Fungicide issues worse in U.S.

It’s not time to panic on boscalid resistance, at least not north of the border.

The U.S. potato industry, including potato producers directly south in North Dakota, have noted a substantial downturn in both boscalid fungicide efficiency and the wider group of SHDI fungicides in general, something that could rob them of one of their main tools against early blight.

In one study of 1,390 isolates across 11 states between 2010-15, about 97 per cent showed some form of SHDI resistance, according to data presented by Neil Gudmestad, a plant pathology professor at North Dakota State University. The researcher was among the speakers at the 2018 Manitoba Potato Production Days.

It’s a trend that has got bad enough that he is now calling for farmers to stop using Endura, a major boscalid-based fungicide and to keep a tether on other SHDI fungicides to limit resistance pressure.

“There are five mutations that can cause resistance to boscalid,” he said, adding that not all those mutations impact other SHDI products.

One of those, designated H278Y, is among the most common in the U.S., and is tied with very high resistance issues and has been a noted problem in the fields directly south of Manitoba.

Among the others, H134R actually originated in North Dakota, although it has since become widespread. H133R is mostly a concern in Idaho, while rising cases of D123E, and its resistance threat to newer chemistries as well as boscalid, have launched it to the top of Gudmestad’s concerns.

Gudmestad pointed to the geographic breath of the mutations, which not only impact different genes, but first emerged in a single region or state before spreading.

“What this tells us is that all of these mutations have been developed spontaneously and independent of each other and, what’s really amazing, about at the same time,” he said.

“What we’re talking about here is just basically evolution, but the selection pressure that is put on this fungus by this class of chemistry is really remarkable for that to happen,” he added.

The problem is not yet as severe in Manitoba, according to Dr. Vikram Bisht, potato and horticulture pathologist with Manitoba Agriculture.

Boscalid fungicides are not as popular in the province compared to the U.S., he noted. Instead, Manitoba producers have opted for chemistries like mancozeb or chlorolthalonil and have been applying them with more worry for late blight than early blight.

Late blight concern has overshadowed early blight in Bisht’s most recent Manitoba disease surveys.

“Both of these fungicides have multiple sites of activity against the fungus, so the fungi do not develop resistance to mancozeb or chlorolthalonil,” he said. “Because they’re part of the program of late blight disease control, early blight also gets controlled. You can say the use of boscalid has been rather limited in our province and maybe that is the reason that we don’t have as much of a problem with resistance as North Dakota does.”

At the same time, he said, air movement across the border still poses a risk for bringing in the resistant pathogen.

“It is on our radar because, in some cases, the variety may be too susceptible or sensitive to early blight and if the wrong fungicides or the fungicide against which the fungus has developed resistance (is used), then the fungicide is not going to work and that could lead to a fair amount of losses,” he said.
Manitoba Agriculture is pushing producers to rotate their chemistries to guard against resistance.

“They cannot continue to use the same product on and on again in the same season and especially not follow two fungicide applications with the same chemistry,” Bisht said.

Gudmestad and Bisht have previously teamed up to try and quantify boscalid resistance in Manitoba. In the study of 200 isolates from both North Dakota and Manitoba, the researchers found that half of those from Manitoba were still sensitive to the fungicide, Bisht said.

A 50-50 chance on resistance may not seem like reason to be optimistic. In comparison, however, the same study found a much more dire picture south of the border with the overwhelming majority of isolates resistant to boscalid.

Adding to the issue, isolates on both sides of the border showed mutations limiting vulnerability to azoxystrobin, another fungicide.

“Most of the isolates that we have in Manitoba and in North Dakota, I would say over 90 per cent have a moderate level of resistance to Quadris,” he said, referring to the current brand claiming azoxystrobin as an active ingredient. “Many of these fungicides have cross-resistance to other fungicides.”

Penthiopyrad might also be caught up in those cross-resistance issues, he said.