Bacterial spot, Xanthomonas perforans, is one of the most serious diseases of tomato in Florida because it can spread rapidly during warm periods with wind driven rains, and because fruit symptoms reduce marketability.
Bacterial spot is caused by several species of Xanthomonas spp. Four species have been identified on tomato: X. euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri. In Florida, the major species encountered is X. perforans.
Symptoms of bacterial spot appear as small, water-soaked greasy spots on infected leaflets. On tomatoes, distinct spots with or without yellowing occur. Individual leaf spots may coalesce with each other, resulting in the browning of entire leaflets. Fruit spots often begin as dark specks with or without a white halo. As spots enlarge, they become raised and scab-like.
Survival And Spread
Entry into the plant occurs through stomata or wounds made by wind driven soil, insects, or cultural operations. Bacterial spot can be seed transmitted, but most inocula comes from volunteer plants or infected plant debris in the soil. X.perforans is seed-borne, which allows for the movement of strains on a global scale.
An integrated approach is needed to manage this disease. Exclusion is the best means of managing bacterial spot on tomato. Unfortunately, even the best bactericidal treatment offers only limited protection when environmental conditions are favorable for rapid disease development, especially during periods of heavy, wind-driven rains.
Sanitation is important. Pepper and tomato volunteers and solanaceous weeds should be destroyed between crops. Transplant houses should be located away from tomato or pepper fields.
Since water movement spreads the bacteria from diseased to healthy plants, workers and farm equipment should be kept out of fields when fields are wet because the disease will spread readily under wet conditions. It is important to apply sprays before and during rainy periods. If conditions are favorable, frequent spraying may not be sufficient to maintain bacterial spot below damaging levels.
The traditional recommendation for bacterial spot control consists of copper and maneb or mancozeb. Attention to application techniques is as important as choice of material in achieving adequate control. The effectiveness of copper is limited because of the widespread occurrence of copper tolerance among strains of Xanthomonas.
There is some evidence the use of organosilicate adjuvants and applications of magnesium might increase the incidence and severity of bacterial spot infections.
In the past few years, several new products have come on the market that have given good results in research trials when used in rotation or together with traditional controls like copper. These include Tanos (famoxadone and cymoxanil, Dupont Crop Protection) as well as the SAR elicitor Actigard (acibenzolar-S-methyl, Syngenta), Regalia (extract of Reynoutria sachalinensis, Marrone Bio Innovations) and Serenade (QST 713 strain of Bacillus subtilis) and Sonata (Bacillus pumilis strain QST 2808) from AgraQuest.
Gene McAvoy is Regional Vegetable Extension Agent IV with the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS)