APSnet Education Center - Plant Disease Lessons - Bacterial Spot of Pepper and Tomato

Disease Cycle and Epidemiology

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The bacteria have a very limited survival period of days to weeks in the soil, and thus their survival is almost always in association with debris from infected or diseased plants. The pathogens have been reported to persist in association with roots of wheat as well as a few weed species; weeds, however, are considered to play only a minor role in pathogen survival. Volunteer tomato plants and possibly pepper volunteers are potentially important sources of inoculum in some locations. In colder regions where vegetative material is killed, the bacteria survive very poorly, if at all. In these areas, reintroduction is primarily on contaminated seed or infected transplants.


Figure 15	Figure 16

The pathogen can survive in association with seed, either externally or internally. On externally infested seed, cotyledons may become infected upon contact with the seed coat and exhibit lesions soon after emerging from the soil (Figure 15). Bacteria are then readily splashed to new foliage and to other plants. Disease is a particular threat in transplant production (Figure 16). Transplant beds favor bacterial spot development because plants are irrigated frequently, crowded together, and humidity is typically high. In a 24-hr period, the bacteria can multiply rapidly and produce millions of cells. The risk of transmission on the seed surface can be reduced by treating seed with sodium or calcium hypochlorite or trisodium phospate (TSP). Killing bacteria that may be carried internally is more difficult. Chemical or heat treatments that kill the pathogen within the seed also can injure or kill the seed.

Although bacterial spot is a disease of warm, humid regions, it can develop in arid, irrigated regions. The bacteria can be spread by rain or by overhead irrigation. Bacteria also may be spread in water droplets when pesticides are applied with high-pressure sprayers.

Bacteria enter through stomata on the leaf surfaces and through wounds on the leaves and fruit, such as those caused by abrasion from sand particles and wind. Prolonged periods of high relative humidity favor infection and disease development. Symptom development is delayed or eliminated when relative humidity remains low for several days after infection.