Thank you for contacting us about your lilac. The timing and pictures are consistent with lilac “rust” mites (aculops massalongoi) – a type of eriophyid mite. This tiny insect (to see, need to use at least 20x magnification) feeds on the lilac leaves, sucking out the juices which results in leaves turning brown (or “bronze”) by late summer. These insects become active early in the season, as soon as new leaves start to develop. As the season progresses, the population builds and damage increases.

Treatment is rarely needed for rust mites in Colorado. Despite being infested with these tiny mites, lilacs seem to thrive – they just look discolored and stressed near the end of summer. Without knowing more about these mites (not much information on research-based websites), it’s difficult to say what would control them, but other similar mites are controlled by a dormant oil application just before the buds open in the spring. An insecticidal soap application or horticultural oil right after the leaves first develop should also help keep the population in check. Since they feed on both sides of the leaves, good coverage would be valuable for good control. Can also try adding carbaryl (an insecticide) to the dormant oil. I have read that sulfur may help, but do not mix oil and sulfur. Starting in mid-spring, look for foliage that is stippled (tiny flecks of white), indicating the mites.

Another thought: This often appears on lilacs in areas where they get less water than a lawn does. Consider making sure it gets as much water as your lawn does.

Good sanitation may help; get rid of the leaves by putting them in the trash (not compost or left on the ground) as they fall. Maybe we can reduce populations that way, too.

If you have specific questions about horticultural oils, dormant oils and insecticides go into a good, local nursery with staff advisors – they are a fountain of information when it comes to the products they have available (both organic and synthetic).

By the way, it is unlikely the maple was affected by the same insect, since most insects feed on specific species of plants.

Karin

Your lilac has sure taken a beating this season. The progressive browning and thinning of leaves is the way the bush is letting go of leaves it can no longer support and can no longer provide photosynthesis.

Yes, the clay can retain water, but most of the Front Range has clay and lilacs thrive here. They get the nutrients they need from our soils. That said, overwatering in clay can create an anaerobic condition where there is no oxygen getting to the roots. Mulches and compost can help the soil by balancing soil moisture, and provide organic matter to your hard soil.

This is not a good time to transplant. Breaking any roots and getting re-established will use up a large part of the plant's resources that it really needs to get through this winter. If you want to move it, wait until you can see that it is strong and healthy. This plant needs good care and less stress; this includes fall and winter watering monthly on warm days when we haven't had at least 1" precipitation.

Recommendation:

• Now - Add organic matter (peat moss, plant-based compost, etc.) to the soil around the bush as best as you can without harming roots.
• Now - Add mulch (bark chips, "gorilla hair", etc.) keeping mulch away from the base of the bush.
• Now and until all leaves are gone - good sanitation - get rid of leaves that fall.
• Fall and winter water
• Treat the spring leaf buds with dormant oil
• Treat the newly emerged leaf buds with horticulture insecticidal soap

This is a situation to "wait and see".

Karin