Bacterial blight caused by Pseudomonas syringae affects every part of the shrub it infects. Brown to black spots form on leaves, which eventually turn completely black and dry on the branches. Shoots develop black stripes or turn black at the ends. Flowers become limp and turn dark brown. Blight is most severe during moist, mild weather, and infects the developing young shoots. The bacteria may enter twigs directly or through blighted leaves. They overwinter in diseased twigs. White-flowered plants seem most susceptible to this disease.
The fungus Phytophthora cactorum causes a similar disease in lilacs. Terminal buds and leaves turn brown instead of black, roll up and droop. Stems are often killed to the ground. This fungus also causes dieback of azaleas, so if you grow both of these plants, place them far apart.
For bacterial blight, prune and destroy diseased shoots, sterilizing your tools between each cut. Spray the entire bush with a copper fungicide. To prevent this disease, avoid overfeeding your plants and promote good air circulation by proper pruning.
For fungal blight, prune the diseased tips well below th infection. In spring, as new leaves emerge, spray the entire shrub with Bordeaux mixture or another copper fungicide. Spray again 2 weeks later.
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