Continuing Education and
Communication Services
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
8/20/01
Contacts:
Paula Flynn, Extension Plant Pathology, (515) 294-0581, pflynn@iastate.edu
Del Marks, Continuing Education and Communication Services, (515) 294-9807,
delmarks@iastate.edu
Yard and Garden Column for the Week Beginning Aug. 24
Wilting Vines
By Paula Flynn
Extension plant pathologist
Iowa State University
Visions of tasty cucumbers, juicy melons, decorative gourds, or Halloween pumpkins might be cut short when a glance at your garden reveals limp, yellowing vines. Weather conditions this year have favored some of the maladies that curse these plants, collectively called cucurbits. Mother Nature, bacteria, insects, and fungi are a few of the agents that have been at work.
Mother Nature
Soil conditions have been dry in much of Iowa. Before that, the weather was cold and wet, and roots had little chance to grow. Cucurbits can wilt after only short periods without rainfall, primarily because their root systems are shallow, with most of the roots located in the upper inches of soil. Compounding the problem are their leaves. They have a very high transpiration rate, which means they lose large amounts of water, moisture that needs to be replaced. As fruits develop, even more moisture is needed.
Growers of cucurbits need to water before wilting occurs. Waiting too long can cause so much stress that plants will yellow and die. The amount of water required depends on soil type, but a general rule of thumb is to provide one inch of water per week until the fruit starts to develop, then 1-1/2 inches of water thereafter.
Bacteria
Bacteria are gummy, slimy organisms that can easily clog the water-conducting vessels of plants and cause wilting. Bacterial wilt is a problem mainly on cucumbers and melons, but occasionally occurs on pumpkins and squash. Cucumber beetles spread the disease. The bacteria overwinter inside adult cucumber beetles. When the beetles feed on young plants in the spring, they create wounds that allow the bacteria to find their way into the plant.
To diagnose bacterial wilt, perform a string test. Cut an affected stem, squeeze the cut ends, push the two cut ends together and slowly pull them apart. Plants infected with bacteria show a string of sticky juice that connects the two cut ends as you slowly pull them apart. Another technique is to place the cut end of an affected stem in a glass of water. If the plant is infected with bacteria, you should see a cloudy ooze flowing from the cut end into the water. These techniques work better for cucumbers than melons.
Once a plant is infected with the bacterial wilt pathogen, there's no way to rid the plant of the disease. With no means of conducting water, plants eventually wilt and die. Preventing the disease requires controlling the cucumber beetle populations. To control cucumber beetles, you must start as soon as the seedlings emerge or are transplanted. Common garden insecticides do a so-so but adequate job if applied regularly and frequently, or you can cover the plants with floating row cover cloth until the plants are well established. Bacterial wilt resistance is available in a few cucumber varieties.
Insects
The squash vine borer can be a menace, especially on squash, pumpkins,
and gourds. The initial symptoms are similar to bacterial wilt -- vines
wilt and die. The two problems can be distinguished readily by examining
the stem near the soil line. The squash vine borer caterpillar tunnels
in the stem, usually near the start of the root system and cuts off the
flow of water and nutrients. Split open the affected stem to reveal the
cream-colored vine borer caterpillar.
Crop rotation, which means planting crops other than cucurbits in problem
areas for several years, can help prevent borer damage. Insecticides can
be used, but must be applied to the stems at the base of plants beginning
when egg laying occurs, which is usually about when the plants begin to
vine.
Fungi
To add yet another foe of cucurbits, fungi found in the soil can infect the roots of plants. Fusarium wilt can be troublesome from year to year because the fungus produces tough structures that allow it to survive over the winter. During the growing season, Fusarium enters the root system and finds its way to the vascular tissues. The story ends the same; the vines eventually wilt and die. To diagnose Fusarium wilt, cut through affected stems and look for a reddish brown color in the cut surface. You might also find a reddish-brown, sticky substance at the base of the plant. Fusarium wilt is best controlled with crop rotation and selection of varieties resistant to the disease.
An awareness of the potential problems that cause vines to wilt may make visions of collecting a harvest seem bleak. However, if you arm yourself with knowledge of the needs of the plants and understand the potential threats, you can increase the likelihood of cucumbers in the refrigerator, pumpkins on the front step on Halloween, and decorative gourds around the house at Thanksgiving.
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ml: isugarden
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