By Julienne Isaacs
In 2013, Sylvie Rioux found stripe rust on a few spring wheat lines and cultivars at Laval University’s Saint-Augustin-de-Desmaures research station. Rioux, a pathologist at the Centre de recherche sur les Grains in Quebec, wasn’t too concerned.
But when she found it again in 2014, this time on winter wheat, she took the discovery more seriously. “When I observed stripe rust again in the spring of 2014 on winter wheat, I suspected that the fungus may overwinter on winter wheat or other perennial wild grasses in regions where the snow cover is deep enough to assure its survival,” she says.
So far, stripe rust (known as yellow rust in Quebec) has only been found in wheat grown at research stations. “All my observations about stripe rust were done in such trials,” Rioux says. “The growers or their agronomists have not reported stripe rust yet, but it does not mean there was no stripe rust in the commercial fields.”
The discovery of stripe rust in Quebec is significant: the disease, which is caused by the fungus Puccinia striiformis, strikes early in the season and can devastate cereal fields via defoliation and shriveled kernels. Until 2013, it had not been discovered further east than Ontario.
Rioux and Benjamin Mimee, an Agriculture and Agri-Food Canada (AAFC) researcher, sent DNA samples from the infected plants to Sarah Hambleton, a specialist in systematics of rust and smut fungi at AAFC’s Ottawa Research and Development Centre.
“We do a lot of DNA barcoding here, so it fell into my workflow,” Hambleton says. “We sequenced the fungal ITS barcode gene using the field-collected DNA, and we could recognize from our results that there were two species in the sample.”
Hambleton processed the samples using real-time polymerase chain reaction (PCR) assays to verify that one of the species was, in fact, P. striiformis. The other was leaf rust (Puccinia triticina), which also infects wheat.
She says a molecular identification of stripe rust – along with morphological identification in the field – can be important at an early stage.
“If the disease is well-developed and producing spores, there are morphological ways to identify it and people can find it in the field. But if you’re at a point where you’re concerned and need to confirm what species you have, you could differentiate at an early stage using genetic markers,” she says.
“It’s another line of evidence: in this case, we identified the species morphologically and molecularly, and that’s standard now for first records. People hope to see both lines of evidence.”
Stripe rust has been increasing in Ontario over the past several years, and Ontario growers are familiar with its management, according to Albert Tenuta, field crop pathologist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).
Tenuta says management of the disease is the same everywhere, regardless of region, and Quebec growers should employ a few basic strategies. The first line of defense against stripe rust is the use of resistant varieties. Stripe rust is only found in Ontario when susceptible varieties are planted.
Though related diseases such as stem rust and leaf rust are often difficult to differentiate, stripe rust can be quickly identified by yellow, blister-like lesions arranged in stripes on leaves.
As opposed to stem or leaf rusts, Tenuta says stripe rust strikes early in the season, and under cool conditions. “From a disease perspective it usually comes in early, and it can be managed pretty early with resistant varieties and fungicides,” he says.
“Being an obligate parasite, needing a living host, it needs a green bridge – that’s where the risk comes in earlier in the season. If it does show up early, you have a susceptible variety, and the weather stays cool and moist or there is high relative humidity, then a fungicide application may be necessary at that point,” he says. “But in most cases, Mother Nature will take care of it when the warm temperatures come.”
Late-season applications of fungicide for Fusarium will take care of rusts and other diseases lower down in the canopy.
These days, diseases can’t move far without pathologists monitoring their progress, Tenuta says.
“Pathologists have a good idea of what’s going on with stripe and leaf rust on an almost weekly basis, so we can usually anticipate if we’re going to see a stripe rust issue if we’re seeing stripe rust in the U.S.,” he says.
From 2007 to 2010, Hambleton, Tenuta and Rioux were part of a team of pathologists involved in a spore-trapping project that monitored Asian soybean rust in several provinces using TaqMan assays. The idea was to capture and screen air and rain samples to get a general sense of disease distribution.
That monitoring project was used as a diagnostic service, but Hambleton says there is potential to test samples against assays in real-time. “The vision is that we can do this sort of monitoring for rusts on wheat and other crops across Canada,” she says. “But it’s early days. The complete range of assays needed is in the development stages.”