Vine Crop Disease and Pest Rx

by National Gardening Association Editors


Being prepared allows you to prevent many pest and disease problems. Here are gardening methods to follow, along with disease and pests to watch out for.

General Practices

Grow resistant varieties. Ask a garden center, a gardening friend, or your cooperative extension service Master Gardeners which diseases are prevalent in your area and start with resistant crop varieties.

Control weeds and debris. Weedy areas often harbor disease and insect populations; weed regularly during the growing season. After harvest, till all healthy vines, leaves, stalks, and discarded fruit into the soil or compost them. Destroy diseased vines to prevent harmful organisms from surviving the winter and attacking next year's plants.

Rotate crops. Shift vine crops to different areas in the garden from one year to the next, and plant another vegetable family in their place. If you have a serious problem with any crop, don't plant it in the same spot for two or three years.

Control cucumber beetles. Because cucumber beetles can cause so much damage to vine crops, protect your plants from this pest. (See control information below.)

Use pesticides sparingly. Injudicious use of pesticides can backfire: it kills beneficial organisms that can help keep pests under control naturally, and encourages pest resistance. (Beneficial organisms do not recover as quickly as pests, and do not develop resistance). Use pesticides very carefully and as a last resort. In addition to the information below, you can check with Master Gardeners for the most up-to-date pesticide recommendations. Always read the instructions before applying pesticides, whether conventional or organic, following them carefully as you go. Read the label again when you put the container away, to be sure you store it safely.

Diseases to Watch Out For

The best means of avoiding vine crop diseases are to plant disease-resistant varieties, clean up crop residues, rotate your crops, stay out of the garden when it's wet, and keep harmful bugs in check. However, if you notice a diseased vine or plant, cull it right away. The remaining plants or vines may be healthy, so you may keep the disease from spreading. Below are descriptions of common vine crop diseases and specific control measures.

Anthracnose infections appear as circular dark spots on leaves, streaked vines, and blackened fruits that drop off. This fungus thrives in humid, wet weather, and spreads via splashing water. It winters over in seed and refuse from diseased plants. Anthracnose affects cucumbers, muskmelons and watermelons. To prevent this disease, clean up gardens well in fall and rotate crops.

Angular leaf spot shows up as water-soaked spots on leaves that turn tan and gray then drop out, leaving ragged holes. Spots on the fruit are rounded and may exude a thick oozing substance. The infection moves into the seed cavity, making the fruit inedible. The bacteria overwinter in seed and plant debris, and spread in wet weather. To test for this disease, cut a stem and rub the two cut ends together. Pull them apart slowly. If a thick, gelatinous strand forms between them, the plant suffers from angular leaf spot. This disease affects cucumbers, honeydew melons and zucchini. Remove and destroy affected plants as soon as you notice the disease. Prevention includes thorough garden clean up in fall and spacing plants well apart to help keep the leaves dry.

The symptoms of angular leaf spot and squash vine borer (see description below) are similar. The stem borer gets inside the stem and cuts off the water supply. If these pests are present, just cut them out.

Bacterial wilt causes leaves, and then the entire infected runner, to wilt. The affected area of the plant dries up and dies; eventually the entire plant dies. The disease is spread by cucumber beetles (see description below). In fact, the bacterium lives in the gut of the beetle, so control control of the beetle is key to avoiding this disease. It does not affect watermelons.

Fusarium wilt causes damping-off of seedlings and stem blight, and vines may develop water-soaked streaks. Growth is stunted, leaves wilt, and vines decay and die. This fungal disease primarily affects muskmelons and watermelons. Fusarium overwinters in infected seed and can build up in the soil. Some varieties are resistant to fusarium, as indicated by an (F) in the variety description. Keeping soil pH between 6.0 and 7.0 and not over-fertilizing with nitrogen can greatly reduce infections.

Powdery mildew thrives in high humidity and warm weather. A gray-brown to white powderlike substance forms on the leaves and young stems. Foliage eventually dries up and dies. The fungus develops best in hot weather. If powdery mildew is an annual visitor to your garden, preventive sprays of potassium bicarbonate (one brand-name product is Remedy).

