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White rot research in alliums

The California garlic and onion industry remains under threat from white rot, a devastating fungal pathogen of allium crops around the world, as researchers attempt to develop plant resistance and to refine uses of available chemical tools and cultural practices.

Progress in the efforts was revealed at the symposium of the California Garlic and Onion Research Advisory Board (CGORAB) held in Tulare.

Colin Eady, plant geneticist at the New Zealand Crop and Food Research Institute in Christchurch, New Zealand, told of his project during 2009 to find allium germplasm having resistance to white rot. His work is being funded by the institute and COGRAB.

Eady said his strategy against the soilborne pathogen is to neutralize its production of oxalic acid. His biotech approach is to introduce genes that break down oxalic acid, which he termed white rot’s “Achilles heel.” He said mutants of the disease are unable to produce the acid and do not infect plants.

Oxalic acid is the naturally occurring compound that makes rhubarb leaves toxic. It can also occur in some humans and leads to the formation of kidney stones. Wheat, barley, and related plants produce enzymes to destroy it.

“Our ultimate goal is to have anti-microbial genes in the allium roots that will resist white rot,” Eady said.

Initial work with transgenic material having resistance developed with wheat and barley plants was promising on tobacco plants. However, it was later found that the resistance was not strong enough to control the disease in alliums.

Subsequently, four other lines of onion plants have been found to show white rot resistance, and two of them will be tested later this year. Once a resistant line is developed, a business model to deliver the benefits to the industry will be worked out.

Eady, known for his “tearless onion” research, has been working with genetic transfers in onions since 2002. He has predicted that it will be several years before white rot-resistant materials will be available for commercial use.

Meanwhile, the home front campaign against white rot continues in the work by Mike Davis, plant pathologist at the University of California, Davis. He has trials in Fresno County and near Tulelake.

Davis has been using the biostimulant diallyl disulfide (DADS), which, in the absence of an allium crop, tricks the fungus into germinating and dying.

The idea is to reduce its fruiting bodies before the crop is planted and thereby lessen the likelihood of it spreading from field to field.

The problem has been that although DADS can reduce the sclerotia by more than 90 percent, in a heavy infestation, what’s left is sufficient to create an infection. Even with a second DADS treatment, some survive. As a result, he has been working with fungicides to deal with the remainder.

“A single sclerotium in a couple of handfuls of soil can lead to 20 to 40 percent of your plants being infected,” he said.

Thus far, his trials show that Folicur fungicide performs well against the disease. However, questions emerged with reports of phytotoxicity of the material at the labeled rate in processing onions. He has since begun trials with different rates and application methods.

“It is possible to see some phytotoxicity with Folicur, but certainly there is a window that we can work in,” Davis said.

Separate from the white rot problem, Davis reported that he has received samples from throughout the state, mostly from onion fields, of infections with basal rot caused by Fusarium oxysporum species.

Another soilborne pathogen, it remains permanently in a field and has the ability to remain active on the outer surfaces of many plants. It is not affected by DADS, and management of it consists of long rotation periods to prevent the build up of spores.

Tom Turini, Fresno County farm advisor, has been collaborating with Davis on white rot studies. He said in-furrow applications of Folicur, Endura, and Cannonball fungicides gave generally better performance, while biological control products were less effective. The trials continue with revisions in drip treatments times and tape placement.

Turini reminded that sanitation of white rot-infested fields remains important in managing the disease, which has spread to some 13,000 acres in the San Joaquin Valley.

He urged growers to keep that practice in mind, even when harvesting tomatoes or lettuce from fields that were in garlic years earlier.

Richard Smith, Monterey County farm advisor, told the symposium the onion industry has a good array of registered materials for several options in weed control.

“The challenge now,” he said, “is to work with manufacturers to retain these registrations and to expand registrations for materials that can provide control of specific weeds.”

The role of herbicides in onions, he noted, is important because of the minimal ability to effectively cultivate weeds in the crop. The use of preemergence products followed by postemergence products can provide excellent control that makes subsequent hand weeding more efficient and economical.

Discussing his work with the preemergence herbicide Nortron, registered in 2008, Smith said at the 24- and 32-ounce per acre rates, it was nearly as effective as Dacthal in a trial where lambsquarters was the main weed.

It was, however, not effective on lambsquarters postemergence. But when followed by postemergence treatments of Goal Tender at the first true leaf stage, the two-step process provided excellent weed control.

Smith added that he saw some reduction in yield with Nortron at the 32-ounce rate in the 2008 and 2009 trials.

In his trials with Outlook for nutsedge, he found that it needs to be applied and activated within a day or so of an application of 7-7-0-7 acid fertilizer.

If the herbicide is not activated quickly, the nutsedge may grow back. Applying Outlook with irrigation is a good strategy to apply and activate it at the same time.

“Do test strips of fertilizers to determine formulation and rate to effectively burn back the nutsedge,” Smith recommended.

His recent work also included testing Basagran as a burn back material for nutsedge. Although it is effective, he said, the manufacturer, Arysta, at present has no plans to register it for use on onions.

TAGS: Vegetables
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