Plant Health Problems
Diseases caused by Fungi:
Phytophthora blight, Phytophthora cactorum.
This blight is very similar to the bacterial blight except that the final color is brown rather than black. Entire suckers or shoots may be killed back to the ground.
Disease may be reduced by pruning and destroying affected shoots. Severity of disease may be reduced by improving drainage around the plant and avoiding excessive irrigation.
Powdery mildew, Microsphaera penicillata, Erysiphe sp.
White mealy appearance of leaves in late summer indicates powdery mildew infection. It is unsightly but does no harm to the bush. This common disease of lilac is favored by hot humid weather.
Severity of disease may be reduced by improvement of air flow around the plant. Many newer varieties are more resistant to infection, and should be utilized. Chemical control is usually not necessary but fungicides can be applied as soon as symptoms are detected. Among the compounds registered for use in Connecticut are potassium bicarbonate, thiophanate-methyl, horticultural oil, chlorothalonil, and triadimefon. Consult the label for dosage rates and safety precautions.
Gray mold, Botrytis cinerea.
In prolonged rainy seasons flowers and some shoots may be blighted with the gray mold fungus. Flowers appear to fade very suddenly, and leaves shrivel. The gray mold fruiting stage is usually found on affected parts. A change in weather will clear up the trouble.
Diseases caused by Bacteria:
Bacterial blight, Pseudomonas syringae.
Blighting of new shoots is black in color, starting as spots on leaves and branches which rapidly involve the entire shoot. Bacteria may be found in abundance in diseased tissues, including the wood.
Pruning and destroying affected shoots serve to eliminate infection from a plant; pruning tools should be disinfested in dilute alcohol or dilute bleach (a solution of 10% bleach in water) between each cut. Many newer varieties are resistant to infection.
Diseases caused by Physiological/Environmental Factors:
Graft blight, non-infectious.
Leaves show what looks like a nutritional trouble. Areas between veins become light green or yellow, and margins turn brown or are flecked with necrotic spots. Leaves are smaller, and growth is slower.
This blight occurs when lilac scions are grafted onto privet stock. After a time an incompatibility between stock and scion shows up. If the lilac can make some of its own roots, the trouble may be overcome in time.
Lilac borer, Podosesia syringae.
The larva bores into the stems of lilac and privet. It is white or yellowish with a dark brown head. The wasp-like clearwing moths have a wingspan of 1 to 1 ½" and emerge late in May. On emergence, the adult leaves the pupal case protruding from the burrow. Females lay eggs in patches on roughened places in the bark. The larvae tunnel in the solid wood and sometimes in the pith for a distance of 8 to 10 inches. Foliage wilts and eventually the branches die back. The insects also cut across the grain so that the branch breaks. The larva pupates in the burrow after cutting a passageway to the bark. There is one generation each year. Woodpeckers are a predator of this borer. For cultural control, do not prune when adults are present as they are drawn to wounds as egg laying sites. Permethrin, which is among the compounds registered for use against this pest in Connecticut, applied to the bark mid-May, early June and two weeks later will control the adults. Consult the label for dosage rates and safety precautions. Beneficial nematodes, Steinernema carpocapsae, can be applied to the bark of trunks and twigs to control larvae in their galleries.
Lilac leafminer, Caloptilia syringella.
The larvae of this little moth begin their existence as miners on the leaves of lilac and privet, and when partly grown, leave the blotch mines, roll the leaf, and feed within the roll. The adult is a tiny ash-gray moth with narrow wings. Spraying with acephate, which is among the compounds registered for use against this pest in Connecticut, when mines start in mid- to late May and again in mid- to late July should give satisfactory control. Imidacloprid applied as a soil drench early in the season will provide systemic control. Consult the labels for dosage rates and safety precautions.
Oystershell scale, Lepidosaphes ulmi.
This scale often infests lilac. A dormant application of horticultural oil will control overwintering scales. Summer application of ultrafine oil can also be helpful. Spraying with malathion, ultrafine oil, or insecticidal soap, which are among the compounds registered for control of this pest in Connecticut, about June 15 kills the young crawlers. Consult the label for dosage rates and safety precautions. See also Apple.
San Jose scale, Quadraspidiotus perniciosus.
Large infestations of the San Jose scale can cause branch and even shrub death. Partially-grown scales overwinter under their circular gray covering or scale on the twigs and the branches of trees. They begin to feed as the sap starts to flow. When apple trees bloom, the males emerge from under their scales to mate with the immobile females. Females are circular and cone-shaped, and their circular scales are about 1/16" in diameter, with a raised center or nipple. The males are smaller and elongate, with the nipple not centered on the scale. Females give live birth to tiny bright yellow crawlers in June. The young crawlers quickly settle, insert their long mouthparts into the twigs, and then suck sap from branches. As they grow, the crawlers secrete a waxy filament that becomes their scale or covering. Overlapping generations are present from June through September. Scales may be controlled by applying horticultural oil at a dormant timing or in June when crawlers are active. Diazinon, which is among the compounds registered for control of this pest in Connecticut, can also be applied against the crawlers in June. Consult the labels for dosage rates and safety precautions. To detect the yellow crawlers, wrap black tape coated with Vaseline around small branches. Adult flights may be detected with pheromone traps.