“All the greatest chemistry in the world cannot make up for poor handling of seed.”
– TRACY SHINNERS-CARNELLEY
Pesticide resistance has grown in the past decade. But on the bright side, new chemistry has kept apace of the bad bugs.
“When I started working in potatoes, almost 10 years ago, we didn’t have a lot of resistance issues that we were dealing with on the fungicide side of things,” said Tracy Shinners-Carnelley, a MAFRI potato pest specialist, speaking at a spring potato production meeting in Portage la Prairie last week.
“In the past two years it seems, we have had a lot of current and new resistance issues that we have to deal with.”
Last year, a national storage survey by AAFC found that of the fusarium samples taken from Manitoba, 54 per cent were resistant to fludioxonil (Maxim), while 15 per cent showed insensitivity to thiophanate-methyl (Senator).
Given the situation, when choosing a seed piece treatment, it’s important for potato growers to know what disease they wish to target, as well as what fungicide treatments were used on the seed lot by the seed grower over previous generations, so that they can rotate modes of action for greater effectiveness.
“That will give you some clues as to what would be a good seed treatment choice when it comes to developing a disease resistance management strategy,” she said.
Many potato growers are unaware that three new variants of fludioxonil have been developed to overcome resistant pathogens. The first, Maxim PSP, contains only the main active ingredient.
The second, Maxim MZ PSP, also contains a shot of Mancozeb for two-shot punch.
The third, Maxim liquid PSP, contains only the fludioxonil active ingredient.
Non-chemical techniques for fighting seed-borne disease include grading out rotted tubers, keeping cutters sharp, disinfecting tools at the end of each day or between seed lots, and not storing seed for too long.
“All the greatest chemistry in the world cannot make up for poor handling of seed,” she said. “The longer you store, the greater the disease risk.”
Last year, a study that looked at the presence of pesticide-resistant Colorado potato beetles from Alberta to the Atlantic provinces, found 43 per cent of the beetle populations tested showed resistance to imidacloprid, and less than 30 per cent of beetle in the samples deemed resistant were killed by the chemical.
The single sample from Manitoba did not indicate resistance to any of the products tested.
“It’s not here yet, but we have to continue to monitor very closely and see what’s happening. It’s definitely an issue in other parts of the country.”
To prevent resistant bugs and help their protective arsenal last longer, producers should “never, ever” follow a seed treatment or in-furrow application of a neonicotinoid with a foliar neonicotinoid.
“At the end of 2008, I heard way too many cases of people doing it, and certain agronomists were recommending it. It is an absolute no-no. It’s the perfect combination for selecting for resistance,” she said.
Last year’s cool spring
brought some head-scratching from potato farmers in Manitoba, she said, mainly in the form of delayed or patchy emergence in their crops.
The culprit, it turned out, was in most cases simply due to the lack of heat early on, but in some areas rhizoctonia infection aggravated the situation by delaying seedling emergence.
If present cool, wet weather trends continue, the same problems might reappear, and seed treatments might be necessary. Little tuber syndrome, which occurs when warmed seed suffers shock by being planted in cold ground, may also present a risk.
Another unusual issue encountered in 2008 was the incidence of purple top, which leaves the tubers dehydrated and limp. Caused by the same phytoplasma that causes aster yellows in canola, it was carried over from the year before.
“Every year, we’ll see a plant here or there that has purple top symptoms. But in 2007, because we had such abnormally high levels of the aster leaf hopper together with the phytoplasma, we had lots of spread late in the season,” said Shinners-Carnelley.
Although it was at elevated risk of late blight for much of the growing season last year, Manitoba’s potato crop made it through unscathed throughout the foliar stage – except for one positive late blight tuber rot sample that was found in October from the Portage la Prairie area.
“You can never let your guard down,” she said, adding that under last year’s high-risk conditions, producers were advised to keep up with a preventive fungicide schedule.
“Just because it’s August and there isn’t any disease, doesn’t mean that it’s time to put the sprayer away.” [email protected]