Cucubit culprits The best plant defense is a good offense

This cucumber leaf shows the early stages of anthracnose.

Lowcountry gardeners who are growing cucurbits, vine crops such as cucumber, muskmelon and squash, should watch for a few diseases and insects that might lower the number, size or quality of the harvest.

After planting, the main method to manage diseases is to protect plants by spraying them with fungicides or biofungicides before disease starts. Cucurbits have large, broad leaves that expand rapidly. To cover new leaf tissue, cucurbits must be sprayed regularly, usually every week.

It is easier to prevent plant diseases than to cure them. Once a disease attacks a vegetable, gardeners may be able to limit the damage on the crop, but the disease probably will not go away.

Powdery mildew is a common disease on summer squash. It is an easy plant disease to recognize and to control, since 95 percent of the mildew grows on the outside of the plant. (More commonly, disease-causing organisms grow inside a plant, where they are hidden and protected.)

White, powdery spots start on the top and bottom of older, shaded leaves inside the leaf “canopy.” As soon as spots are noticed, squash should be sprayed with sulfur, potassium bicarbonate, or a fish oil-plus-sesame oil product (Organocide). All three biofungicides are approved to use on organic vegetables.

Most hybrid varieties of cucumber and muskmelon are resistant to powdery mildew. Heirloom varieties are susceptible and will need to be sprayed.

Downy mildew is a serious threat to cucumber and muskmelon. Leaf spots start as yellow, angular spots that turn brown. Diseased cucumber leaves have a characteristic “checkerboard” look. When leaves are wet with dew, a purplish black mildew grows on the bottom of the leaves.

Downy mildew usually shows up in the Lowcountry in May. The last two years, however, downy mildew did not appear until June, because of cool spring weather across the South.

If you want to know how close to Charleston downy mildew has spread, the Cucurbit Downy Mildew Forecast Homepage (cdm.ipmpipe.org) displays a map of outbreaks. The first report of the 2015 season came in this past week from Miami-Dade County, Fla.

Once downy mildew is reported from Georgia, Lowcountry gardeners should start spraying cucurbits. Sprays with chlorothalonil (e.g. Daconil) or mancozeb do an excellent job of preventing downy mildew, if the spray is applied before downy mildew starts. Organic gardeners can use fixed copper.

Yellow summer squash and zucchini tolerate downy mildew. Although the top of squash leaves will turn yellow from downy mildew, the plants usually keep producing squash and don’t need to be sprayed.

In the past few years, anthracnose has been a problem on watermelon, cucumber and muskmelon. On watermelon, small, dark, angular spots appear on leaves. Short, narrow, sunken tan spots appear on vines. Small, round, tan blisters or sunken spots appear on fruit. Leaf spots on muskmelon look similar. Leaf spots on cucumber are tan and larger, about a half inch in diameter.

Sprays of chlorothalonil or mancozeb control anthracnose well if they are applied before the disease starts. Spraying every other week is often enough. Unfortunately, no organic products control anthracnose.

A useful book to identify insects on vegetables is “Garden Insects of South Carolina,” which can be purchased from the Lowcountry Biodiversity Foundation (lcbiodiversity.org).

The main insect pest in the spring is the cucumber beetle, which chews up tender, young plants. The beetles are about a quarter inch long and yellow with either dark spots or stripes. Beetles like straw mulch, so use other types of mulch in the garden, such as coarse compost or pine straw.

Newly planted cucurbits should be checked twice a week for cucumber beetles. If there is an average of one beetle per two plants, apply neem oil, an organic-approved biopesticide that repels beetles.

In summer, the squash vine borer does just that: It bores into the main stem of a summer squash and causes the entire plant to wilt. An old-timey remedy is to slit the damaged stem far enough to find the borer, stab it, and then cover the stem with moist soil and hope the plant will heal itself.

By checking cucurbits regularly and acting quickly, gardeners can protect their crops and enjoy a good harvest of healthy produce.

Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He also is an avid gardener. Contact him at tknth@clemson.edu.