Fusarium wilt is an expanding threat to coastal California lettuce, even with use of resistant varieties, according to findings of Thomas R. Gordon, professor of plant pathology at the University of California, Davis.

Gordon disclosed results of his continuing 2008 studies of the fungal disease during annual reports of the California Leafy Greens Research Board meeting near Coalinga.

Fusarium wilt has been a greater concern in Arizona and in California’s warmer lettuce-growing areas, but it also occurs in Santa Cruz and Monterey counties. New infections have been found around King City.

It destroys the vascular system of susceptible lettuce plants leading to collapse, although it survives in other species without symptoms. The only defense is planting to clean fields and field sanitation to avoid new infections.

As he has done in the past, Gordon advised growers to rotate infected fields out of lettuce. Except in the case of strawberry, where fumigation can significantly reduce the wilt, the degree of reduction depends on how the disease can survive on the rotation crop.

Iceberg cultivars vary in susceptibility to the wilt, although all will show symptoms if disease pressure is high enough. However, certain romaine and leaf cultivars show complete resistance to even very high levels of disease.

With this in mind, Gordon has collaborated with plant breeders at UC Davis to develop crosses between Valmaine, a romaine type, and the common iceberg type, Salinas.

Gordon tested crosses in a June planting at Davis under high amounts of the fungus in the soil, and some lines showed resistance, along with traits approaching acceptable iceberg commercial standards. Additional crosses were made, and the material has been made available to breeders for development of improved resistance.

“Disease resistance will be increasingly important as Fusarium wilt becomes more widespread, and although resistance will be central to disease management, efforts to minimize soil inoculum should not be neglected,” Gordon said.

During 2008, he repeated a previous trial and confirmed the earlier results. Examination of broccoli and cauliflower plants grown on infected plots indicated the same effect on the pathogen as if the soil were fallowed.

“But spinach and resistant romaines had a relatively small impact on the disease although they did allow the pathogen some opportunity to increase.

“On the other hand, if a moderately resistant lettuce variety is grown and disease pressure is low enough and temperatures are moderate, you may not see any disease even though the pathogen population is increasing,” he said.

Specifically, he explained, in the case of the resistant romaine, “King Henry,” the resistance “may allow the pathogen to increase and thereby elevate the risk to subsequent plantings of more susceptible cultivars.”

In comments about his research in lettuce herbicides, Steve Fennimore, UC Cooperative Extension weed specialist at Salinas, noted that development of new herbicide formulations for vegetables are not as attractive for manufacturers as products used on millions of acres of soybeans and field corn. As a result, he predicted herbicide-tolerant varieties will be the answer for weed management.

“Since Roundup Ready varieties were introduced in the late 1990s, the number of new herbicides has been only one or two per year. During the same time the number of manufacturers has reduced to what you can count on one hand,” he said.

His view is that the future will require plant breeders to select lettuce and spinach plants — without using genetic engineering — that will perform with existing, successful herbicides such as Prowl or Lorox that are adaptable to a system.

Particularly for among spinach herbicides, he said, “we have screened and screened and screened, and there’s nothing there. We need to continue some screening. It probably isn’t going to work in the short term, although it possibly will work in the long term.

“Somehow we will have to find mutants or other breeding lines that will tolerate available herbicides. It will take time; it’s a numbers game.” But, pointing to the number of years it took for a 24(c) label for Dual Magnum on spinach, he said, “No matter what we do, it’s going to take time.”

In reporting on his recent trials with full-season drip irrigation on lettuce to avoid the costs of sprinklers for emergence, he said Kerb herbicide was not successfully activated by drip into a layer to protect germinating seeds.

Kerb also had shortcomings in sowthistle, which is of the same composite group as lettuce. Although Prowl controlled sowthistle, it proved to be injurious to lettuce.

Fennimore said he is working with agricultural engineers at UC Davis on a robotic cultivation system for lettuce and is seeking research funding from the industry. The celery and tomato industries are participating in the effort.

In another project supported by the board, Erik Nelson, a post-doctoral researcher at UC Berkeley, reported on his series of trials for biological control of lettuce aphids that attack romaine crops.

His work focused on identifying the best habitat plants to support syrphid flies, a natural enemy of aphids. Insecticide sprays are used in conventional fields for control, but the elusive lettuce aphid resides deep within romaine heads, difficult to reach.

Although organic lettuce growers plant strips of nectar producing, habitat species to attract and feed adult syrphids for pest aphid control, Nelson’s approach is to find plants that attract non-pest aphids to support larval stages of the beneficials early in the crop cycle so populations are in strength when pest aphids appear on the crop.

Barley and alyssum provided the best supply of alternative aphids, and Nelson said it is the one most likely to boost the syrphid populations.

The plots of bell beans, vetch, and barley intended to provide syrphids did not indicate which plants were best because of large variations in pest aphids determined by time and location of plots. Nelson said the highly mobile syrphids were difficult to count. Alyssum was found to be a source of pollen and nectar for the syrphids.

For his 2009 research, which will be funded by USDA, Nelson said he hopes to solve the problem of syrphid counts with larger plots.

“We will be comparing barley alone, alyssum alone, and a combination of the two. If this system works, we think it could be ultimately implemented by growing barley and alyssum in the same habitat strip,” Nelson said.