Root rot is caused by several soil-borne fungi including Rhizoctonia, Pythium and Fusarium, among others. Nematodes are microscopic roundworms that feed on plant roots causing symptoms of plant decline similar to fungal root rot. Both fungi and nematodes thrive on moist, warm soil.
Both fungal root rot and nematodes feeding on roots cause the plants to grow poorly. Leaves turn yellow or brown and sometimes drop. Stems die. Plants fail to respond to fertilizer because of damaged roots. The plants decline slowly and die--usually over the course of several years, but sometimes the plants die suddenly.
The roots of healthy plants should have a yellowish-white interior; the lateral roots should be white with white cores.
Roots suffering from rot are dark brown to black; the discoloration starts at root tips and moves up to larger roots.The roots often look water-soaked and are squishy.The surface material of the roots may slip off easily.
Horticulturists at the University of California, Davis, say a fungus is almost always the cause of root rot. Nematodes in the roots of seedlings at planting make the plants susceptible to fungal root rot. When nematodes feed on roots, fungal rot infections are more severe.
Keep soil well drained. Do not over-water.
Make sure your soil is free of fungi before you plant.To test your soil put a sample in a 1-quart plastic bag and take it to your county extension agent.
Remove plants showing severe symptoms. Horticulturalists at Virginia Tech recommend applying fungicides to infected soil at one-month intervals. Drench the soil around remaining plants with fungicides containing mefenoxam, metalaxyl, fosetyl-Al or etridiazole plus thiophane methyl.
Nematodes often cause galls or swellings on the roots; these lesions can encircle the roots and kill them. Roots infected by nematodes can branch abnormally or be stubby in appearance. Nematodes usually infect feeder roots, and a massive invasion can kill them.
The most common nematode, root knot nematode, is commonly found in warm, moist sandy soils and environments that are congenial to fungal root rots.
There are no effective insecticides registered to control nematodes. Doses of pesticides large enough to kill nematodes around the roots usually kill the roots.
Some plants are more susceptible to nematodes than others. Plant nematode-resistant varieties whenever possible. Do not plant susceptible plants in places where nematodes are known to occur.
To have roots tested for nematodes contact your local agricultural extension service. Take a root sample in the late spring, summer or early fall when soils are moist. Take several samples of actively growing host roots and seal them in plastic bags. Put samples of the soil in separate bags. Label all bags so they can be identified.