Processing tomato growers fight soil-borne disease

Processing tomato growers face the difficult job of trying to stay ahead of a long-lasting, soil-borne disease that can devastate entire fields of susceptible varieties.

While there are finally enough plants resistant to Fusarium wilt race 3 pathogen to meet demand, some growers are still asked to plant susceptible varieties because they produce fruit with qualities desired by their cannery. Other growers contend with ground infested with a related disease, Fusarium falciforme, for which there are no truly resistant varieties.

"The supply of race 3-resistant varieties is now fairly good," said Brenna Aegerter, University of California Cooperative Extension farm advisor in San Joaquin County. "If you look at the top 10 varieties in terms of loads from 2020, three of them were resistant to race 3. If you look at the number of loads of the top 50 varieties, one-third of those loads were of race 3-resistant varieties."

Fusarium wilt infects susceptible tomatoes through the roots and plugs the water-conducting tissue, causing the plants to wilt and die or survive to produce a subpar crop.

Aegerter continues to study strategies for managing this disease in conjunction with Cooperative Extension plant pathology specialist Cassandra Swett at UC Davis.

The challenge for growers is no longer finding plants that can produce in ground with the latest race of Fusarium wilt, as plant breeders have caught up with the demand.

"The host resistance is quite good," Aegerter said. "You can still see an occasional affected plant, but not enough to justify chemical treatment or avoiding the field."

The problem now is that some growers are asked to grow susceptible varieties and cannot find enough ground that is free of wilt race 3.

"A cannery may still request a particular nonresistant variety for its desired fruit chemistry characteristics, which may not be race 3-resistant, so we will continue to see problem fields planted with nonresistant cultivars," Aegerter said.

While the normal strategy would be to plant these nonresistant varieties in disease-free ground, the increase in tree and vine planting in the Central Valley has made that ground harder to find.

"Rotation out of tomato is certainly done in some cases," Aegerter said. "However, with the increase in permanent crops, there is limited open ground for growing tomatoes, so some growers do not have the option of avoiding infested fields while still meeting their desired acreage of cannery contracts."

When there is enough ground to rotate out of tomatoes, that strategy may still be ineffective because the disease can survive in the soil for years without host plants and infect the next susceptible variety.

"Even after multiple years of rotation out of tomatoes, we still see Fusarium wilt when tomatoes are replanted—so crop rotation is not as effective as we might hope for this particular disease; the fungus is quite long-lived in the soil, or is reproducing on other crops or on weeds," Aegerter said.

There may be one more tool, however, for reducing disease damage in infested ground plants with susceptible varieties. Trials show that chemical controls can reduce crop loss.

For the past four years, Aegerter, in cooperation with Swett's lab, has studied applying the fumigant K-Pam through the buried drip line at least three weeks before planting or fungicides at planting and found they give at least enough protection to more than pay for themselves.

"If we use an average increase of 8 tons in yield, and assume a commodity price of $80 per ton, then the economic advantage of the fumigation is $640," Aegerter said. "As the costs of fumigation are around $150 to $400 per acre, we can conclude that on average, in fields with soilborne disease pressure, the costs of drip fumigation are likely to be repaid in increased yield."

Fumigants and fungicides are not normal management strategies, but a last resort where resistant varieties or long-term rotation out of tomatoes are not viable options.

"Chemical control should not be considered the first line of defense against these diseases, but rather the focus should be on resistant or tolerant tomato varieties and avoiding infested fields when feasible," Aegerter said. "However, in situations where resistant or tolerant varieties are not an option, or when the disease tolerance is not sufficiently high, chemical control might be an option to consider."

Some promising news is that the fumigant and fungicides appeared to also provide at least some relief from Fusarium falciforme, a relatively recently identified disease for which there are no fully resistant tomato varieties.

This disease, which has been identified in at least 100 commercial tomato fields, causes vine decline from 45 to 60 days after planting and severely reduces yield.

In a San Joaquin County trial, chemicals run through the buried drip line provided some protection against yield loss due to Fusarium falciforme.

"We have observed some efficacy of K-Pam and fungicides to reduce incidence of Fusarium falciforme and, yes, this would be the first effective tool for this disease," Aegerter said.

Swett has spent years studying Fusarium falciforme and screening processing tomato varieties for their ability to produce a crop in ground infested with this disease.

"Some growers are using more tolerant cultivars with reported success," Swett said.

These plants are not resistant to Fusarium falciforme—there are no resistant varieties—but show the ability to delay symptoms long enough for the plants to set a crop.

(Bob Johnson is a reporter in Sacramento. He may be contacted at bjohn11135@gmail.com.)