FL Late Blight update, Feb 7th 2012

Late blight is now showing up in multiple locations in all west coast tomato and potato production areas.  Incidence and severity is moderate to high in some locations.

Growers would be well advised to scout susceptible crops carefully as the weather forecast for next several days (high humidity - cloudy weather with possibility of showers along with with cool nights and warm days) will be conducive to disease development.

The disease can spread quickly and devastate a tomato or potato field within a few weeks if not properly controlled.

The disease thrives under cool wet conditions. Temperatures between 50 and 80 F 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.

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.

Late blight symptoms on leaves appear as irregularly shaped brown to purplish lesions with indefinite border lesions that 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 found anywhere 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.

See attached photos of symptoms provided courtesy of Dr Gary Vallad, Plant Pathologist at UF/IFAS GCREC in Balm. Photos coming soon.

Begin a spray program with fungicides if late blight is in your area or weather conditions are suitable for late blight development. At harvest, potato growers should kill infected foliage to minimize tuber infection.

I have seen a lot of freeze damaged abandon crops and or double crops
with numerous tomato volunteers - these can serve as a source of
inoculum of late blight and many otehr diseases and insects.  These
should be destroyed to protect nearby crops

Tomato growers should purchase disease-free transplants. Observe your fields thoroughly each day, especially when cool and wet weather prevails.

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.

Numerous fungicide products are registered for late blight control.
Protectants, as the name implies, protect foliage from infection by spores.
Protectant chemicals must be well distributed over the leaf surface and must be applied before spores land on leaves.  They are ineffective against established infections.

Systemic products become distributed locally within plant tissues and protect foliage from infection by spores. They may kill some established infections and may suppress production of new spores.  Even a short break in spray schedules, despite what is said regarding some of the newer fungicides, can result in a dramatic increase in blight when conditions are conducive to disease development.

Here are some choices for late blight control

Curzate 60DF, Previcur Flex, Revus Top, Ranman, Acrobat 50WP, Headline, Amistar,  Tanos, Gavel, phosponics,

Use these in a resistance management program, i.e., always mix with a protectant fungicide (e.g., EBDC or chlorothalonil-based products) and never apply consecutive treatments of the same product.

Consult current UF/IFAS recommendations for labeled fungicides for the control of late blight.

Hope this helps.

All the best


Gene McAvoy
County Extension Director
Regional Vegetable Agent IV
UF/IFAS Hendry County Extension
PO Box 68
LaBelle, Florida 33975

863-674-4092 office
863-673-5939 cell