US (MI): The case of watermelon wilt

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 favour Fusarium wilt.

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.

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For more information, visit www.msue.msu.edu.