MSU Extension

During the 2014 growing season in Michigan, watermelon fields were challenged by Fusarium wilt, a well-known enemy for some growers and a new threat for others. Fusarium wilt is a common disease name in cucurbit crops, but the symptoms are caused by several types of Fusarium that differ based on the host. For example, Fusarium oxysporum f.sp niveum (FON) infects watermelon only. (F.sp is short for “forma specialis,” a term that refers to a species of fungi pathogenic to specific host.) Four races of this pathogen exist and are referred to as 0, 1, 2 and 3. Watermelon varieties may be resistant to one or more Fusarium races (Table 1).

Variety selection is the most important approach to limiting losses to Fusarium. This pathogen produces long-lived spores that can reside in soil. The pathogen spreads through movement of infested soil particles by machinery, soil wash off or flooding, or on the bottom of shoes. Light sandy soil, acidic soil (pH ranging from 5 to 6), dry weather conditions, low to moderate soil moisture, ammonium-based fertilizer, and plant parasitic nematodes favor Fusarium wilt.

The first symptoms of Fusarium wilt occur on older leaves that change color to dull green and yellow. Leaves can become dry, brittle and dark brown and die. Early in the stage of disease, plants wilt early in the day, but recover in the evening. As infection advances, wilting becomes permanent. The vascular tissue often appears brown as it becomes infected and water transportation from the roots to the foliage is reduced, causing plant wilting.

In young seedlings and plants, Fusarium can cause damping off (pre- and post-emergence) and stem collapse. Symptoms on older plants include yellowing, stunting and wilting. Fusarium can cause plant death at any growth stage, but older plants are more tolerant to infection than younger plants. Characteristic symptoms of Fusarium wilt of watermelon include browning of vascular tissue in the crown and tap root. Fruit produced on infected plants may split or become sunburned.

Himmelstein and collaborators recently published research that studied the role of green manures in suppressing Fusarium wilt of watermelon in northeastern United States. They found that fall planted cover crops such as hairy vetch (Vicia villosa) and crimson clover (Trifolium incarnatum), tilled in the spring as green manures, reduced Fusarium wilt of watermelon. Michigan State University Extension encourages growers to try this approach under their field conditions (FON race, soil type, rotation, etc.), which may vary widely and affect cover crop benefits. Using cover crops as green manure has benefits such as adding nitrogen credit to the fields. Keep in mind that killing hairy vetch often requires multiple till passes, mowing or use of a roller crimper during full bloom as it can be good at reseeding, and viable seed can be present in the soil for several years. Additionally, hairy vetch can be host of plant parasitic nematodes, like northern root-knot nematodes, and should be avoided in fields with a history of nematodes.

A. Keinath and R. Hassell recently published results of grafting research in the South Carolina. Watermelon was grafted onto bottle gourd or interspecific hybrid squash rootstocks and planted into fields infested with FON race 2 and a mixture of races 1 and 2. Grafted plants had significantly less Fusarium wilt symptoms and produced more fruit than susceptible controls. However, the authors mentioned the cost of production increases from three to six times the cost of conventional non-grafted transplants.

To manage Fusarium wilt of watermelon, a combination of cultural tactics is required. To help decrease the amount of Fusarium spores in the soil, use a long rotation (five to seven years) away from watermelon. Adjust soil pH in acidic soil by adding lime to keep soil pH at 6.5 to 7.5. Select nitrate (NO₃)-based fertilizer instead of those with ammonium (NH₄). Fall planting of cover crops such as hairy vetch and clover combined with green manure tillage in the spring may help suppress the pathogen. Take advantage of Fusarium resistant watermelon varieties; more than 51 different varieties are available (Table 1).

It is helpful to know the Fusarium race that is prevalent in a problem field. If race testing is not available, consider a field testing approach by planting ‘Sugar baby,’ Charleston Gray,’ ‘Calhoun Gray’ and ‘PI296341-FR.’ These varieties have different responses to Fusarium races and can help identify the races in your field.

Table 1. Watermelon varieties listed as resistant to Fusarium wilt

(wt)= western type, (et)=eastern type
Adapted from Cornell University Vegetable MD online

For more information and pictures:

• Fusarium wilt of watermelon and other cucurbits
• A Diagnostic Guide for Fusarium Wilt of Watermelon