Michael E. Matheron, Extension plant pathologist at the University of Arizona’s Yuma Agricultural Center, says a series of three plantings on infected soil will reveal how 41 head lettuce varieties, plus lesser numbers of other types, stand up to the fungal disease.

In the first completed trial, the disease claimed 95 percent of the plants of the head lettuce varieties. Losses were 75 percent for two butterhead types, more than 60 percent for three greenleaf types, 50 percent for four redleaf types, but only 18 percent among 15 romaine cultivars.

"Romaine is the bright spot, and some of the romaine cultivars did better than the average," Matheron said.

Aimed to show susceptibilities at different times of the season, the trials have broad support from a coalition of grower groups, seed companies, and governmental agencies.

Variety rankings

Matheron said susceptibility rankings by variety will be announced in March and April of 2003 and seed company breeders can then proceed to develop resistance.

In an account of the first trial during the recent Desert Crops Workshop in Yuma, Matheron said the disease was discovered in the Yuma area in six fields in 2001 and in another five in 2002.

"The important thing for growers in these sites is to avoid transporting any soil from a diseased field to another where the disease has not yet occurred," he said.

"Once this fungus gets into a field, you can’t get rid of it. The only thing you can do is manage it, and that’s why we want to minimize the spread of it."

He was one of 20 speakers at the event sponsored by Western Farm Press, University of Arizona Cooperative Extension, and University of California Cooperative Extension.

The subspecies Fusarium oxysporum f. sp. Lactucae that attacks lettuce can comprise 40-70 percent of the Fusarium population in a soil, depending on the location.

A very active saprophyte that produces spores that over-season from one crop to the next, it can live either as a pathogenic or a nonpathogenic form on other host plants, and that’s why it is so difficult to destroy, he explained.

Darkened tissue

The distinguishing feature of the disease is darkened tissue seen when an infected lettuce plant is split down the middle. Infected plants show the wilting symptoms soon after thinning, and the disease continues to develop as the crop matures until the infected plants die.

As with Fusarium diseases in general, management of this pathogen depends solely on genetic resistance, Matheron said. "So with this in mind, we set out to evaluate all the lettuce cultivars grown in the Yuma area to see if there are any differences in susceptibility."

Fusarium wilt was first reported in lettuce in Japan in 1960. When it appeared 30 years later at Huron, Calif., it was not regarded as a serious threat, but when it showed up in both Huron and Yuma in 2000, plant pathologists began taking a closer look.

Joining Matheron in observing the trials are Barry Tickes, Yuma County Extension agent, and James McCreight, USDA geneticist, Salinas, Calif.

Workshop participants also heard from John Palumbo, University of Arizona entomology research scientist at Yuma, who found that overhead sprinklers, used two or three times per week, without pesticides, can reduce western flowers thrips populations up to 50 percent.

The sprinklers act to dislodge thrips, repel adults, drown immature forms, and suffocate pupae in the soil.

Season-long use

He concluded that season-long use of sprinklers on romaine can benefit suppression of the major lettuce pest in the area, particularly in organic systems where effective insecticide alternatives are not available.

Palumbo also said insecticide sprays consistently provided more than 80 percent suppression of thrips, along with higher yields and quality.

Another speaker, Tom Turini, plant pathology farm advisor, at Imperial, Calif., said he learned in his research in 2001 and 2002 that a third race of powdery mildew appears be present in muskmelons in the Imperial Valley. That word came after laboratory testing of samples.

"The significance of this is although we have cultivars that resist races I and II, we probably don’t have any that resist races III, IV, and V," he said.

Turini said also melon vine decline appeared in his Imperial Valley trials in 2002. "Vines started to collapse about three weeks before commercial harvest. We dug up the roots and found the black reproductive forms that are diagnostic of Monosporascus cannonballus."

In general, he added, honeydews and mixed melons had lower vine decline infection than cantaloupes. The variety with the least vine decline severity was Syngenta Seed’s Esteem.