Aster yellows in wheat: a positive in the negative

By Clare Stanfield

Sometimes, finding out something isn’t a problem is just as important as finding out it is.

Take the case of aster yellows (AY) in wheat A well-known and serious problem of canola, AY was not really thought of as a major issue in wheat. “But one year, we had all these unexplained yield losses in the field,” says Pierre Hucl, wheat breeder and professor at the University of Saskatchewan’s Crop Development Centre. “I knew it was also a high aster yellows year in canola, so we wondered if that was the cause.”

To find out, Hucl joined forces with Tyler Wist and Chrystel Olivier, both entomologists with Agriculture and Agri-Food Canada in Saskatoon, to conduct some field and lab studies and take a closer look at AY in wheat.

At the time, very little was known about AY disease expression in wheat, whether certain varieties were more susceptible than others, if there was a critical threshold for aster leafhopper populations, or what kind of yield loss AY was capable of causing.

U of S wheat breeder Pierre Hucl.

Indeed, aster yellows in wheat was a veritable blank slate of knowledge With funding from the Sask Wheat, the Western Grains Research Foundation, and the Saskatchewan Ministry of Agriculture through the Agriculture Development Fund, Hucl, Wist, and Olivier devised a series of lab and field studies to fill in some of those blanks.

“We looked at durum and common wheat – 25 varieties in total,” says Hucl. The bulk of that was CWRS (14 varieties), with CWHW, GP, CWSW, CPS and four varieties of CWA durum rounding out the list.

The results of the three-year study were decidedly mixed and perhaps a little anticlimactic, according to Hucl, but not without value. “We grew the field trials abutting a commercial canola field,” he says.

This would ensure the trials were well exposed to leafhoppers and therefore AY. “We had all these unexplained symptoms in the field, and I was hoping to separate this out indoors, but we were unable to do it.”

Those symptoms included bleaching, stunting, tiller dieback and empty heads in wheat with known AY infection. But since other pests can also cause these symptoms, Hucl and the entomologists tried to replicate them under controlled disease conditions in the lab.

There, under wet soil conditions with high light intensity, they could see that AY-infected plants showed more of these symptoms and had a small drop in yield, but it was not consistent.

“In the indoor experiments, entomologists had colonies of insects, infected and not infected,” says Hucl. “We had randomized, replicated trials, and we were still unable to see much in the way of definitive symptoms.”

What about varietal differences? “We did find that durum is twice as likely to get the disease as common wheat,” says Hucl. It sounds alarming, but he says that the average infection rate in common wheat is one percent, so a doubling to two percent in durum is pretty insignificant.

From 2015 to 2017, Hucl and Wist studied AY in wheat from every possible angle, conducting seven separate lab experiments and three seasons of field trials all designed to uncover the interactions between aster leafhoppers and AY in wheat. Everything from how many insects fed on plants, to how long they fed, to crop staging at time of feeding to the possible roles of soil moisture and light intensity – all was thoroughly investigated.

“We wanted to know if this disease was a breeding objective,” says Hucl. Turns out, it’s not. And that’s okay, he says. Proving a negative is just as valuable as finding a positive, and growers had been asking if AY was causing yield losses. “The research did answer that question.”