Tomatoe Diseases Bacterial Spot (bacterial – Xanthomonas campestris pv. vesicatoria) Infected leaves show small, brown, water soaked, circular spots about one-eighth inch in diameter. The spots may have a yellow halo. This is because the centers dry out and frequently tear. on older plants the leaflet infection is mostly on older leaves and may cause serious defoliation.
The most striking symptoms are on the green fruit. Small, water-soaked spots first appear which later become raised and enlarge until they are one-eighth to one-fourth inch in diameter. Centers of these lesions become light brown and slightly sunken with a rough, scabby surface. Ripe fruits are not susceptible to the disease. but the surface of the seed becomes contaminated with the bacteria The organism may also alternate hosts over winter, by getting on volunteer tomato plants and on infected plant debris.
Moist weather and splattering rains are ecential to disease development. Most outbreaks of the disease can be traced back to heavy rainstorms that occur in the area. the Infection of the leaves may occur through natural openings. The infection of other fruits may occur through insect punctures, sandblasting and other mechanical injury means. Bacterial spot is difficult to control once it appears in the field. Late Blight (fungal – Phytophthora infestans) Lesions produced on the leaves are at first large, greenish-black, and water-soaked.
These areas enlarge, becoming brown, and under humid conditions, develop a white moldy growth near the edge of the diseased area on the lower surface of the leaves or on stems. The disease spreads rapidly under humid conditions, destroying large areas of tissue. Fruit lesions occur as large, green to dark brown, mostly on the upper half of the fruit. Also, a white moldy growth may appear on fruits in humid conditions. The fungus produces an abundant number of spores which may be splashed by rains or be airborne. These spores infect healthy leaves, stems and fruit if weather conditions are good. Ideal conditions for late blight development are cool nights, moderately warm days, abundant moisture. Hot and dry weather reduces disease development. Gray Leaf Spot (fungal : Stemphylium solani) First infection appears as small, brownish-black specks on the lower leaves that extend through to the under surface of the leaf.
These spots usually remain small, but may enlarge until they are about one-eighth inch in diameter. The spots become glazed and the centers crack. Infected leaves usually die and fall off. spots may also form on the stems of the host plant. Leaf-Mold Causal Agent(s): (fungal – Fulvia (Cladosporium) fulvum) Leaf mold is usually first observed on older leaves near the soil where air movement is poor and humidity is high.
At first, diffuse whitish spots appear on the upper surfaces of older leaves; these rapidly enlarge and become yellow. Under humid conditions, the lower surface of these spots become covered with a gray, velvety growth of the spores produced by the fungus. When conditions are proper for fungal development, large areas of the field are infected, plants are weakened and the crop is greatly reduced. The fungus produces abundant spores during periods of high temperature and very high relative humidity. Infection occurs readily, and the disease becomes established in the fields quickly. The best control of this disease is by using a preventative fungicide program at 7 to 10 day intervals, the same as used for late and early blight control. Buckeye Rot Causal Agent(s): (fungal – Phytophthora parasitica) This disease occurs on tomato mainly on the fruit, particularly where it touches the soil. The fungus is different from the one causing late blight, which affects both leaves and fruit.
Buckeye rot is first noticed as a light green water-soaked area on the fruit. Later, dark zonate bands can be seen on the surface of affected areas. The surface of the lesion is usually smooth and firm. With time, the entire fruit will rot. The fungus lives in the soil and it can also affect pepper.
The disease is more troublesome in heavy, poorly drained soils during prolonged warm wet weather. Nailhead Spot Causal Agent(s): (fungal – Alternaria tomato) Leaf symptoms are the same as those caused by early blight on fruits; however, spots are smaller, with slightly sunken centers and dark margins. As the spots become older, the edges become roughened. On ripe fruit, the tissue immediately around the spots often remains green. Control is the same as for early blight.
Anthracnose Causal Agent(s): (fungal – Colletotrichum sp.) At first, infected fruit show small, slightly sunken, watersoaked spots. These spots enlarge, become darker in color, depressed and have concentric rings. Masses of the pink fruiting fungus can be seen on the surface of the lesions in moist weather. Under warm and humid conditions, the fungus penetrates the fruit, completely destroying it. The fungus persists on infected plant refuse in the soil. Fruit may be infected when green and small, but do not show any marked lesions until they begin to ripen.
Fruit becomes more susceptible as they approach maturity. Control of this disease involves the use of well-drained soil, crop rotation and a preventative fungicide program as recommended for other diseases. Fusarium Wilt Causal Agent(s): (fungal – Fusarium oxysporum f. sp. lycopersici) The first indication of disease in small plants is a drooping and wilting of lower leaves with a loss of green color followed by wilting and death of the plant. Often leaves on only one side of the stem turn yellow at first; yellowed leaves gradually wilt and die.
The stem of wilted plants shows no soft decay, but when cut lengthwise, the woody part next to the green outer cortex shows a dark brown discoloration of the water conducting vessels. The fungus is soilborne, passes upward into the xylem of the stem. Blocking of the water-conducting vessels is the main reason for wilting. The fungus is most active at temperatures between 80 and 90 degrees F., seldom being a serious problem where soil and air temperatures remain low during most of the growing season. Control can be obtained by growing plants in disease-free soil, using disease-free transplants, and growing only resistant varieties. Verticillium Wilt Causal Agent(s): (fungal – Verticillium albo-atrum) The first symptom is yellowing of the older leaves, followed by a slight wilting of the tips of the shoots during the day.
Older yellowed leaves gradually wither and drop, and eventually the plant is defoliated. Verticillium wilt does not show the one-sided effect as does Fusarium wilt. Leaves from Verticillium infected plants sometimes show brown dead spots that may be confused with those caused by other fungi. However, they are lighter in color and do not show concentric zones as in early blight. In late stages of the disea …