No controls, but new reports out on lettuce disease trio

Although controls remain distant, studies by University of California and USDA scientists have accumulated more facts about lettuce drop, verticillium wilt, and phoma basal rot, three diseases of continuing concern to the lettuce industry.

Mid-year progress reports on those and other projects funded by the California Lettuce Research Board were made at a recent meeting of the board at Seaside.

Krishna Subbarao, UC plant pathologist at Salinas and leader of a team of experts working on lettuce drop, said they did not observe significant control from the biocontrol agents, Plant Shield, Companion, and Contans in 2004 trials.

“All of the treatments,” Subbarao said, “were imposed on corresponding plots established last year. Unlike in previous years, there was little drop caused by S. sclerotiorum in all treatments. Hence, treatment efficacies could not be ascertained this year.”

Earlier research showed that Rovral does not control the disease, while Botran reduced the disease in the spring but not the fall.

The investigation of biocontrol materials stemmed from reports they controlled the Sclerotinia sclerotiorum pathogen elsewhere, even though earlier studies with such products failed to show control of infections of S. minor, the species most common in the Salinas area.

Subbarao said the recent studies also showed that two applications of Rovral and Botran, once at thinning and again two weeks later, by harvesttime degraded to less than 20 percent of their original concentration. This, he added, confirmed the reasons for the lack of efficacy of the two fungicides.

Experimentals success

Some prospects emerged however, from trials with two experimental fungicides, Endura and Switch, which retained 45 percent to 80 percent of their concentrations in the soil by harvest and provided good control of S. minor.

Subbarao said several new fields with heavy losses to verticillium wilt in Salinas and Watsonville were identified during the 2004 season. One difference from previous years was a higher degree of infections during spring and early summer, which he said may be due to warmer temperatures during the spring.

The V. dahliae pathogen again appeared in lettuce following strawberries in fields that had been fumigated prior to the strawberry plantings. This has researchers puzzled because their test plots thus far do not show that strawberries contribute significant amounts of infection to the soil.

Subbarao also said the seed borne nature of verticillium wilt in lettuce and the role of certain weed seeds in spreading the pathogen are well-established. He is continuing fumigation of test plots.

Subbarao is also studying how the onset of the wilt might be related to plant development. After evaluating both early and late maturing cultivars in greenhouse and field trials, he said symptoms are dependent on plant development.

Disease spreads

Phoma basal rot, a disease that was first discovered in the Watsonville area on romaine lettuce in 2000, has since spread to both romaine and iceberg fields in other coastal lettuce producing areas.

Steve Koike, Monterey County farm advisor, said his group of specialists studying the disease has identified Phoma exigua as the likely cause, although the source of it remains unknown.

Since outbreaks of the rot occurred in fields of lettuce following strawberries, soil samples from strawberry transplants were tested, but no conclusions were reached. Koike is continuing tests with soil and roots from strawberry fields where Phoma infections were found on lettuce.

Other lab trials, he said, “showed that romaine isolates were able to consistently cause disease on romaine and crisp head cultivars, and crisp head isolates consistently caused disease on crisp head and romaine cultivars.”

Koike's trials showed the fungicides Quadris, Endura, and Switch were very effective in preventing the rot from developing, although timing of applications may be critical. The three materials did not perform well in a plot treated with them eight days later than the first treated plot.

Meanwhile, lettuce breeders at the USDA station in Salinas are working on resistance to the three diseases. In the case of lettuce drop, they have made progress with new crosses with heirloom cultivars, and additional work is under way to confirm resistance and select for desired horticultural traits.

In trials on verticillium wilt, they have been working with the cultivar La Brillante and wild lettuce germplasm. In tests for susceptibility to the disease among more than 100 lettuce cultivars of all types, they have found that Latin and butter cultivars are the most resistant, while green and red leaf cultivars are the most susceptible.

Several sources of resistance to Phoma basal rot have been identified and crosses have begun with them and modern romaine germplasm.

Continue field study

The breeders are continuing field studies, although the rot was not evident in 2004 in plots that previously were infected. They suspect this might be due to weather that did not support the disease's development. They are seeking additional sites for continuing tests for screening cultivars and breeding lines.

Among other continuing CLRB projects is that of a group of scientists led by Carolee Bull, USDA plant pathologist at Salinas, pursuing ways to manage corky root, another economic disease of lettuce.

Her three-phase plan began with a grower survey that revealed the greatest interest in resistance for romaine types. Interest was shown in developing both chemical and biological controls.

The second phase is finding ways to gauge infection of the disease. Trials in fields where it has occurred showed variability, and Bull wants to find a method of inoculating plots for uniform disease expression. No significant differences were found in four methods used in 2004, and trials are continuing.

In the third objective, a search for potential biological controls, she said the team has identified several bacterial predators which are being evaluated further.

Tom Gordon, plant pathologist at UC, Davis, told the board “the most effective means of managing fusarium wilt diseases, is through genetic resistance in the host.” The wilt appeared near Huron in 1990 and since has spread elsewhere in the San Joaquin Valley, as well as coastal counties and Arizona.

Gordon's research confirmed that the variety Salinas is significantly less susceptible to the warm-season disease than other cultivars. Fifteen others, including five romaine, three leaf, and seven iceberg cultivars, are in that group.

On another front, Gordon said growers are likely to rotate lettuce out of fields having the disease to allow it to decline before lettuce is again planted. He is testing potential rotational crops, including tomato, melon, pepper, onion, and cotton, to learn how they differ in the amount of fusarium wilt that colonizes on them.

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