How do I identify a pathogen effecting an established lilac?
Question:
I am looking to identify the pathogen effecting half of an established lilac in a coastal town.
Answer:
None of the typical lilac ailments leap out as an obvious culprit. But the combination of necrotic leaf tips and margins, and the obvious chlorosis (the pronounced yellowing of the leaves while leaving streaks of green) you have on the leaves make me wonder about the soil conditions. This could be a nutrient deficiency or a pH outside the range where the lilac can absorb soil nutrients. Have you ever had the soil there tested? And have you changed anything in the fertilizing or supplemental irrigation (anything in addition to rainfall)?
My advice:
1) Get the soil tested through the UMaine Soil Analytical Lab . Their website has clear instructions for how to take the sample (we have a nice video here, too), and they can mail you a test kit or you can pick one up in any Extension office. Please note on the paperwork that you are growing lilacs in this best, so they can give the best amendment recommendations for that particular species.
2) Make sure you are following best practices recommendations for growing lilacs. In particular, they like consistently moist, well-drained soil. If the ground is staying too wet, that could be contributing to these problems, as well. This includes pruning, as well–overgrown lilacs can grow stressed, which makes them vulnerable to other environmental, pest, and pathogen conditions.
3) Scout the base of the plant for holes or anything like sawdust that might indicate the presence of insect pests.
4) While I’m leaning toward an environmental issue as the underlying problem, there is always the possibility that there is a pathogen going that I’m unable to recognize from the photos. As a result, I am also recommending that you submit your photos (and possibly a physical sample) to the UMaine Extension Plant Diagnostic Lab for evaluation. There is a small fee for their service, but our plant pathologist is better qualified to evaluate the photos or conduct lab testing on a sample–NB: they are very busy this time of year, so it can take up to a week to hear back once they’ve received your submission.
Best of luck, and happy gardening.