Bacterial leaf spot and stem rot, caused by Pseudomonas and Xanthomonas bacteria, is a common flower and vegetable disease. Pseudomonas syringae causes a devastating leaf spot and blight disease of lilacs. Leaf spot bacteria, which can live in plant debris for 3 to 6 months, are spread by garden equipment and splashing water. Mild, wet spring weather favors the progression of bacteria leaf spot, as does rapid plant growth.
Geranium, zinnia, begonia, pepper, tomato, beans, brussels sprouts, celery, cucumber, and other flowers and vegetables; also English ivy, cherry laurel, and lilac.
Symptoms vary, depending on the pathogens at work and the affected plant. On brussels sprouts, tiny black to purplish spots with yellow halos appear on outer leaves. These spots eventually grow together to form light brown, papery areas. On celery leaves, a water-soaked spot appears, then becomes bright yellow. The center of the spot gradually turns brown, with a yellow halo. On English ivy, brown and black spots with red margins appear on leaves. Leaf stems turn black and shrivel.
On geranium cuttings, a black rot may develop at the cutting base. Leaves may have slightly sunken, 1/4 inch dark spots. Leaves may remain on the plant or may drop. Blue-black rotting occurs on many or all stem parts. The stems may partially recover and produce new leaves at the terminals. If an infected stem is cut open, a thick yellow liquid filled with millions of bacteria may ooze from the cut surface.
Inspect plants regularly to detect the disease early. Avoid sprinkler or overhead irrigation. Do not handle transplants when they are wet. If possible, plant only in dry weather.
Remove small plants that are infected. Prune infected branches and stems from larger plants. Do not replant the area with susceptible crops the following year. Disinfect cutting tools after use. Spray affected plants with copper or fixed-copper fungicidal soap (Soap Shield) or in combination with mancozeb as soon as the disease appears. Repeat every 7 to 10 days during rainy weather.