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Caution advised for dry bean growers

Be conservative and resist the temptation to spray first and ask questions later. That is what plant pathologists are telling growers in Western Canada.

The issue emerged in Manitoba in early 2008 as the concept of preventive applications for the sake of plant health was promoted. The concept is straightforward: spray early while the crop is healthy and looks healthy, to protect yield potential as a minimum and to possibly increase the yield.

Ontario research has not found an advantage
In the past, dry bean growers were advised to “think twice” about a protective application. Chris Gillard, University of Guelph dry bean researcher at Ridgetown College, says he has no evidence supporting the position that protecting a healthy bean crop from disease will pay for itself, but the cost is substantial and the practice poses a risk. Like weeds, fungi can become resistant to a product. “The jury is still out on the plant health benefits,” Gillard says. “We have two good years of data now from spraying Headline and Quadris on a healthy crop, trying to determine if there is a health benefit to the crop. To date, we haven’t got a single trial that shows a benefit. Even when you combine the data over the eight trials so far, there’s no benefit.”

North Dakota caution
North Dakota State University extension plant pathologist Sam Markell is the broadleaf crops specialist for dry beans in North Dakota. “We are very cautious of this “plant health” concept,” Markell says. “Companies have been promoting it for corn and soybeans for three or four years, but for plant pathologists that I communicate with frequently, we would much prefer our growers to go scout and manage disease, and to not use a fungicide if the disease is not a problem.”

Like Gillard, he points to two issues. First, in the absence of disease in soybeans, research work shows little to no yield response from a protective treatment. Second, by putting down fungicide applications for sub-economic disease levels, growers seriously increase the risk of developing fungicide resistance. “The problem is that suddenly you may have a very significant disease outbreak and if you have resistance, that fungicide is no longer able to manage that disease. This approach is like taking one tool and throwing it out of the toolbox.”

However, preventing white mould in dry edible beans may be an exception to the rule. Because it is a common and serious problem, many edible bean growers in this region will do a protective fungicide application at the flowering stage, without waiting to see disease symptoms. “To manage white mould, we use products like Topsin, Proline, Endura and Rovral in a very narrow window, because that’s the only time you can protect your crop.”

Manitoba looking at the issue
Bruce Brolley, pulse crop specialist with Manitoba Agriculture, Food and Rural Initiatives in Carman, Manitoba, is also cautious of the concept. “I’ve seen a lot of data, and to date pulse crops have not been responsive to the whole plant health concept,” Brolley says. “When looking at growers’ data you will sometimes see a yield response but when you factor in the cost of the chemical and application cost, it’s rarely more than break-even at best.”

Plant health support
According to some manufacturers, plant health is an additional reason to use a fungicide. Both BASF and Syngenta advise growers to do proactive treatments to sustain the health of susceptible crops. “Most fungicides are protective, few are curative. Because they are protective, most fungicides need to be down in advance of disease,” says Robert Hornford Sr., technology specialist, BASF Canada in Winnipeg. “Some new fungicides have plant health or growth regulatory benefits that are separate from the fungicide activity. The plant health concept is a combination of the disease-control product and some other additional benefit. The main benefits are improved efficiency of growth or improved plant tolerance to some stresses.”

Eventually, additional research will help to answer whether the ‘spray first and ask questions later’ approach is an appropriate approach to managing disease in dry beans. Meanwhile, as with any technology, a cautious approach to plant health will help to preserve the use of valuable fungicides for when they are needed most, when disease prediction models warrant the use of protective fungicide applications.