Vigilance Required to Tame Verticillium Wilt of Tomato
In spite of the name Verticillium wilt, the initial symptoms of the disease on tomato consist of characteristic V-shaped lesions on the lower leaflets or leaves, with yellowing in a fan-shaped pattern that narrows down from the leaf margins. As symptoms progress, leaf veins become brown, and brown necrotic spots appear on the leaves. These spots may be confused with other diseases like early blight, but they do not develop the concentric rings characteristic of Alternaria.
Diurnal wilting and recovery typically commences at fruit set. Initially, infected plants usually show mild to moderate wilt during the warmest part of the day but recover at night. The leaves may wilt and eventually die and drop off. Disease symptoms progress up the stem, and plants become stunted with just the upper leaves remaining green. Fruits on affected plants remain small with yellow shoulders and may be sunburned because of loss of the canopy.
Verticillium wilt can be confused with Fusarium wilt and other wilt diseases of tomato and pepper. Vascular discoloration is evident in lower stems when they are cut open. In contrast to Fusarium wilt, Verticillium wilt discoloration seldom extends more than 10 to 12 inches above the soil. The discoloration is said to be a lighter shade of brown than Fusarium wilt diseases, but this is not a reliable characteristic for diagnosis.
Survival and Spread
Verticillium wilt is a cool weather disease. It occurs in South Florida during the winter months when daytime temperatures are 68°F to 75°F. Since Verticillium occurs primarily in neutral to alkaline soils, it is more of a problem in areas around Homestead and other lower East Coast Florida production areas. It is rarely seen in other parts of the state.
Infection takes place when Verticilliurn albo-atrum penetrates root hairs. The fungus invades the xylem interfering with the normal upward movement of water and nutrients. The fungus also produces a toxin that contributes to the wilting and spotting of the leaves.
Its wide host range permits verticillium to persist in soils for long periods. It can attack and multiply in many common weeds. In addition, the fungus also produces tiny black resting bodies (microsclerotia), which help it survive between seasons.
The pathogen thrives in wet soils. Tomatoes and potatoes must experience at least a day of saturated soil before infection occurs.
Cultural methods are important in controlling this disease. Long rotations between susceptible crops (four to five years), good drainage, and proper soil moisture management are recommended to aid in control.
Soil fumigation provides control and is recommended on high-value crops grown on plastic.
The most feasible and economic control is the use of Verticillium-tolerant tomato cultivars. Most commercial hybrids incorporate Verticillium resistance, and a wide range of cultivars with varying maturities and superior horticultural qualities are available.