Leafy greens studies seek to ward off mildew threats

An article published by Ag Alert

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  • Researchers say new resistant spinach varieties, such as these growing in a greenhouse at the University of California, Davis, may help curtail the spread of downy mildew in California farm fields.

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"Downy mildew is a primary disease problem for organic and conventional spinach growers in California.

While those growers turn to cultural management practices or chemical applications to protect their crops, researchers are looking for resistant varieties to ward off the disease.

Tom Ikeda, a vegetable farmer and owner of Ikeda Bros. in the Arroyo Grande Valley, said such new research approaches are also critically important to growers.

"The rate of new (downy mildew) races coming out has gone up exponentially in the last few years," Ikeda said. "Breeding resistance is probably the biggest tool that we have to combat the new races that keep coming, and if you can stack on different types of resistances into the same variety, that helps to keep that resistance longer."

Ikeda said he is interested in alternatives to chemicals, the main tools currently available to growers. "It's not something that we want to do, but we have to do, and if we can breed to reduce chemical applications, that's a plus," he said.

"Without the good genetics, we have to spray more, especially if resistance breaks down in current varieties. We would much rather have varieties that are resistant and more durable, so we don't have to spray as much," Ikeda added.

Researchers are now looking to find solutions. Charlie Brummer, a professor in the Plant Science Department and director of the Plant Breeding Center at the University of California, Davis, is working in collaboration with Allen Van Deynze, director of the Seed Biotechnology Center at UC Davis to develop open-pollinated varieties that have broad-spectrum resistance to downy mildew. The project is funded by the California Leafy Greens Research Board.

Brummer said downy mildew is a big challenge for conventional and organic growers.

"The pathogen is evolving rapidly, and it's hard to keep up with resistance," he said. "The hope is that we could develop something with broad-spectrum resistance that, in combination with the current major-gene resistance, could at least combat the problem to some degree."

The research focuses on baby leaf spinach for salad mix and working with commercial level germplasm. The main goal of the breeding program is to develop something that may be commercialized with better broad-spectrum resistance to downy mildew and an improved overall downy-mildew package, Brummer said.

The research is being conducted during the growing season in the Central Coast region from March through early November and in the Desert Research and Extension Center in Holtville during the winter. A trial was also undertaken in far Northern California at the Intermountain Research and Extension Center in Tulelake, and more research may be done there in the future for seed production.

In looking for ways to curb disease spread and cut costs, researchers are studying potential benefits of incorporating more open-pollinated seeds as an alternative to hybrids, Brummer said.

"Most seeds that farmers buy are hybrids, and the beauty of hybrids is that they're very uniform," he said. But, he added, "It's more expensive to produce seed of hybrids, and seed is a major cost in the overall cost of production."

Brummer said seed is as much as half of the cost of production, and open-pollinated varieties could reduce seed costs.

With open-pollinated varieties, Brummer doesn't need to develop inbred lines that are used to make hybrids, and it's not as tricky to produce seed.

Open-pollinated varieties are actually a population of plants, rather than a very uniform hybrid where all the plants have the same genotype. But, with open-pollinated varieties, it's harder to get complete uniformity. That's the challenge, Brummer said.

There are other benefits to open-pollinated varieties. From a purely pathogen evolution standpoint, having a population of spinach out there that's not uniform can help overcome a disease outbreak, Brummer said.

A few plants may get infected, he said, but there are many plants that will be resistant. Compare this to a uniform hybrid, or an inbred line, if a pathogen can overcome the resistances, then all the plants are susceptible.

"The challenge, though, from a practical standpoint is that consumers don't want to see any downy mildew on their leaves when they open the bag of spinach," Brummer said. "And so with open-pollinated populations, it's challenging to make sure that you can get the level of resistance so that you don't see any downy at all."

He added, "The ideal situation would be that we would have something that would have the major gene resistances to the predominant races of the pathogen, or isolates of the pathogen, and then have broad-spectrum resistance that would provide a sort of insurance."

He said the hope is that "the plants will still have a measure of resistance" as new races arise "and at least we could delay the onset of symptoms beyond the harvest window."

The ideal outcome, Brummer said, would be to find something that can make spinach immune to downy mildew. Until then, the best approach for managing downy mildew is with resistant varieties, cultural practices and chemical control.

"When you have challenging problems like downy mildew, the more different ways you can attack the problem through both management and genetics, the better off you are," Brummer said.

Brummer and Van Deynze have been working on the research for about five years. "Breeding is one of those open-ended things that we can always do better and always keep improving," Brummer said. "We're at the point now where we're starting to develop new populations that have some potential."

He said those include a variety with promise for broad spectrum downy mildew resistance. But it could be four to five years before it's available to growers, he said.

"We're starting to test our first materials coming out of the program while we continue to develop in the pipeline new materials and new germplasm with hopefully better characteristics," Brummer said, adding that researchers will be producing seeds this summer for testing.

Ikeda said this research may give growers more options for controlling downy mildew with inputs that enable them to compete better in the market.

"I think that's the bottom line for every grower, is to have your crops survive, so you can harvest and market them," he said. "If I can reduce the risk of losing a crop to downy mildew, ultimately that helps me stay in business."

(Kathy Coatney is a reporter in Bend, Oregon. She may be contacted at kacoatney@gmail.com.)"