Downy mildew a new threat for corn salad

Minor crops tend to fly under the radar when it comes to disease identification and mitigation. In the Salinas Valley, main staples of the vegetable world seemingly garner all the attention, but that's not necessarily the case, according to Steven Koike, Monterey County farm advisor.

“There are 30 to 50 specialty crops that are grown here in addition to the mainstream vegetable crops,” he says. “They don't get very much press or attention at times, but that doesn't mean we're ignoring them.”

Those include crops such as arugula, beet leaves, chervil, cilantro, corn salad (mâche), escarole, fennel, mizuna mustard, radicchio, rappini, red mustard, tatsoi and others. Specialty crops are relatively small acreage commodities, but they have problems just like lettuce, spinach, strawberries and the other crops that are normally associated with the Salinas Valley. The minor crop element is an important component of the overall agricultural landscape in the Salinas Valley.

“Corn salad (mâche) is one of the more recent minor crops,” Koike says. “It's only been grown here five or six years, and in terms of larger acreage, only the last couple years. It's a good example of a small commodity that is the matrix of our Valley.”

Limited acreage often precludes industry commodity boards and university programs from focusing adequate research resources on specialty crop needs.

“As you bring in these new commodities, you bring in new problems with it,” Koike says. “Corn salad is new and the downy mildew strain it brought with it is also new to us.”

Luckily, it wasn't unheard of overseas. “In Europe where they've grown corn salad for a long time, they know about this disease,” Koike says, “so we were able to reference some of the work in Europe because they've already done the research on the same disease. In the U.S. and California, no one knew what it was. Originally, out in the field, it was misdiagnosed as powdery mildew so they were spraying the wrong chemical for a while.”

It wasn't a difficult disease to diagnose in the laboratory, but it was difficult to diagnose in the field. “People working in the field don't have microscopes and the equipment that we have to identify it,” Koike says. “So when they looked at it, they thought it kind of looked like powdery mildew, which was already known to be a problem on that crop, so they said ‘let's spray this’ but it didn't work. They brought it in to us, and we looked at it under a microscope. We could tell that it was downy mildew — an entirely different fungus than powdery mildew and thus needed to be treated differently.”

Weather is a significant factor in the expression of downy mildew on corn salad. Cool, wet, humid weather will allow the fungus to survive and thrive. “Last year was the first time we saw downy mildew on corn salad, but I had worked with Sclerotinia on the crop in the past,” Koike says. “Due to staggered plantings, there was corn salad in different stages of maturity during the season. We saw early spring plantings starting in February that began to have downy mildew problems in April, May and June.”

Disease symptoms include a downward folding of leaves under humid conditions, yellow and necrotic foliage later in the disease cycle and brown spots on the infected foliage.

“Once we got into August, when it was warmer and dryer, the disease went away,” Koike says. “Or more precisely, it was hard to tell when it really went away, because by then, the growers were using effective fungicides targeted to the real disease culprit.” Sometimes, disease problems in minor crops can carry over into lettuce and other widely planted crops in the Salinas Valley such as lettuce. Corn salad is susceptible to the same soil borne fungus (Sclerotinia minor) as lettuce. However, what affects one crop may or may not affect another.

“It depends on the disease,” Koike says. “You have to look at it on a case-by-case situation. For downy mildew, it's not a problem. So even if corn salad has a massive downy mildew problem, you could plow it down and plant anything you wanted. It won't make any difference because the downy mildew will not spread to any other crop. It's pretty clear from our research and the research that was conducted in European countries that this strain of downy mildew stays pretty much with corn salad or closely related species.”

Corn salad is susceptible to Sclerotinia minor which is a big problem on lettuce and also on fennel. “If a grower rotates from corn salad where he had some Sclerotinia minor and follows with lettuce, then the lettuce will also have the disease,” he says. “That's why rotation needs to be carefully thought out depending on the disease.”

Sclerotinia is difficult to control because sclerotia are viable in the soil for at least three to five years, whether or not a host crop is planted during that time.

“The other problem is that we grow too many crops that are susceptible to this disease,” Koike says. “If you look at the Valley, there's just a ton of lettuce, and lettuce is its preferred host. And somewhere in the Valley there's always back-to-back lettuce.”

Even rotation into something like broccoli for a couple of years, would not eradicate Sclerotinia, according to Koike. “It would impact it, but not eradicate it,” he says. “Keep in mind that cabbage, cauliflower or spinach do not even suppress Sclerotinia.”

Managing disease in Salinas Valley vegetable production is a partnership, Koike says. “We have the facilities, the diagnostic tools and we are here to support the growers.”

TAGS: Management
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