August 2020: Green Grow the Lilacs – NOT!

LilacLilacs are one of my favorite spring-flowering shrubs.  When sited properly (in full sun) and pruned/thinned regularly, they reliably produce a bounty of beautiful blooms each spring in a myriad of colors.  And oh that scent!  Is there really anything that smells better than the scent of lilacs in full bloom?

In most years, lilacs tend to be relatively disease free.  However, this year, I have seen a fair number of disease issues on lilacs that have ranged from cosmetic to lethal.  Here’s a round up of common lilac diseases to watch for.

Powdery MildewLilacs are the poster child (in my mind) for this disease.  They routinely exhibit the white, powdery growth on their leaves that is characteristic of powdery mildew, but tend to show no adverse effects from the disease.  The best management strategy for powdery mildew on lilac is to develop a healthy ability to ignore the disease during the growing season.  Then in the fall, be sure to collect up the leaf debris from your lilacs, and burn (where allowed), bury or compost the leaf debris.  Even with diligent leaf clean up, don’t expect a powdery mildew-free lilac next year.  The pathogen has a sneaky way of surviving in the overwintering buds of the plant.

Bacterial BlightThis disease tends to be more of an issue early in the growing season, often just as lilacs are beginning to leaf out.  The bacterium that causes bacterial blight (Pseudomonas syringae pv. syringae) often is part of the normal microflora that lives on lilac leaves, occurring at a low enough level not to cause any disease issues.  If the population increases however (often in response to wetter weather), the bacterium can lead to leaf tissue necrosis (i.e., death).  The dead areas (sometimes in discrete spots, other times in larger blotches) typically have yellow halos.  To complicate matters, Pseudomonas syringae pv. syringae has a protein in one of its membranes that mimics an ice crystal.  So, if the bacterium is present on lilacs in high numbers and a cold snap occurs, plants tend to be more prone to cold injury, particularly branch tip dieback.  Management of bacterial blight includes good fall clean up (as described above) and pruning in the case of branch dieback.  Prune four to six inches below obviously dead areas on branches.  Be sure to prune only when it is dry, and be sure to disinfest your pruning tools between cuts by dipping them in 10% bleach or (even better) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants can be used as a source of alcohol as well.  Just be sure to check the ingredient list of the disinfectant that you select to make sure it contains roughly 70% alcohol.  If you use bleach, be sure to thoroughly rinse your tools after you are done pruning and oil them to prevent rusting that can be caused by the use of bleach.  You can dispose of branches by burning (where allowed) or burying them.

Septoria Leaf Spot:  This has been the number one disease this season on lilac (and is towards the top of my disease list for all plants).  I have received numerous inquiries about lilacs with leaves that have partially or fully browned starting at the bottom of the shrub and working up the plant.  The culprit in this browning appears to be a species of Septoria, a fungus related to (but distinct from) the organism that causes Septoria leaf spot of tomato.  While the browning caused by this disease is quite dramatic, the disease is not lethal.  If you look carefully at the branches with symptomatic leaves, you should be able to find healthy leaf buds at the base of the petioles of this year’s leaves.  These buds are ready to sprout next spring to produce a new crop of leaves.  Good fall clean up (see above) is the place to start in managing Septoria leaf spot.  Routine pruning/thinning to open the canopy and promote rapid drying of leaves when they get wet will also help keep this disease at bay.  If we continue to see wetter summers, this disease may become a chronic, severe problem, and use of fungicides to manage the disease may become necessary.  However, I suspect we will cycle into a series of drier summers over the next couple of years, and if we do, I would expect the serverity of this disease to decrease, and no fungicide treatments will be needed.

Lilac Witches’ Broom:  This disease is caused by the same bacterium-like organism (called a phytoplasma) that causes ash yellows.  The phytoplasma is introduced into a lilac’s food-conducting tissue (i.e., phloem) by leafhoppers.  Once in the plant, the phytoplasma leads to a yellowing of foliage, stunting of the entire plant, and over-production of lateral branches (i.e., brooming).  Infected plants typically decline and die over a period of several years,.  There is no way to get rid of phytoplasmas, so removal and replacement of infected shrubs is the management strategy of choice.

Verticillium WiltVerticillium wilt is the most rapidly lethal of the lilac diseases, and the disease that I most commonly see on Japanese tree lilac.  The fungus that causes this disease (tyically Verticillium dahliae for lilac) is found in the soil and infects plants through the roots.  It colonizes the water-conducting tissue (i.e., xylem) inside the plant and blocks it off.  This prevents water movement from the roots to the branches, leading to wilting.  Typically the wilting starts in a single branches.  As the disease progresses, a localized cluster of branches wilts, and eventually, the entire tree/shrub dies.  Verticillium dahliae has a wide host range, so if you have a lilac with this disease, you need to be very careful about what you choose as a replacement.  Conifers are your best best, although a limited selection of deciduous trees and shrubs (tricolor beech anyone?) ban be used.

NEED HELP?  If you need help diagnosing any of the lilac diseases described above (or diseases of any other kind of plant for that matter), feel free to contact the PDDC.  For the PDDC’s current policy on sample submission, including submission of digital photos, check out the following link.  As always, be sure to check out the PDDC website for timely information on plant diseases.  Also, feel free to follow the clinic on Twitter or Facebook (@UWPDDC) to receive timely PDDC updates.  Or alternately, put in a request to subscribe to the clinic’s new listserv (UWPDDCLearn) by emailing pddc@wisc.edu.

Hang in there, be safe, and stay healthy everyone!

P.S.:  Bonus points if you recognize where this month’s article title comes from.