Do new buds and branches on your lilac bush look blackish, like they’ve been scorched by a blowtorch? They 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, according to Oregon State University Extension plant pathologists.
Known to scientists by the complete name of “lilac bacterial blight,” this disease is caused by the bacterium Pseudomonas syringae pv. syringae. The same organism is the source of bacterial blight on pear, blueberry, cherry, maple and many other woody plants. The symptoms of lilac blight are similar in appearance to fire blight in fruit trees.
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 turn black and die shortly after opening. Symptoms are similar to those of winter injury.
Lilac blight is difficult to control. You should buy blight-resistant varieties whenever you plant new lilacs.
It also helps to space and prune your lilac plants so they don’t rub against each other, and air can circulate freely among them.
Do not fertilize late in the growing season. Do not over fertilize young plants—high nitrogen favors disease development, says Melodie Putnam,
OSU Extension plant pathologist.
If your lilac bush has an infection, prune and burn all infected parts as soon as you notice them. A spray of an organic copper-based pesticide 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 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.
Explore more resources from OSU Extension at https://tinyurl.com/5t6jwcb4 and https://tinyurl.com/5n7wahkc.