Numerous pathogens of Fusarium wilt can affect many vegetables. But for many commercial growers, the fungi that inflict damage on tomatoes and watermelons are most troublesome. Both fungi can persist in the soil for approximately 10 years.

The Fusarium wilt fungus, Fusarium oxysporum f. sp, lycopersici, invades tomato plants through the rootlets. The pathogen progresses up the xylem, clogging the vascular tubes. The first signs are yellowing and droopy lower leaves, often on one side of the plant. The plant can wilt during the day, but recover at night. As the disease continues upward, more leaves yellow, wilt and die. The symptoms can appear before the plant reaches maturity, but often occur during fruit sizing. As the disease spreads throughout the vascular system, the entire plant can die. When cut, brown discoloration extending throughout a stem is visible.

Soil and air temperatures around 82 degrees Fahrenheit are ideal for Fusarium wilt to develop in tomatoes. Soils above 90 degrees and below 60 to 68 degrees retard wilt development. Acids and sandy soils favor Fusarium wilt. High potassium and soil moisture enhance the pathogen’s virulence.

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is the most economically important disease affecting watermelons. Loss in marketable yield resulting from misshapen, undersized, sunburned or inferior quality fruit coupled with the expense of soil treatment or new land preparation obviously impacts growers.

Dull, grayish-green leaves that soon yellow and wilt, and loss of turgidity in the vines precede plant necrosis. As with tomatoes, the younger foliage becomes affected first. Also similar to the one-sided pattern in tomatoes, only one vine runner may show symptoms. A brown necrotic streak may be visible externally in watermelons. In severely infested soils, seedlings may damp off as they emerge. The heaviest pathogen infestations often appear in light, sandy, slightly acidic soils and during temperatures of 77 to 81 degrees Fahrenheit. But, as the temperature increases later in the season, more severe symptoms may be displayed.

Confirming a Fusarium diagnosis

Because Fusarium symptoms often resemble Verticillium wilt, a laboratory culture may be necessary for positive identification in both tomatoes and watermelons.

As noted, Fusarium oxysporum is soilborne. Once in the soil, unless fumigated by chemicals or soil solarization, controlling its spread becomes important. Tillage practices, flooding, windborne and waterborne infested soil, contaminated farm equipment, workers’ footwear, stakes and other field implements can spread the pathogen. Infected seed and transplants can disseminate the disease widely. Cover crops, particularly those that suppress root knot nematodes, may mitigate the damage. Regardless, avoidance of heavily-infested nematode fields is advisable because nematode feeding can overcome plant resistance to Fusarium wilt.

Employing effective strategy

A combination of strategies typically produces the best controls. Crop rotation and resistant varieties top the list.

Both of the Fusarium oxysporum pathogens cited are host-specific. Nevertheless, rotation recommendations for tomato still advise avoidance of the solanceous crops of eggplant, pepper and potato. Likewise, watermelon rotation advice generally excludes the cucurbits, such as cucumber, squash, muskmelon and pumpkin. Four to six years is typically recommended to reduce inoculum levels.

Although new races continue to surface in these fungi, vegetable breeders continue to develop resistant new cultivars and to improve older varieties by instilling resistance.

Modern tomatoes resistant to race 1 and race 2 of Fusarium wilt are widespread; some have resistance to race 3 as well. Professor Mary Hausbeck, Michigan State University, notes, “Resistance has held up for our commercial growers.” However, she added that for the heirloom tomatoes that lack Fusarium resistance, “We may see a bit of resurgence.”

Crop advisor Gene Miyao, University of California Cooperative Extension, Solano and Yolo Counties, recalls how decades ago Fusarium wilt race 2 killed young plants and surfaced in the late growth just before ripening prior to the resistant varieties. Race 3 started in the Sacramento Valley, but now numerous processing varieties have resistance plus improved quality. “On the commercial level, genetic resistance has been holding,” Miyao said. “High temperatures in the Central Valley summers can break down the resistance,” he adds, “but this is not widespread except in fields with the root knot nematode.” Miyao concludes, “For the most part, resistant varieties and crop rotation work well.”

Tom Pagels, salesman for Stokes Seeds, said that he’s unaware of problems among the resistant varieties in the East. Mark Willis, vegetable seed manager for Harris Seeds, points out that numerous tomato varieties have resistance to races 1 and 2; many also resist race 3. Willis reports that the development of race 3 appears to be currently limited to the southwest and the south.

Seed catalogs denote a variety of resistances. With respect to watermelon, Willis notes that some cultivars resist Fusarium wilt race 3. Pagels advises growers to consult with their seed salesman, as some of the breeders’ varieties show stronger resistance than others.

In watermelon, Fusarium wilt race 1 is predominant, although race 2 is highly aggressive. Race 3 developed in the eastern United States. The Agriculture Research Services’ Watermelon Research and Development Working Group in Charlestown, South Carolina, has been given research priority by USDA. Research Scientist Shaker Kousik reports that the group has developed many lines. Working with wild watermelons, their resistance to high loads of Fusarium wilt is being introduced into the genes of modern watermelon cultivars by Kousik and ARS research plant pathologist William Wechter. Various seed companies have accessed the lines, but new varieties are not yet commercially available.

Consider use of grafting

Dr. Kousik noted the work on grafting by professor of vegetable crops, Richard Hassell, Clemson University. A desirable watermelon grafted onto a Fusarium wilt-resistant rootstock performs well. Bottle gourd and squash rootstock outperform the rootstock of another watermelon. Although expensive at $1.25 per seedling, small niche growers for specialty markets who demand quality yields may find the grafted seedlings advantageous.

Grafting has also been successfully accomplished with tomatoes. Miyao reports that on a commercial basis of processing tomatoes, to break even the growers would need to produce over 100 tons an acre. Again, a specialty heirloom grower who commands higher prices might profit.

Because resistant varieties, cover crops, grafting sources and other factors differ among regions, growers should consult their local extension educators for recommendations.

Researchers continually discover and develop tools to combat diseases. Scientists at the Agricultural Research Services (ARS), USDA, experimented several years ago with nonpathogenic strains of Fusarium to fight the harmful strains. Plant pathologists Deborah Fravel and Robert Larkin exposed roots to benign saprophytic Fusarium strains. In greenhouse studies, the helpful fungi reduced Fusarium wilt by up to 80 percent in both tomato and watermelon seedlings. Larkin observed that the microbes of the benign Fusarium fungi colonize root systems better than their pathogenic brethren. He explained, “They live on and in the vicinity of the roots. There they crowd out the competing pathogens for sugars, amino acids, and other nutrients both need in order to flourish.” Although this phenomenon has not been duplicated on a commercial level, Larkin sees promise in the future to discourage disease, particularly with biocontrols.

COVER PHOTO BY KEITH WELLER, COURTESY ARS, USDA.