In many areas, it’s been a dry summer… just the condition powdery mildew (PM) likes… and cucurbits ( cucumbers, melons, and squash, including pumpkins, zucchinis and pattypans,) get more than their fair share of the disease, especially the latter.
It’s pretty much the only plant disease that actually develops best on dry leaves. The perfect conditions? Humid air, no rain and moderate temperatures: 68-80°F (20-26°C).
It’s a late season disease, rarely noted before August. On cucurbits, powdery mildew first shows up when what appears to be a powdery residue covers the older leaves. It actually first shows up on the underside of the leaf, sight unseen, before migrating to the top side. Affected leaves then die back and that can sometimes lead to sunscald on the fruit underneath. Often it spreads to younger leaves and sometimes kills back the stems.
It’s worth noting first that PM is not one disease, it’s many diseases. In cucurbits alone, there are at least 2 different PM species in 2 different genera and each has several strains, all strictly adapted to cucurbits. You therefore needn’t worry that it will spread from your infected cucurbits to your other garden plants.
Many modern cucumber and melon varieties are naturally resistant to powdery mildew, but there are fewer such choices in squashes. Even so, if you’ve had a problem one year, ideally you’d look for a PM resistant variety for the following year’s harvest.
What to Do
Powdery mildew often appears so late in the season that often it doesn’t much affect the crop. With summer squash, for example, you’ve often pretty much finished harvesting before it shows up (or you have so many zucchinis you really don’t care if you lose a few). Therefore, sometimes there is no need to react.
Also, winter squash often continue to mature in spite of the disease. You’ll often see fields of pumpkins with damaged or dying leaves, for example, yet the fruits themselves will be fine.
PM is rarer on cucumbers and melons because so many modern varieties are highly resistant to the disease. Heritage varieties, though, may need treatment.
With powdery mildew, the important thing is to slow down its progress. You can do so by spraying with any number of fungicides available at your local garden center (sulfur, horticultural oil, neem, etc.). Make sure the fungicide is approved for edible plants.
Home remedies like baking soda (5 mL/1 tsp. in 1 quart/1 liter of water, plus a few drops of insecticidal soap) or milk (1 part to 9 parts water) can also be quite effective.
Prevention Worth A Pound of Cure
With powdery mildew, it’s easiest to put your effort into prevention rather than treatment.
- Destroy infected plants at the end of the harvest and practice crop rotation, not planting any cucurbits in the same spot for at least 4 years.
- Plant only resistant varieties. (This is such a simple solution that I’m always amazed so few gardeners look into it.) Good seed companies will mention which varieties are mildew resistant.
- Plant cucurbits in full sun and space them appropriately (don’t overcrowd) to ensure good air circulation.
- Avoid excess fertilizer, especially ones rich in nitrogen.
- Water regularly in case of drought, as drought-stressed plants are more susceptible to PM, and, for once, don’t worry about moistening the leaves when you water, as PM develops best on dry leaves.
Seeing cucurbit leaves turn powdery white can be quite a shock the first time you see it, but since it doesn’t always affect the harvest to any degree, most gardeners learn to take it in stride. Personally, just choosing resistant varieties has given me all the mildew protection I need. That may be enough for you as well!