Don’t Make Light Of Bacterial Leaf Blight

Bacterial leaf blight of tomato

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Identification

Typical symptoms of bacterial leaf blight on tomato are brown to black spots developing at infection sites. These leaf spots can become enlarged and coalesce causing leaf blight and typically develop bright yellow chlorotic margins. Necrosis usually occurs on leaves, but can occur in plant stems, flowers, flower buds, and on fruits. Stem and petioles also may display a superficial brown discoloration and flower buds may abort.

Since P. viridiflava produce symptoms similar to other Pseudomonas species, such as bacterial speck
(P. syringae), it was not uncommon in the past for symptoms of this pathogen to be confused with other Pseudomonas pathogens, leading to misidentification.
P. viridiflava may occasionally cause significant crop damage, though it is commonly isolated as a sub-population in the investigation of more vigorous pathogens. When environmental conditions are conducive for development, damage can be severe. This spring, growers in a number of locations around Florida also reported severe leaf and stem necrosis symptoms on watermelon, squash, and cantaloupe.

Survival And Spread

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Bacterial leaf blight is favored by cool, moist environmental conditions. The bacteria are spread mechanically and by wind-driven rain. A period of stormy weather, followed by overcast days with cool temperatures, increases risk of outbreaks. Past outbreaks also occurred under conditions of unusually high winter rainfall and cool temperatures. When these conditions occur in the future, epidemics may be anticipated. Another important factor in the epidemiology of bacterial speck is the survival of the organisms in either soil or host debris and on native weeds. P. viridiflava is distinguished from many other plant pathogens in being able to infect a large variety of host species including many weeds. In addition to tomato, the disease can cause leaf blights on a wide range of plants including beans, cantaloupe, cauliflower, Chinese cabbage, eggplant, onions, squash, tomatoes, and watermelon.

Management Methods

Several cultural practices may help reduce the spread and severity of the disease. Growers should strive to minimize leaf wetness and avoid field operations when foliage is wet. Because of its wide host range, rotation may be of little use as a control method.

At present, no chemicals are registered specifically for bacterial leaf blight. Materials applied for bacterial spot control, also should provide bacterial leaf blight control. Chemical controls for bacterial diseases on tomato include copper-based fungicides combined with mancozeb/maneb and the use of compounds such as acibenzolar-S-methyl (Actigard, Syngenta) to activate a systemic acquired resistance response in the crop. Some growers have reported some success using bacteriophage (AgriPhi Inc.), but this requires sampling and development of a mixture of the appropriate strains of bacteriophage for the bacterial population present in the field. Phosphite salts such as K-Phite (Plant Food Systems) and biopesticides such as Serenade (QSTW 713 strain of Bacillus Subtilis, Bayer Crop Science) also may provide some control.

Consult UF/IFAS recommendations for currently labeled products for bacterial disease control in Florida vegetables.

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