Q: My peonies have powdery mildew and I'm wondering what I can do to get rid of it. What causes this? I'm planning to divide them this fall but I don't want to spread it into a different flower bed. — Pat Johnson, Valley City, N.D.
A: Powdery mildew is a disease that causes a grayish-white coating commonly found in midsummer on the foliage of peonies, lilacs, zinnias, roses, vine crops and many other plants.
Caused by many different species of fungi, the disease develops most severely during weeks of high humidity. Although the disease does little long-term damage to plants like peonies and lilacs, it's still unsightly. Powdery mildew is more serious on squash, cucumbers, melons and pumpkins because the disease can deteriorate the foliage, reducing plant vigor, fruit production and yield.
Remove and discard affected vines or peony tops during fall cleanup. Powdery mildew fungi are ever-present, and are easily carried by wind, so sanitation alone isn't enough to prevent powdery mildew.
Prevention is the key, because once a plant is infected, there's no reversal for the current growing season. Increase air flow around plants by pruning or spacing plants. Avoid wetting foliage when watering, as wet foliage increases plant humidity.
The best way to prevent powdery mildew on disease-prone plants is by applying an all-purpose fungicide before the disease starts, or at the earliest symptoms. A common active ingredient is chlorothalonil.
Q: I finally had chokecherries on my tree and was looking forward to making jelly, but when I looked, every berry was stripped from the tree. What would eat the berries, and how do I prevent for next year? — Janet Johnson.
A: Chokecherries are quickly consumed by wildlife, mostly birds. It's common for the tree to be loaded with chokecherries one day, with few remaining the next.
Years ago, when chokecherries were planted in shelterbelts, or growing naturally along creeks and rivers, it seems there was enough fruit for both humans and birds, as people had little problem finding chokecherries to pick. Now, on single, in-town trees, it's difficult to harvest a crop without bird prevention. Garden centers offer bird netting that can be placed over the tree as fruits are forming.
Other less effective methods involve hanging "scary" objects in the tree, such as aluminum pie tins that rattle in the wind or plastic owls and hawks.
Q: I'm a new gardener. How can you tell when cabbages are ready to harvest? — D. Hanson, Alexandria, Minn.
A: Cabbage cultivars can be divided into early, midseason or late, based on the approximate part of the growing season in which they're ready for harvest. Generally, the later cabbages are larger than small, early varieties.
When heads appear to be well-formed, test by squeezing the head with both hands. If the head is solid, it's ready to harvest. If the head gives under pressure, wait a few days and test again. If heads are left in the garden too long after they've reached the solid stage, they can split open, especially after a rain.