Bacterial spot, caused by the bacterium Xanthomonas campestris pv. vesicatoria, infects both tomato and pepper. Spots that appear on leaves and stems are small (up to 1 ⁄8 inch across), circular to irregular in shape, and have a slightly greasy feel. Unlike similar-sized spots caused by the fungus Septoria lycopersici, those caused by the bacterial spot pathogen do not develop grayish brown centers. As lesions enlarge, they often become surrounded by a yellow halo. If spots are numerous, they begin to grow together, and leaves wither and turn brown.
Fruit symptoms are more distinctive than leaf or stem symptoms. Spots on green fruit first appear as black, raised, pimple-like dots surrounded by water-soaked areas. As the spots enlarge to 1 ⁄4 to 1 ⁄2 inch, they become gray-brown and scabby with sunken, pitted centers.
The bacterium overwinters on the surface of seeds, in infected debris, and in soil. It is commonly brought into fields on infected transplants. Warm, rainy weather favors rapid spread of bacterial spot.
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