IFIS Reports on Late Blight in Florida

Dear Vegetable Grower, Vegetable Industry Representative,

Please be advised that late blight has been found on tomato southwest of Immokalee. This initial find is confined to one location where a number of infected plants have been detected. Given the recent cool foggy weather and forecast of precipitation this weekend, conditions are liable to be conducive to further spread and tomato and potato growers would be well advised to pay strict attention to spray schedules and choice of materials and adhere to a preventative spray program.

Growers and scouts in West Central Florida indicate that late blight has been reported from scattered locations across the area for the last couple of weeks. Respondents not that blight has increased in incidence and occurrence in the last few days, most likely due to the weather which has featured some cool nights and dense fog almost every night the last week.

There have been no reports of late blight from other South Florida growing areas to date but many growers report increasing spray schedules and going into a proactive mode in light of favorable weather conditions over the past few weeks.

Late blight is caused by the fungus Phytophthora infestans, which is a pathogen of potato and tomato. Very few vegetable diseases cause more concern to growers. 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 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 fifties with daytime temperatures from the mid-fifties to mid-seventies are ideal for this disease. Temperatures in the lower range stimulate the formation of many swarm spores (zoospores) from the sporangia. This situation dramatically increases the potential for disease spread.

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.

Inoculum of Phytophthora infestans can originate from diseased seed tubers; cull piles, and volunteers as well as infected plantings of potatoes or tomatoes. For tomatoes, infected transplants can serve as a source of inoculum.

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 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.

Several control measures plus observation are absolute necessities if late blight is to be properly controlled. Potato growers should purchase certified, disease-free seed pieces and store seed in a dry location before planting.

Other important cultural controls include destruction of cull piles and volunteer potato or tomato plants. Plant resistant varieties. Begin a spray program with fungicides if late blight is in your area or weather conditions are suitable for late blight development. After harvest, kill infected foliage to minimize tuber infection.

One cannot but wonder if sanitation of picking crates and bins moved from field to field may also help reduce the spread of innoculum to new locations.

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. Newer products such as Curzate (DuPont) boast “kick back” action that may kill some established infections and can suppress production of new spores and can help arrest infestation if applied within 48 –72 hours of initial infection. However, 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 under the conditions we have had during the past two weeks.

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