If a root rot is suspected, Pataky recommends removing the plant from the ground carefully, placing it in a bucket of water to gently dislodge the soil. Then examine the roots for indications of rotting. If roots are washed too vigorously, all of the rotted tissue will be washed off, often leaving a white root interior that appears healthy, but is actually much thinner than healthy white roots. A healthy plant has numerous white fibrous roots. Roots of a diseased plant show various degrees of mushiness and usually are some shade of brown or black, both externally and internally.
Healthy roots feel firm.
The Plant Clinic can help to determine which fungus, but symptoms can also help suggest the fungus involved. Pataky recommends preventing rots from becoming a problem in our gardens by using sound horticultural practices. This includes use of healthy transplants, proper site preparation to provide good water drainage away from roots, use of balanced fertilizer, and rotation in the garden plantings for two or three years with unrelated plants to help prevent the buildup of pathogens in one area. It is also important to remove crop residue at the end of the season to help reduce pathogen survival. Once a pathogen is identified, try to find and use resistant varieties when available.