July is the season of grey on the foliage of some popular garden plants... cucumbers, roses, bergamot, peonies. The symptoms often start as dusty grey spots on the leaf tops, that expand into a more wide-spread grey coating over the foliage. This is called powdery mildew, a group of fungal pathogens that show up regularly during the heat and humidity of mid summer. The mould is unsightly, and when severe, it can certainly reduce the vigour and productivity of garden plants.
Powdery mildew on Bee-Balm (Monarda / Bergamot)
As with most problems in the garden, their is not a single solution for attending to powdery mildew, but rather a tool-kit approach that tackles the situation holistically. The first thing to know is that powdery mildew is not necessarily terrible... in fact many plants can be counted on to carry on just fine after their annual bout with the grey coating. Secondly, powdery mildew is very specific as to which plants it can grow on, so if it shows up in the garden, it is not going to spread indiscriminately. Third, while some plants often get powdery mildew, the occurrence of the mould is a clue that plants are looking for more light and air penetration, and may be under some stress (often simply associated with the season, for example blooming requires a lot of resources for the plant, which puts it under stress).
So what to do?
- Trim off afflicted leaves. This advice is most applicable to large foliage plants such as squash, pumpkins and cucumbers. Obviously not so easy to do on roses, bergamot or ninebarks. Dispose of the leaves in the garbage rather than the compost. Use rubbing alcohol to sterilize your clipping utensils before moving to other plants.
Powdery mildew on squash plants
- Do what you can to improve light and air penetration. If plants are getting big and possibly a little over grown, reign this in. If plants are spaced too close to one another, try to correct this (for perennials, this may be a question of doing some pruning now, then reorganizing them in fall). For larger plants, such as ninebark shrubs, feel confident that a good pruning is in their best interest!
- Avoid watering the foliage. It is always best to water at the base of plants, to target the water but also not have moisture sit on the foliage; excess moisture on the leaves is one factor that contributes to powdery mildew.
- Apply an appropriate fungicide. Powdery mildew is effectively controlled by a variety of safe to use fungicides, although this tactic works best when the infection is in the early stages. At Sage Garden we offer three types of fungicides for powdery mildew, each with a different mode of action. The first is Green Earth sulfur spray; the second is Safer's 3 in 1 which also contains sulfur, but also something called potassium salts of fatty acids (sometimes more than one active ingredient is helpful); and lastly a product called Natria, which is made from a bacteria which eats up the powdery mildew... how cool is that!
Note that none of the fungicides change the conditions that lead to powdery mildew, so the suggestions from earlier in this article are an important part of the control process.
Lastly, it is worth making a couple of notes now, for future planning. For example, are your afflicted plants growing in their correct exposure? Powdery mildew is much more common when full sun (6-8 hours direct sunlight) plants find themselves in part shade. And remember that sometimes shade creeps up on the sunny perennial bed, as trees fill in over time, or new structures are built around the garden. Along a similar theme, plants that are not getting enough nutrients are more likely to get mildew. A final detail to keep track of is the mildew resistance of certain varieties; for all of the popular plants that get mildew, some are much more susceptible and others highly resistant... see if you can work towards including only the resistant ones.