If your zucchini plants have root rot, their stems and roots will turn brown and mushy, and their leaves will turn yellow. The plant will then wilt and die prematurely, sometimes very quickly. The causes of root rot include excess water, fungi and the larvae of the squash vine borer moth. If you do not know the exact cause, you can still use several techniques to prevent root rot from destroying your plants and cutting short your zucchini harvest.
Make sure your planting area is well-drained. Waterlogged soil, roots and stems will encourage growth of root-rotting fungi. This is especially important if your soil is rich in clay or very rocky, both of which will tend to hold on to more water. To reduce the amount of water collected by the soil, try planting on a slope. This will direct some of the surface water away from your plants.
Many commercial farmers use chemicals to kill some types of plant-harming organisms. To achieve similar results from an all-natural approach, try biofumigation. Biofumigation involves growing a cabbage-family crop (such as mustard or rapeseed) with the sole intention of turning it over into the ground instead of harvesting it. When the crop decays, it releases natural anti-fungal substances into the soil. To experiment, try planting a cover crop of rapeseed in late summer or early fall in the spot where you would like to grow your zucchini plants. Let the crop grow through the winter. About four weeks before planting your zucchini, mow or clip the crop to soil level, then till or dig it into the top few inches of soil. Water the new soil lightly until it is evenly moist but not too wet.
Squash Vine Borer
Squash vine borer moths lay their eggs in young squash vines. When the eggs hatch, the larvae burrow inside the vine where they feed for several weeks, eventually killing the plant. As their name indicates, squash vine borers love squash plants such as zucchini and pumpkin. They leave your plants wilted and severed from its root system, often just as you've begun enjoying your harvest. Once this decay has begun, it is very difficult to reverse.
If squash vine borers are a problem in your area, try successive planting throughout spring to ensure continued harvest. In successive planting, you plant a new crop every two to three weeks throughout spring and early summer. Because it takes several weeks for the larvae to kill a plant, you will usually get some zucchini fruits before each plant dies. If you time your plantings right, you will always have a new zucchini plant ready for harvesting.
Providing extra fertilizer early in the season can also help you beat the vine borer. The extra fertilizer will help your plants grow faster and will help them reach fruit-producing age before the pest has time to kill it.