CORVALLIS, Ore. - Do new buds and branches on your lilac look blackish, like they've been scorched by a blowtorch? Your bush might have a bacterial plant disease called lilac blight.
A cool, wet, rainy, spring season favors development of lilac blight, especially if rains follow a late frost or winter injury. Oregon State University Extension plant pathologists are warning that this might be a favorable year for the disease.
Actually known to plant pathologists by the complete name of "lilac bacterial blight," this disease is caused by a bacterium Pseudomonas syringae pv. Syringae. The same organism is the source of bacterial blight on pear, blueberry, cherry, maple, and many other woody plants and the symptoms of lilac blight are similar in appearance to fire blight in fruit trees.
At first, leaves look perfectly healthy and then a short time later they look as though someone has placed an open flame near them. The dark black streaks on one side of young shoots show the progression of the disease. The flowers will wilt and turn brown and unopened flower buds become blackened.
Lilac blight is difficult to control and it is recommended that you buy blight-resistant varieties whenever you plant new lilacs.
It also helps to space and prune your lilac plants so they are not rubbing against each other and air can circulate freely between the plants. Do not fertilize late in the growing season. Do not over fertilize young plants because high nitrogen favors disease development, explained Melodie Putnam, OSU Extension plant pathologist.
If your lilac bush does have infection, prune and burn all infected parts as soon as you notice them. A spray of copper sulfate during the early spring each year should help prevent the problem before the buds begin to break.
Lilac blight bacteria over-winter on diseased twigs or on healthy wood. Factors that weaken or injure plants – wounds, frost damage, soil pH, poor or improper nutrition and infection by other pathogens – predispose them to the disease.
Sources of this disease can include old cankers, healthy buds, leaf surfaces and nearby weeds and grasses. Wind, rain, insects, tools and infected nursery stock spread the bacteria.
Some species of lilacs have shown resistance in western Washington including S. josikaea, S. Komarowii, S. microphylla, S. pekinensis, and S. reflexa. Most cultivars of S. vulgaris are susceptible, but some have been observed with less disease when planted in a garden; those include 'Edith Cavell', 'Glory', 'Ludwig Spaeth', and 'Pink Elizabeth'. Note that 'Ludwig Spaeth' is highly susceptible under intense nursery production systems.
The disease starts as brown spots on stems and leaves of young shoots as they develop in early spring. A yellow halo may also be around the spot. Spots become black and grow rapidly, especially during rainy periods. Further infectious development depends on the age of the part of the plant attacked.
On young stems, infection spreads around the stem and girdles it so the shoot bends over at the lesion and the parts above it wither and die. Infections on mature wood occur only on cherry trees, not on lilacs.
Young, infected leaves blacken rapidly starting near the margin and continuing in a wedge-shaped pattern down to the petiole. Eventually the entire leaf dies. On older leaves, spots enlarge slowly. Sometimes, several spots will run together, and the leaf may crinkle at the edge or along the mid-vein. Flower clusters also may be infected and rapidly blighted and blackened. Buds may fail to open or may turn black and die shortly after opening. Symptoms are similar to those of winter injury.
To see photos of this disease, visit OSU Extension's PNW Plant Disease Management Handbook.