Fusarium wilt of watermelon is the most widespread and destructive disease of watermelon in the southeastern United States and other areas of the world. The causal fungus is named Fusarium oxysporum forma specialis (“special form”) niveum, which is commonly abbreviated as FON. FON persists indefinitely in soil in the form of resting spores that survive heat, cold, and drought.
Fusarium wilt reduces the number of fruit and the weight of individual fruit. When Fusarium wilt is severe, many fruit may not reach marketable size (ten pounds for seedless, fifteen pounds for seeded watermelon). Death of leaves and vines exposes fruit to the sun and may increase the number of sunburned fruit (figure 1).
Over the years, FON has been able to adapt to resistant varieties by developing races, which are new genetic variants that can infect previously resistant varieties. Currently there are four FON races: race 0, the original race, which can attack heirloom varieties of watermelon; race 1, which developed after the first resistant varieties, like Charleston Gray, were released; race 2, the most widespread race, which causes the current resistant varieties to wilt; and race 3, a new race found in Maryland, Florida, and Georgia. Race 2 is present in most watermelon fields in South Carolina where growers have observed symptoms of Fusarium wilt. Race 3 has not yet been found in South Carolina.
Symptoms appear starting three weeks after transplanting. In the early stage, the oldest leaves wilt one after the other, starting at the crown, or all at once on a diseased vine (figure 2). Wilting of random leaves on the same plant is likely due to leaf damage from wind or other causes, not Fusarium wilt. In the middle stage, one or more vines per plant die and turn brown, while the rest of the plant appears healthy. In the final stage, severely diseased plants are stunted or are killed.
To diagnose Fusarium wilt in the field, cut the main stem or a main vine near the crown and look for reddish brown discoloration in the vascular bundles around the edge of the stem. Discoloration often appears on only one side of the stem (figure 3). If the discoloration is not clear, make a 2-inch cut lengthwise up the stem to expose more vascular tissue. Vascular tissue more than twelve inches away from the crown may not be discolored, even in diseased plants.
Five practices to manage Fusarium wilt include growing resistant varieties, delaying transplanting until soil has warmed, grafting, applying fungicides, and cover cropping. Combining as many practices as possible provides the best results.
Most seeded watermelon varieties and newer varieties of seedless watermelon are resistant to FON races 0 and 1 but susceptible to FON races 2 and 3. Although resistance to race 2 is being crossed into commercial varieties, as of 2019, no varieties have resistance to races 2 or 3. Lists of resistant varieties are available in the Southeastern Vegetable Crop Handbook.1 Regardless of which race is present in a field, varieties resistant to race 1 are recommended, as they also reduce the number of plants showing symptoms caused by race 2.
Delaying transplanting until the soil has warmed to 81°F at 4-inch depth will reduce the number of plants with Fusarium wilt by 50%-80%. In South Carolina and Georgia, soil in plastic-mulched raised beds has warmed enough to reduce Fusarium wilt by April 15 to 20. Delayed transplanting does not affect the sizes of seedless watermelon fruit or the net return per acre. Growers can use delayed transplanting by transplanting severely infested fields last. Growers should make sure, however, that they can maintain their desired market window.
Grafting is a horticultural technique in which a crop plant, known as the “scion,” is joined to the roots of a related plant, the “rootstock.” Watermelons may be grafted onto bottle gourd or interspecific hybrid squash as rootstocks. Both rootstocks are very resistant to FON and prevent infection of the watermelon scion.
Several disadvantages of grafting should be considered before adopting this technique on a large scale. Bottle gourd and interspecific hybrid squash rootstocks are very susceptible to root knot nematode, so nematicides must be used when these rootstocks are planted in infested fields. A different rootstock, ‘Carolina Strongback’ citron, is resistant to both Fusarium wilt and root knot nematode. This rootstock should be used in fields infested with both pathogens.
Grafted transplants cost approximately four times as much as non-grafted transplants. In severely infested fields, grafted plants yield about twice as much as non-grafted plants, so the increased cost can be partially recovered. Grafting is a recommended practice for organic watermelon production. Grafted watermelon transplants are available commercially.
As of 2019, Proline (prothioconazole, Bayer) is the only fungicide registered to reduce Fusarium wilt on watermelon. It is not recommended in South Carolina because it has not been effective in most field trials in naturally infested soil.
Winter Cover Crops
Hairy vetch (Vicia villosa) or hybrid common vetch ‘Cahaba White’ (Vicia sativa × V. cordata) grown as winter cover crops may reduce Fusarium wilt in spring crops of watermelon. Vetch is thought to stimulate beneficial microorganisms in soil that promote watermelon growth, even when plants are infected by FON. The average net return was $800/acre greater with vetch compared with rye as a cover crop, based on 2008 prices. Vetch cover crops must be killed with herbicide or rolled and disked several weeks before plastic mulch is laid.
The five practices to manage Fusarium wilt differ in efficacy, labor, and cost (table 1). Growers should calculate the costs and benefits of each practice for their business. Contact your local Extension office for assistance.
Table 1. Comparison of five management practices to reduce the effects of Fusarium wilt on watermelon
|Management practice||Relative efficacy||Combine with other techniques||Extra labor required||Relative cost|
*Nematicides are necessary when root-knot-susceptible rootstocks are used.
Kemble JM, senior editor. Meadows IM, Jennings KM, Walgenbach JF, editors. Vegetable crop handbook for the southeastern US. Willoughby (OH): MeisterMedia Worldwide. 2019. Growing Produce. https://www.growingproduce.com/southeasternvegetablecrophandbook/.
Keinath AP, Coolong TW, Lanier JD, Ji P. Managing Fusarium wilt of watermelon with delayed transplanting and cultivar resistance. Plant Dis. 2019; 103(1): 44-50. https://doi.org/10.1094/PDIS-04-18-0709-RE.
Keinath AP, Hassell RL, Everts KL, Zhou X-G. Cover crops of hybrid common vetch reduce Fusarium wilt of seedless watermelon in the eastern United States. Plant Health Progress. 2010; 11(1): 8. https://doi.org/10.1094/PHP-2010-0914-01-RS.
Keinath AP, Wechter WP, Rutter WB, Agudelo PA. Cucurbit rootstocks resistant to Fusarium oxysporum f. sp. niveum remain resistant when co-infected by Meloidogyne incognita in the field. Plant Dis. 2019; 103(6): 1383-90. https://doi.org/10.1094/PDIS-10-18-1869-RE.