A Greener View by Jeff Rugg

Q: We have a large pumpkin patch in our community garden and some of the leaves have turned white or yellow. Will this cause a problem, or can we let it go?

A: It sounds like some of the leaves are getting powdery mildew. It is common on many vine crops, but can also affect beans, peas, strawberries, tomatoes and other vegetable crops. There are several different species of fungi all lumped into the name "powdery mildew."

It can start in dry weather, but it spreads rapidly in humid conditions after rains or from dew. It seems to be more common in cooler weather. Often it doesn't start affecting northern gardens until later in the fall, but this summer many areas have been cooler, so it is starting earlier. In southern states, where many gardens are planted for winter and spring crops, the mildew comes after periods of light rains.

If it starts appearing on plants after the fruit is growing and getting to a mature size, there may not be much of a loss in yield. If a lot of leaves are lost as the fruit is forming, there may not be as much sugar in the fruit — melon crops may lose quality. A crop of pumpkins used for jack-o'-lanterns may not be affected. By the time a loss of yield would be noticed, it would be too late to start treating the plants, so it is better to protect as many green leaves as possible.

Spray programs could include copper- or sulfur-based fungicides alone or in combination with lime fungicides known as Bordeaux mixtures. These products have been around for many years and some powdery mildews are resistant to their use. Check the label to see if it can be sprayed on pumpkins. If it doesn't say pumpkins on the label, it is illegal to use.

The next product is one based on neem oil. It will have insecticidal properties; it can be used to help reduce the striped cucumber beetles also found on many vines, but needs to be applied late in the evening, near dark, to avoid harming the bees pollinating the pumpkins.

Products with chlorothalonil are among the most common fungicides used on food crops.

Safe products containing sodium bicarbonate (baking soda) might work, but potassium bicarbonate works better on powdery mildew.

Either kind needs to be used with a small amount of horticultural oil or insecticidal soap to stick to the leaves better. Potassium bicarbonate is available in Armicarb 100, Kaligreen and Remedy.

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

All of the products need to be sprayed on the top and bottom of all leaves. They work better to prevent infection; therefore, they need to be sprayed on the leaves without the problem first. All of them, except the bicarbonates, will require waiting periods after spraying — waiting to go into the area and waiting to harvest. The bicarbonates are acceptable for use in organic gardening programs.

Some of the plants seem to be more susceptible and should be sprayed first. Then spray the plants near them, followed by all the rest. Since powdery mildew affects all the vine crops and some other vegetable crops, other community garden plots may have plants that would benefit from spraying.

Florida and other southern gardeners should begin looking at crops listed as resistant to powdery mildew for the gardens they will be planting in the next few weeks and months. Northern gardeners will need to wait until the gardening catalogs arrive this winter to look for resistant varieties to plant next summer.

E-mail questions to Jeff Rugg, University of Illinois Extension at jrugg@illinois.edu. 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.

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