Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu and Karen Rane, Plant Diagnostic Clinic Director, University of Maryland; rane@umd.edu
Traveling around the Maryland and Delmarva area it is obvious that this is a particularly good year (bad for growers) for wilts in watermelon and pumpkin fields. The previous two seasons had been relatively dry, with last year being a drought. This year has seen moderate to heavy rains on a weekly basis. Overall this is good for crops, but in some fields for some cucurbits it has led to an increase in root and crown rot problems. 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 these 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. The weather conditions we had back in June when we had temperatures over 95° F for several days followed by heavy rains (with water sitting in fields) probably stressed young plants and allowed root rot diseases to get started.
Figure 1a. Watermelon rows with and without Fusarium crown rot
Figure 1b. Watermelon plant with crown rot
In pumpkin fields, the wilting symptoms can look a bit different than in watermelon. Leaves usually turn yellow and then necrotic at the margins (Fig 2a) or may simply wilt down with little foliar discoloration (Fig 2b). Both Fusarium and Phytophthora have been recovered from these plants.
Wilting and foliar symptoms in cucurbits also can be caused by squash bug feeding or squash vine borer (SQVB) damage. Close examination of the plant will show whether it is insect damage or disease. If squash bugs are responsible, you will see many of them on the plant and actually smell the insects as they release a bitter scent. If it is SQVB you will often see leaves with yellow and necrotic margins, as in Figure 2a, and an enlarged cracked stem at the base of the plant (Fig. 3a). If this stem is split the area within the stem is usually entirely brown and a fat SQVB larva can be found (Fig. 3b). With Fusarium crown rot or Phytophthora blight the crown of the plant near the soil line will turn brown (Fusarium) or become soft and water-soaked (Phytophthora). Both diseases will allow the stem to detach easily from the roots, which may or may not be decayed.
Figure 2a. Pumpkin with yellow and necrotic leaf margins
Figure 2b. Pumpkin plant wilting
Figure 3a. Swollen cracked pumpkin stem due to SQVB
Figure 3b. Same stem now split showing SQVB and brown area
Wilting due to root diseases and/or insect damage can look similar to symptoms caused by Fusarium wilt. Fusarium wilt is caused by a different fungal species than Fusarium root and crown rot. Discoloration of the vascular system within infected stems is a diagnostic symptom for Fusarium wilt. Plants with Fusarium wilt usually have no external discoloration of the crown, and the roots usually appear healthy.
There are a few things that can be done to alleviate some of these wilt problems during the season. I talked about chemical control for squash bugs in the article titled Squash Bugs in Pumpkins in WCU 16:14. 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 may help (next year), 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. Kate Everts talked about these and other controls for Phytophthora blight in cucurbits the article titled Phytophthora Blight Management in Cucumber in WCU 16:17. For squash vine borer little can be done this year, but if the grower can rotate next year’s field >¼ mile away from this year’s field it will greatly reduce SQVB problems. If rotation is not possible then sprays (Warrior or bifenthrin) directed at the base of the plant 2-3 weeks after plants are up and repeated for the next 3 weeks will reduce SQVB damage. Pheromone traps that trap adult males can help determine when moths are flying and when insecticide sprays should start and how long they should continue.