development if the soil is alkaline. Some development takes place at pH of 5.0 but all growth is stopped at a pH of 4.0. Control involves the use of long rotations. Cotton gin trash should be avoided or be well composted if used.
Yellows (Tobacco Ring Spot Virus): The disease causes yellowing and whitening of upper leaves. Later, entire plant becomes yellow and may die. Avoid planting in fields where yellows have occurred and, if warranted, fumigate soil to control nematodes. The dagger nematode is a known vector of the virus.
Bacterial Blight (bacterium - Pseudomonas syringae pv. pisi): The bac- terium attacks all parts of the plant. Infected stems become olive brown in color, while the leaflets become yellowish or water-soaked. Young infected pods drop prematurely. Well formed pods become water-soaked.
The bacterium is seed-borne. Rain or irrigation water is necessary for movement of the organism. After infection takes place, four to six days is required before lesions are visible. Optimum temperature for development is 82 degrees F.
Control of this pathogen is through a combination of practices such as drainage, row spacing, seeding rates, weed control, and restricting irrigation only to that needed for maximum plant growth.
Ascochyta Blight (fungi - Ascochyta pisi, A. pinodella, A. pinodes): All three fungi are known to attack peas. Affected leaves have spots which are large, pale brown to dark brown in color. Lesions are papery in appearance and have gray to tan centers marked by small black pycnidia. Infection by some species cause purple lesions. On pods, deep lesions are formed which may have purple margins. The center is tan with black pycnidia.
Planting infected seed results in poor stands. All three species of fungi are seedborne and carry over in crop residue. Rainfall and heavy dews are necessary for infection. No infection occurs when the relative humidity is below 80 percent. Optimum temperatures for the fungus are between 68 and 82 degrees F.
Control is achieved by using clean seed, long rotations (four years or longer), planting in well-drained soil, and deep plowing to remove old crop residue.
Aphanomyces Root-Rot (fungus - Aphanomyces euteiches): Early infection causes complete crop loss due to seedling death. Late infection results in poor plant growth and reduced seed formation. Tissue decay does not develop above the soil line unless the weather is extremely wet. Infection occurs in both wet and dry soils, but is most destructive in wet soils. Optimum temperatures for infection are between 65 and 75 degrees F. The use of high levels of fertilizer will encourage continued root development. Nitrogen acts as a suppressant to fungal growth.
The use of three year rotation, well-drained soil, and the liberal use of fertilizer will help reduce losses from this disease. Mosaic (virus): Irregular, light and dark, greenish areas and puckering occurs in leaves. Control aphids that transmit the virus.
Chlorosis: (See section on Chlorosis.)
See Charcoal Rot
See Root Knot Nematode: