Late Blight Alert - 11/20/2006

Gene McAvoy
County Extension Director
Regional Vegetable/Horticulture
Extension Agent III
UF/IFAS Hendry County Extension Office

I am writing to inform you that Glades Crop Care has reported a confirmed finding of late blight on tomato in the Immokalee area. The infection is in a fairly large planting which has multiple infections widely scattered throughout the field. Numerous infected plants are present with symptoms ranging from single lesions to several hotspots throughout the field where plants display multiple late blight lesions.

The disease can easily devastate a tomato or potato field within a few weeks if it is not properly controlled.

Since the disease can spread so rapidly, growers should scout their fields thoroughly each day, especially when cool and wet conditions conducive to disease development prevails. Since late blight symptoms may be confused with symptoms of other diseases, the following diagnostic pointers may help growers distinguish between the late blight and other diseases.

Late blight symptoms on leaves appear as irregularly-shaped brown to purplish lesions with indefinite border lesions can span veins. The lesions may be seen any time of day, on any stage of plant growth and on leaves of any age. Velvety, white fungal growth may appear on the lower surface of affected leaflets early in the morning before leaves dry and/or in the lower canopy.

On stems, purplish lesions may be seen any time of day and may be found any where on the stem. Cottony, white growth of fungus on stems with lesions can often be seen early in the morning and/or in the lower canopy. Stems with lesions are brittle and break easily. Lesions are confined to epidermis and cortex. Leaf rolling and wilting is often associated with stem lesions and purpling of leaflets may occur in some varieties.

The disease thrives under cool and wet conditions. Temperatures between 50 and 80oF combined with moist conditions such as rain, fog, heavy dews, or relative humidity above 90 percent are conducive for disease development. Night temperatures in the mid-fifties with daytime temperatures from the mid-fifties to mid-seventies are ideal for this disease.

Currently, fungicides are the most effective means of controlling late blight and will remain the primary tool until cultivars with resistance to this disease become available. Fungicides slow the rate at which the disease develops in the field by creating a protective barrier on the foliage. Just applying a chemical, however, does not necessarily equate with effective disease control. Relative effectiveness of a product, coverage, and timing must be factored into the equation for maximum benefit.