Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Last week Gordon Johnson wrote an excellent article, “Tough Year for Early Peas,” about problems with peas due to the wet soil conditions we have had in our area resulting in “soil rots”. We have had anywhere from 2-5 inches above average rainfall for the month of May throughout the Delmarva area, which has resulted in the second wettest May over the last 50 years for many areas. Last year we also had a very wet May; it was the third wettest May in many areas of Maryland, which resulted in many more root rot problems in cucurbits, i.e., watermelons, cucumbers, cantaloupes, pumpkins, etc. We can probably count on similar problems this year in the field as the June forecast calls for wetter than normal conditions.
The symptoms in watermelon fields usually begin with leaves flagging on a few plants down a row and then a few days later a total collapse of those same plants. Sometimes the wilting occurs within certain rows while in adjacent rows the watermelon plants look fine (Fig 1a). If wilted plants are dug up you can see reddish-brown discoloration of the crown of the plant (Fig 1b). The roots will be decayed as well. There are several fungi that can cause crown and root rot diseases, including Fusarium, Pythium and Phytophthora. To identify the specific fungi involved, samples should be sent to a diagnostic laboratory for testing. The disease often starts with a few plants in one row then moves down that row. Water, either through irrigation or heavy rains is usually responsible for the movement of the disease down a row. High plant loss can occur in the lower areas of fields where water stands after heavy rains. In the last month we have had several days of heavy rains with water sitting in fields, which will stress young plants and allow root rot diseases to get started.
Fig 1a. Watermelon rows with and without Fusarium crown rot
Fig 1b. Watermelon plant with crown rot
One thing growers can do for root and crown rot diseases is to be sure to not over water the plants or apply excess nitrogen. Rotation helps somewhat, especially for Fusarium wilt, but the root and crown rot pathogens can infect many hosts, making crop rotation less effective in reducing disease. Environmental conditions are probably the most important component for the development of root and crown rot diseases. Well drained fields will have less of a problem than poorly drained fields.
Besides seed treatments containing fungicides which will protect the seed from rots, there is a biological control that can be seed applied (preferred application method) or drenched onto the transplant that will help protect the plant from soil rots. The product is T-22 a naturally occurring fungus, Trichoderma harzianum, strain T-22. Trichoderma grows on the surface of roots, where it provides disease control and enhances root growth. Its spores survive in the soil, but the food it exists on is secreted from the root surface. The fungus multiplies on its own, protecting the roots over the growing season. The fungus, however, does not work very well if fields have standing water in them over a period of days, so it is important to keep your fields well drained.