Bacterial leaf spot affects many different types of plants, flowers and vegetables such as geraniums, poinsettias, celery, cucumbers and many others. The bacteria can live in plant debris from 3 to 6 months and spread to plants through tools or splashing water. They thrive in mild, moist conditions and spread quickly during the time of fast plant growth. The spots start out tiny but soon grow larger with yellowish halos around them. They form papery areas.
Disinfect all tools regularly.
Symptoms vary depending on the plant. The spots may be of different colors. Plants from the cabbage family the spots are black to purplish; tomatoes exhibit tiny water-soaked spots that later become angular and turn black and on celery the spots look like water-soaked areas that later turn bright yellow.
Avoid overhead watering that can splash the bacteria up onto plants. Try to avoid handling plants when they are wet to prevent the spread of the disease. Inspect plants often and remove infected leaves or plants when necessary. Destroy infected pruned plant material. If the disease continues to spread, destroy the whole plant. Do not plant susceptible plants in the same area again.
Watering is necessary when transplanting, but be careful not to over water.