Downy mildew appears as irregularly shaped yellowish to brown spots on the tops of leaves; in moist weather (when it thrives), a purplish mildew may form on the underside of these spots. Leaves die as the spots grow larger. It affects cucumbers and muskmelons. Control as you would powdery mildew.

Mosaic virus causes mottling, dwarfing, and distortion of leaves. Each vine crop has its own type of mosaic virus: cucumber mosaic virus (CMV), watermelon mosaic-2 (WM-2), and squash mosaic virus (SMV). If mosaic virus has overwintered in perennial plants it can be carried by insects, such as cucumber beetles and leafhoppers. It can also be spread by gardeners working in moist garden conditions. It's best to plant disease-resistant varieties, control insects, and stay out of the garden when it's wet.

Scab appears as dry, corky spots on cucumbers and muskmelons. Unlike angular leaf spot, scab shows a dark olive-green velvety growth on the disease spots, which can cover young stems and leaves. The entire plant eventually dies. Scab thrives in foggy, cool weather and cool night temperatures. To prevent scab, remove and destroy diseased fruit, and clean up the garden well in fall.

Common Insect Pests

Cucumber beetles. There are two forms of cucumber beetles: one with black stripes, the other with black spots. Their feeding causes damage to blossoms and other plant parts, and they spread mosaic virus and bacterial wilt, both serious diseases. As soon as you transplant or seedlings emerge from the soil, cover them with floating fabric row covers to exclude beetles. It's important to remove the covers when plants blossom to allow pollinating insects to reach the flowers. Otherwise, you'll have no fruit!

If beetles are already present, spray with an effective pesticide, such as pyrethrin. Treat the base of the plant, because beetles attack there. In some regions, beetles hatch four broods of eggs, so you have to repeat spraying every five to seven days. Respray or dust after a rain, especially the undersides of leaves. (Apply pesticides when bees and other pollinators are least likely to be present, such as late afternoon/early evening.)

Aphids. These tiny, spidery-looking insects gather in colonies on the undersides of leaves and on succulent new growth. As they feed, they injure plant tissues and spread disease. Encourage beneficial insects, such as ladybeetles and lacewings -- their larvae are voracious aphid predators. Plant flowers and herbs in the umbel family (Queen Anne's lace, dill) in your vine patch -- they are favorite nectar sources for adults. Or, spray with insecticidal soap.

Squash Vine Borer. The larvae of this moth bore into stems near the ground, causing the plants to wilt and die. When you see the signs of boring (a hole on the main stem with yellow "sawdust" around it), cut out the pest with a razor blade or sharp knife, then quickly cover the wound with a mound of soil. The plant should recover and continue growing. You can also inject Bt var. kurstaki, a biological control, sold under brand names such as Dipel and Thuricide. Use a syringe and inject the pesticide into the stems where the borers are active. (This is most effective on very young larvae.)

Cutworm. These caterpillars dwell in the soil, can do incredible damage to young seedlings or transplants. Active at night, they chew through tender plant stems, causing entire plants to topple over and die. One way to control cutworms is to wrap the stem of your transplanted seedlings with a paper collar (newspaper strips are fine) one inch below to one inch above the ground. You can also apply predatory nematodes to the soil prior to planting.

Squash Bug. Adult squash bugs are flat, brownish-gray bugs with an X-mark on their backs. Nymphs are pale and pear-shaped with dark legs and antennae. Both life stages suck juices from vine leaves and stems. Adults overwinter on garden debris, so a thorough clean-up helps reduce the population. If squash bugs appear in the garden, lay boards on the ground near affected plants. The bugs will gather under the boards at night. Early in the morning, lift the boards and destroy the bugs. You can also pick off any bugs or egg masses (which range in color from copper to gold) present on the leaves (check the undersides, too). If you use an insecticide like pyrethrin, spray it on the top and bottom of the leaves.

Photography by Suzanne DeJohn/National Gardening Association



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