Spore numbers are not enough to infect.
By Ralph Pearce
In late 2004, word of the spread of Asian soybean rust from Venezuela to the Gulf Coast of the US put researchers across the US on high alert as to the potential for the disease. Yet despite presentations, publications and other printed materials, the threat never materialized much beyond states such as Tennessee or Louisiana.
|Although its effects can be devastating, the spores that landed in Ontario in 2007 were not viable, posing no immediate threat.|
However, the 2007 growing season has seen the arrival of Asian soybean rust (ASR) in some of the mid latitude states. Oklahoma and Kansas had confirmations of ASR in August and September, respectively, and it was confirmed in Indiana on October 23, yet discounted as posing no significant threat. Yet as of October 29, the disease was confirmed in two counties in Illinois and 14 counties in Iowa, with the northern-most of those confirmations at roughly the same latitude as southern Ontario.
While that might cause concern for growers in the Great Lakes Basin, there is still a cushion: Iowa and Indiana are still several hundred kilometres away.
The fact is, ASR spores were detected in Ontario during the 2007 growing season. However, thanks to dry weather conditions in the southern US and the size and concentration of spores found in Ontario, growers have been spared any ill effects, at least for now. “Conditions were not favourable for soybean rust development in the southern US early on in the spring and throughout most of the summer in 2007,” details Albert Tenuta, field crop pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs.
“They’ve gone through three incredibly dry periods in the southeastern Gulf states, all the way through to Texas. The result has been that spore development in the US was not exceptionally quick and early, but as we see, spores still make their way north during the summer.”
Tenuta agrees the hope is that ASR will never be an issue for Ontario growers, yet conditions during at least one year since 2005 indicate there is a need for greater diligence. “If we get back to a normal situation where those southern states have a normal spring with good rainfall and half-decent weather conditions, there’s the potential for those spores to be produced early on and make their way across into the corn belt and potentially into Ontario,” says Tenuta. “We can see that just by our detections this year, there is the ability for those spores to make it into the province.”
Detection methods improving
Although spores were detected using PCR or polymerase chain reaction molecular techniques, the potential for infection in Ontario was limited. Spore numbers were either so small as to not be a threat, or they were effectively sterilized by higher levels of UV radiation or cool temperatures in the atmosphere.
“More than likely, these were small events or small clusters or single spores that migrated, because we didn’t have a lot of inoculum down in the southern US,” says Tenuta, noting the weather’s impact on the size and collective nature of the spores as they were blown northward. “But with significant storm fronts where they’re sheltered from the sunlight and with cloudy, rainy conditions, you get a lot of spores clumping together. So you can get 100 or 200 or more spores surviving together and with that, you do have a zone of protection in the centre of that cluster and that increases the likelihood of those spores surviving.”
Ultimately, the detection levels in Ontario in 2007 were very low in terms of the number or amount of DNA that was extracted from collectors. “In Ontario, we used three types of collectors: a funnel which is open to the elements at all times, another that is automated to open only when rainfall is detected, and a unit that continuously samples the air, regardless of weather conditions.
|Collectors used to trap spores were placed in various locations across Ontario.|
“Locations where we have all three of those units, whether it’s Ridgetown, Ottawa, downtown at the University of Toronto or at Harrow, throughout the summer at different times we found that all three of those units would show up positive for soybean rust spores,” explains Tenuta. Indications were that spores were being collected, but not enough to make it a ‘major event’.
Confirmation came when some of the isolates from the collectors were tested using PCR technology and as a further check, the DNA was cloned or sequenced, confirming the Asian soybean rust detection. Tenuta stresses this work could not have been done without the assistance of Dr. Sarah Hambleton and Ray Tropiano at Agriculture and Agri-Food Canada’s Eastern Cereal and Oilseed Research Centre (ECORC) in Ottawa.
Glass half empty or half full?
How a grower reacts to news like this depends on his personal level of optimism. Yes, ASR spores have been detected in Ontario; no, the levels are not significant, for the time being. And it is worth noting that detection methods have improved considerably since the alarm on ASR was first sounded in 2004/05.
“With the new protocols and DNA techniques, we’re able to fine-tune the detection process more,” confirms Tenuta. “That’s something we’d like to continue in the future, as well. That and sentinel plots, all of these things are integrated to provide Ontario producers and the soybean industry with a co-ordinated early warning system for soybean rust across the whole continent.”
Funding for many of the Ontario soybean rust projects was provided in part through the Canada-Ontario Research and Development (CORD) program, administered by the Agricultural Adaptation Council (AAC), as well as the AAFC Pest Management Centre, the Ontario Soybean Growers and the Ontario Soybean Rust Coalition through AAC’s CanAdvance program. -end-