Bacterial Blight on Lilac

Are you wondering why some lilacs have sick-looking leaves and twigs at this time of the year? In early spring when the weather is cool and wet, the bacterium Pseudomonas syringae may attack newly emerging shoots, flower buds, and leaves.

It causes water-soaked brown spots on leaves and twigs. Eventually, infected plant parts turn completely black, then wither and die.

Buy only disease-free plants. Good sanitation will help prevent the spread of bacteria to nearby healthy lilac plants. Immediately remove and destroy diseased plant parts. Remember to dip your pruners in a 10% bleach solution between each cut. Prune only when the weather is dry and no rain is expected within the next few days.

Sanitation combined with other cultural strategies can provide a good control of bacterial blight. These include spacing or thinning plants to provide for good air circulation, adequate fertilization (do not overfertilize), and proper watering to avoid wetting of foliage. Also, avoid wounding plants as bacteria enter through open wounds.

There are chemical sprays available for bacterial blight but are often not feasible from a homeowner's perspective. Most chemicals require a full coverage spray before disease appears in spring at an interval of 7-10 days and especially after each rain. Sprays can also cause phytotoxicity in some lilac varieties and does not guarantee complete disease control.

For more information on bacterial blight on lilac and its control, come and visit us at the Plant Pathology Extension website .

This article originally appeared in the May 7, 1999 issue, p. 51.

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