Q: We have a large pumpkin patch, and most of the leaves have turned white. It appears to be powdery mildew. Is this a problem, or can we let it go? We don't want to spray the plants if we don't have to because we may make pies out of the pumpkins.

A: Powdery mildew is common with many vine crops, but it can also affect beans, peas, strawberries, tomatoes, other vegetable crops and perennials like peony and phlox. There are several species of fungi all lumped into the common name of powdery mildew.

Powdery mildew can start in dry weather, but it spreads rapidly from dew and in the humid conditions after a rain. It seems to be more common in cooler weather. Oftentimes it doesn't start affecting gardens in the north until later in the fall. In Southern states, where many gardens are planted for winter and spring crops, the mildew comes after periods of light rain.

If mildew shows up on plants while the fruit is growing and getting to a mature size, there may not be much loss in the yield. But if many leaves are lost as the fruit is forming, the fruit may have less sugar, so certain crops like melon may not taste as good. But this wouldn't matter for a crop of pumpkins used for, say, Jack-o'-lanterns. It would be too late to start treating the plants after a loss of yield, so it is better to protect as many green leaves as possible.

You could use a copper- or sulfur-based fungicide as a spray treatment. Or you could combine a fungicide with lime, which is called a Bordeaux mixture. These mixtures have been used for centuries. Some powdery mildews are resistant to them. Check a product label to see if it says the product can be used on pumpkins. If it doesn't, the product is illegal to use.

Another product you could use is based on neem oil. It has insecticidal properties, so it can be used to help reduce the number of striped cucumber beetles (also found on many vines). But it needs to be applied late in the evening to prevent harming the bees that pollinate the pumpkins.

Fungicides with the ingredient chlorothalonil are among the most common used on food crops, and they are commonly available.

Safe products containing sodium bicarbonate (baking soda) might also work, but potassium bicarbonate works better on powdery mildew. Both ingredients need to be used with a small amount of horticultural oil or insecticidal soap so they stick to the leaves better. Potassium bicarbonate is available in garden center products, such as Armicarb 100, Kaligreen, and Remedy.

If you want to try baking soda, the recipe is 4 teaspoons baking soda per gallon of water with an equal amount of liquid dish soap. Too much of either ingredient will damage the pumpkin leaves.

All of these products need to be sprayed on the tops and bottoms of all leaves. They work better to prevent infection, so they need to be sprayed on the uninfected leaves first. And all of them — except the bicarbonates — require waiting periods after spraying before going into the area. The bicarbonates are acceptable for use in organic gardening programs.

If some plants seem to be more susceptible to powdery mildew, they should be sprayed first — then the plants nearest them, and then all the rest. Since powdery mildew affects all vine crops and some other vegetable crops, the nearby, uninfected vegetable plants might benefit from a preventative spraying, especially if your pumpkin patch is large and there will be a large amount of spores available to infect other crops.

Gardeners in Florida and other states in the southern U.S. should look for crops that are resistant to powdery mildew for the gardens they will be planting in the next few weeks. Northern gardeners will have to wait until the gardening catalogs arrive this winter to look for resistant varieties to plant next summer.

Email questions to Jeff Rugg at info@greenerview.com. To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.