Gail Ruhl, Interim P&PDL Director, Senior Plant Disease Diagnostician, Botany and Plant Pathology, Purdue University
Lilacs can be damaged by several different pests and disorders. One
infectious disease which you might see at this time of year is bacterial
blight. Chinese, Japanese, Persian and common lilacs, especially
white-flowering types are susceptible. Leaves, flower clusters and shoots
are killed by this disease, which is caused by the bacterium Pseudomonas
syringae pv syringae. The first symptoms on leaves are usually irregular to
circular dark brown spots with yellowish halos. Under moist conditions
infected parts die rapidly and become dark brown to nearly black. The
symptoms look very similar to late frost injury.
Infection is favored by mild, wet weather, during which time the bacteria multiply quickly and are spread by splashing rain. The new succulent leaves and shoots are most susceptible. Infected leaves may become distorted because of the dead parts.
Dense clumps of lilacs should be thinned to promote air circulation. Infected shoots should be cut out and destroyed as soon as they are noticed, preferably when the plants are dry, to avoid spreading the bacteria. When pruning out infected shoots, it is important to disinfest the shears after each cut by dipping them in 70% denatured alcohol or Lysol.
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Bacterial blight of lilac
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