Zucchini, a widely cultivated summer squash, grows vigorously in warm weather. These vining- or bush-type plants are often grown in mounded hills. They prefer thorough watering and the fruits can be eaten at any size. A splitting zucchini plant stem may simply be the result of rough handling or evidence of a pest. If the stem problem is noticed and remedied quickly, the zucchini may recover and vigorously produce fruit.
Zucchini stems may split if the plant has been moved, twisted, mishandled or its own weight. This can be easily addressed. If striped cucumber beetles, which carry bacterial wilt or fungal infections are common in the garden, treat the split area with a fungicide or cover it with a bandage of tape or impermeable adhesive. Even if the main stem is damaged, the plant can survive. Press the stem together and heap moist soil over the stem joints. This permits the growth of new roots along the covered vine.
Squash Vine Borer
Winter squash, pumpkins and zucchini are especially susceptible to squash vine borer damage. The larvae tunnel into the stems. Vines wilt and may be completely girdled, causing part of the stems and leaves to rot. To determine if the borer is present, look for splits or small holes in the stem with sawdust-like frass coming out of the stem. Cut this area of the stem partly open, locate and kill the larvae. Alternatively, use a sharpened wire to kill the larvae. Entomophagous nematodes can be injected into the squash plant stem to control the squash vine borer larvae.
Gummy Stem Blight
Gummy stem blight is caused by the fungi Didymella bryoniae or Phoma cucubitacearun. The infection is noticeable on low, older foliage, leaving brown lesions. It also causes brown stem lesions that often split open and become lighter in colour with age. There may be small, black, fruiting structures in this stem canker. Gummy stem blight can kill the plant. It is usually seed-borne and spreads by water splashing and strong winds. It can be avoided by using fungicide-treated seeds and infected plants can be treated with a fungicide.
Striped Cucumber Beetle
The striped cucumber beetle (Acalymma vittatum) is rarely a serious pest on zucchini but may occasionally exist in high enough numbers to be damaging. The adult beetle is 1/5 inch long with yellow-green wing covers, three black stripes and a black head. The larvae feed on roots and underground stems but cause minimal damage. Adults emerge late in the summer and feed on foliage. If populations are high enough, they also feed on roots and stems. If plants suffer more than 25 per cent defoliation during the plants' first-to-third true leaf stages, treat them with an insecticide labelled for cucurbits like permethrin, carbaryl or esfenvalerate.