“Powdery Mildew on Lilacs” continues to be one of the most read articles on Gardening in the Mud. However, anyone located in Zones 5 and 6 hoping to treat powdery mildew at this time of year has only one control option available–and even that control will only help contain spread of the disease for next year’s growing season: collect all fallen leaves and either burn them or otherwise permanently dispose of them. Do not compost infected leaves. Powdery mildew fungus overwinters in the leaves, so you want to keep the fungus away from affected plants.
Regarding plant diseases more generally, there are four principles of plant disease management:
Notice that plant pathologists use “disease management” rather than “cure”. That’s because there are no cures for plant diseases caused by biotic agents. This is such an important concept that I will repeat it: There are no cures for plant diseases caused by biotic agents. Each new growing season requires repeating prevention strategies for susceptible plants.
Plant diseases caused by abiotic agents, such as insufficient sunlight, too much or too little water or fertilizer, insufficient airflow around plants, or transferring a fungus by handling wet plants, can be addressed by altering the circumstances leading to plant disease. Although the plant disease will still not be cured, rapid improvement in health is likely. For biotic diseases, Avoidance and Protection are the most commonly used individual management strategies. Avoidance includes purchasing a disease-resistant variety of lilac (avoiding varieties more susceptible to powdery mildew). Protection includes a chemical spray program for the lilac (chemicals encompass any product from sulfur to horticultural oil to regulated fungicides). Otherwise, there is Exclusion (don’t grow lilacs) or Eradication (remove all existing lilacs that are susceptible to powdery mildew).
Although I’ve used lilacs and powdery mildew to illustrate the different disease management options, depending on the disease, all four options may not be available. For example, virus-stricken plants need to be eradicated–there is no treatment that will improve the plant’s health. Also, if you’re living in certain Canadian provinces, fungicides may not be an option (although you can still use a horticultural oil treatment in the case of lilacs). Mostly, however, we can choose among different strategies based on trade-offs between cost, aesthetics, time commitment to caring for particular plants, and personal preferences.