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CLRB lettuce drop projects advance

Although lettuce drop caused by Sclerotinia species continues to be a tough foe for growers on the Central Coast of California, researchers are gaining new information on how to deal with the fungal disease.

Krishna Subbarao, University of California, Davis plant pathologist stationed at Salinas, reported on his work with lettuce drop during the California Lettuce Research Board’s annual meeting held near Coalinga.

One of his projects for the board was a study of reported resistance of Sclerotinia minor to the fungicide Endura, a mainstay for control for the past several years.

Subbarao told the gathering the reason for recent reported failures of control with Endura is likely its degradation by soil microbes in fields where it was repeatedly used.

He came to that conclusion after his laboratory trials applying recommended rates of the fungicide to 30 samples of the fungus collected from fields where failures occurred and to another 30 samples from fields where Endura had never been used. Even at the lowest rates, he observed no resistance in either group of samples.

Noting that the same field degradation applies to other fungicides, Subbarao said that the materials are expected to last about 60 days. However, in fields where they are repeatedly used, failures begin to appear in about two weeks, potentially because soil microbes consumed fungicides as a food source.

“So, the best solution if you have observed such failures in your fields is to stop using Endura in those fields.

Although this may not be of much help to growers, it will at least save the costs of the fungicide,” he said.

In another series of studies, Subbarao has been collaborating in a novel approach to develop lettuce drop-resistant plants using the “slow-dying” resistance to both S. minor and S. sclerotiorum found in some romaine and iceberg lettuce lines.

The trait allows plants to stave off the disease for 10 to 15 days, while susceptible plants die in as little as two days after infection. It has been known for years, but only recently have breeders taken a closer look at it in an attempt to use it in a breeding program.

Subbarao is supplying disease inoculum used in field screening for USDA lettuce breeder Ryan Hayes’ efforts at Salinas to incorporate the resistance into new breeding lines.

Although the research has found the resistance is linked to early flowering, transferring it to lettuce plants with horticultural-superior backgrounds has proved to be difficult. Even so, a few families of plants are being evaluated in the field.

Subbarao is continuing a project with Contans, a biological fungicide manufactured by the German company, Prophyta, for control of S. minor in the Salinas Valley. Although the material was very effective against S. sclerotiorum in his trials, it was not effective against S. minor, and he plans to investigate using higher rates of the material.

The objective is to find a balance between the amount needed and the costs compared to other fungicides. Contans is known to be most potent against the mycelial stage of the pathogen, and Subbarao explored the best time to apply it.

The manufacturer’s projections, he said, put the cost per application at about $60 per acre, nearly the same as Endura, but he hopes to learn if application costs can be reduced.

Subbarao evaluated Contans with other fungicides for the past two years with funding from the manufacturer and has begun another two years of trials with it under USDA funds.

Subbarao’s research on Verticillium dahliae, the cause of lettuce wilt, was spurred by spread of the disease in the Salinas Valley, plus an outbreak in marigolds, a relative of lettuce, in Lompoc, during 2007.

About 1,200 acres of lettuce, including 58 fields, has been determined to be infested thus far in Watsonville, Salinas, and King City districts. The disease was thought to be limited to coastal California and Crete until last year, when it also was reported in lettuce in Greece and Italy.

Although it is managed by fumigation, the wilt returns after a typical rotation cycle of two crops of lettuce and one of strawberries and requires fumigation every other year.

Subbarao observed that the disease is most prevalent around the edges of an infected field, and he said it could be aggravated by reintroduction of it by contaminated field equipment.

He has been working with seed companies which submitted seed and soil samples for analysis. He noted that the levels of soil infestation were low enough to trigger extensive development of the disease, and that seed infestation could possibly have come from airborne spread of the pathogen. The monitoring of seed lots is continuing.

On USDA’s breeding front for lettuce wilt, Hayes has released material resistant to Race 1 and is evaluating material for resistance to Race 2.

Meanwhile, researchers are mapping fields to learn the relative distribution of the two races as a guide for use of new cultivars when they become available.

Fusarium wilt is another pathogen threatening lettuce fields of California and Arizona. Tom Gordon, UC, Davis plant pathologist, is investigating the disease in a CLRB project.

“Although Fusarium wilt has yet to become a major problem in coastal growing areas, it is becoming more widespread,” Gordon reported. “During the summer of 2007 it was identified in six fields, based on symptoms and recovery of the pathogen from diseased plants.”

Gordon explained that iceberg cultivars differ in susceptibility to Fusarium wilt, but all will die from it if disease pressure is severe enough. On the other hand, some romaine and leaf cultivars appear to be completely resistant, even under the most severe cases.

Geneticists at UC Davis are evaluating inbred lines from a cross between Valmaine and Salinas, both of which have some resistance to the wilt. Gordon said it appears possible to achieve a greater resistance than either has now.

He went on to say that while resistance will be central to management of Fusarium wilt, suppression of soilborne inoculum is also important.

“The most cost-effective means of reducing pathogen populations in soil is to rotate out of lettuce,” unless the rotation crop is strawberries, where fumigation reduces inoculum of all fungi in the soil, he said.

The amount of reduction, he added, depends on the rate of attrition of existing disease and the extent it is able to increase in the rotation crop.

Gordon’s investigation also showed that among rotation crops, except for spinach, most support little development of the pathogen. The disease was recovered from spinach in only a slight fraction of the amount needed to infect Salinas, but that was twice as much as was found on resistant romaine.

“The significance of these findings,” Gordon said, “lies in the potential for the pathogen to sustain its population in the absence of a susceptible crop and thereby to compromise the effectiveness of crop rotation.”

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