This is certainly the season when we think of the glory of changing leaves and the glory of colorful foliage, but the excessive rainfall this year has caused gardeners to pay attention to the leaves on their perennials and shrubs. And while we associate fall with the moment of leaves dropping to the ground, this year many lilacs defoliated in midsummer after their leaves first succumbed to powdery mildew and then to Pseudocospora leaf spot. This left gardeners worried for the long-term survival of their lilacs, especially the common lilacs (Syringa vulgaris), which seemed more susceptible. Varieties like ‘Miss Kim’ and ‘Paliban,’ which belong to different species, seem to have survived relatively unscathed, though they, like most lilacs, prefer well-drained soil and suffer from exposure to excessive moisture.
This leaf blight is usually not fatal, but plants with less air circulation or in humid and wet situations are more prone to getting it. And in a year with excessive rain, many lilacs in the region had been infected and lost most of their leaves by August. In a few cases, where the plants lost their leaves early in the season, a second bloom has occurred in the past month, confusing gardeners, or at least making us question our sense of time. These blooms were triggered by the premature loss of leaves and the production of new foliage, which tricked the plant into thinking it was spring. This flowering will likely reduce the floral effect of the lilac next spring, as more buds will not have time to form in the weeks ahead. Fortunately, though, it should not impact the plant’s long-term health.
This leaf spot is not typically fatal, but it can survive the winter on fallen foliage and reinfect the plant next spring, so it is wise to rake up and dispose of the leaves to prevent its return. Like the botrytis-covered leaves of peonies or the leaves of German iris that may contain borers, it is best to get rid of such leaves, not by putting them in the compost, but in the trash. Removing the foliage and stems of peonies with blackened leaves reduces the chance of reinfection next year; the same principle applies to the disposal of the fallen leaves of diseased lilacs. I often even discard leaves with powdery mildew (of which there were many this year due to humidity and high rainfall), although some scientists claim that proper composting will prevent these spores from spreading as they need a living host in order to survive.
Plants are not dissimilar from humans, in that environmental conditions can impact infection. And just like us, plants are likely to survive these infections, but also just like us, plants benefit from having their beds cleaned of evidence of their illness so that they can get back into full health. Disposing of diseased foliage is the equivalent of changing the sheets after a flu, and I imagine the plant feels just as happy as we do to be rid of the evidence of our recent illness.
A gardener grows through observation, experimentation, and learning from the failures, triumphs, and hard work of oneself and others. In this sense, all gardeners are self-taught, while at the same time intrinsically connected to a tradition and a community that finds satisfaction through working the soil and sharing their experiences with one another. This column explores those relationships and how we learn about the world around us from plants and our fellow gardeners.