Fig. 1. Formation of a "shepherd's crook", blackened leaves, and water-soaked lesions are key symptoms of Ascochyta blight.

Fig. 2. Fungal fruiting bodies may appear during wet weather.

Ascochyta blight is considered a little known and seldom seen disease that affects lilacs. In Minnesota, however, it is both common and very destructive. Ascochyta blight is a fungal disease that has two distinct phases: A shoot-infecting phase in the spring, and a leaf-infecting phase in the summer.

The infection cycle begins in the spring. Key symptoms include shoots that are "crooked" with wilted, blackened leaves (Fig. 1.). If the branch has gotten wet, pale orange "fungus-balls" may appear (Fig. 2.). These fungus-balls darken to a grayish-brown. Eventually, the infected branch dies back. This disease can be easily confused with bacterial blight of lilac. Key differences are the water-soaked lesions that occur with bacterial blight, and the absence of the fungal signs.

The leaf-infecting phase appears in late summer through early autumn. Water-soaked, olive-colored lesions spread and coalesce. Lesions eventually turn brown and are more or less round with indefinite margins. Fruiting bodies of the fungus can be seen on the leaves as small, darker brown spots against the tan colored lesion.

Weather plays a prominent role in the occurrence and severity of this disease. Wet weather, coupled with cool temperatures create conditions that favor the development of this disease. Rain splash is the primary mechanism of spread.

To manage Ascochyta blight, prune out infected branches when the lilac is dry, to prevent disease spread. Dispose of infected branches. (Do not compost!) Pruning in early spring after shoot infection will prevent foliar disease spread, and will not affect next year's flower production. Rake and dispose of any infected leaves in the fall. For severe infections, Bordeaux mixture, wettable sulfur and fungicides containing Chlorothalonil (Daconil) are effective in controlling this disease. As always, follow label instructions.

References:
Sinclair, W.A., Lyon, H.H. and Johnson, W.T. 1987Diseases of Trees and Shrubs.

Agrios, G. N. 1978. Plant Pathology.

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