Is the foliage yellowing in places and dropping prematurely, growth is slowed?
Are the leaves covered with white powder?
Are there any black stripes on young shoots or leaves with black spots?
Do the leaves have tan or brown blotches or serpentine tunnels?
Do the trunk or branches have small holes in them, limbs may die and break off?
Are the leaves completely yellow and covered by small bumps?
These symptoms usually indicate a fungal disease - Verticillium wilt. This soilborne fungal disease makes leaves yellow, wilt, and fall off before the entire branch dies. Avoid planting trees near strawberries, potatoes, tomatoes, and eggplants as they are susceptible to Verticillium wilt. Soil solarization may prevent the disease.
White, powdery patches on the leaves are caused by a fungal disease Powdery mildew. Powdery mildew usually appears in damp places. Encourage good air circulation around the affected plants by thining them and any surrounding plants. Water plants from below to keep foliage dry. Any affected leaves should be cut off and removed. Spray foliage thoroughly with wettable sulfur once or twice at weekly intervals starting as soon as the whitish coating of the fungus is visible.
Black stripes on young shoots or leaves with black spots are caused by bacterial blight. The spread of the disease is greatest during humid, wet weather conditions. Practice good garden hygiene at the end of the season and discard, do not compost, possibly diseased plants and plant parts. Pull out and destroy infected plants.
These symptoms are usually caused by insects - Leafminers. These tiny pale green fly larvae feed between the upper and lower surfaces of the leaves. Prune off and destroy all infested leaves. Remove debris in fall because it might contain overwintering leafminers. The weekly application of insecticidal soap should keep their numbers low. Also, introducing parasitic wasps is a very effective method of controlling leafminers.
These symptoms usually indicate caterpillars - Borers.
Yellow leaves covered by small bumps are caused by scales. These tiny, shelled insects often feed on the undersides of the leaves, causing an unhealthy appearance with yellowish blotches on the upper leaf surfaces. Scrape off minor infestations with your fingernail. Prune out badly infested growth, or use a soft brush and soapy water to gently scrub the scales off the stems. Control scales by spraying insecticidal oil during the growing season.
Poor growing conditions may have caused your lilac unhealthy look. Too much nitrogen promotes soft, leafy vegetative growth. Excess irrigation can reduce blooms and encourage soft growth.
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/japanese-tree-lilac-problems.htm
Ellis, B. W., Bradley, F. M., & Atthowe, H. (1996). The Organic gardener's handbook of natural insect and disease control: a complete problem-solving guide to keeping your garden & yard healthy without chemicals. Emmaus, Pa.: Rodale Press.
Sreten null
Hi! I’m Sreten Filipović. I graduated from the Faculty of Agriculture at the University of Belgrade, with a master's degree in Environmental Protection in Agricultural Systems. I’ve worked as a researcher at Finland's Natural Resources Institute (LUKE) on a project aimed at adapting south-western Finland to drought episodes. I founded a consulting agency in the field of environment and agriculture to help farmers who want to implement the principles of sustainability on their farms. I’m also a founding member of the nonprofit organization Ecogenesis from Belgrade whose main goal is non-formal education on the environment and ecology. In my spare time, I like to write blog posts about sustainability, the environment, animal farming, horticulture, and plant protection. I’ve also published several science-fiction short stories.
You can find me on LinkedIn at https://www.linkedin.com/in/sreten-filipovi%C4%87-515aa5158/
