Although still not a major concern for many pea growers, researchers in Saskatchewan have initiated an extensive survey to try and find out exactly what diseases are out there and how much of a problem they could pose in the future.
The 2009 field pea disease survey is the first provincially co-ordinated study done for at least eight years, says Faye Dokken-Bouchard, provincial plant disease specialist with the Saskatchewan Ministry of Agriculture. Surveys were taken at 141 different field pea crop locations throughout the province during last July and August.
“The idea was to start doing surveys so we would have a benchmark to start from, so that if any new diseases show up or any diseases start to appear more, we have something to compare with,” she says. “As an example, we had started to get reports in the last few years of root rot particularly, and didn’t really know how much of a problem it is or if it is just showing up in some areas. It helps to do a survey because then you have an idea of what the picture is across the province.”
There were a few surprises, especially in terms of some old diseases that were thought to have largely disappeared and some regional variations in the amount of others that were recorded.
WHAT’S OUT THERE
Downy mildew appeared to be more prevalent in the east-central and northern parts of the province than in other areas. “We weren’t too surprised because the weather in those areas had been conducive (to disease growth), with lots of cool rainshowers last summer,” says Dokken-Bouchard. “But downy mildew has become an endemic problem in central Alberta. I think it is an important one to look out for because we know that it can be a problem.”
Powdery mildew also showed up, but only in around nine per cent of the crops surveyed, which was consistent with expectations, as most pea varieties have good resistance to the disease.
An interesting finding was the presence of Ascochyta pisi (A. pisi) leaf and pod spot and the fact that it was most prevalent in the southwest, where it occurred in 44 per cent of the fields surveyed as compared to 13 per cent across the province in general. “It warrants further study to find out why it seems to be showing up more in the southwest,” says Dokken-Bouchard.
She explains that this finding is significant because it was generally thought that A. pisi had been largely subdued since the introduction of the highly resistant Century pea variety in the 1960s. Since that time incidences of A. pisi have been rare as mycosphaerella blight (a disease caused by Mycosphaerella pin-odes, a pathogen that is part of the same complex as A. pisi) became more dominant.
“The fact that ascochyta leaf and pod spot is showing up more in one area makes you wonder if the pathogen is shifting again or if mycosphaerella blight will continue to be the more dominant disease,” says Dokken-Bouchard. It is particularly important to watch for A. pisi, she adds, because most farmers are now so familiar with the symptoms of the very-prevalent mycosphaerella blight they might confuse the two, as they can occur at the same time on the same plant.
DISTINGUISHING BETWEEN ASCOCHYTA AND MYCOSPHAERELLA
Why should farmers really care about which disease is present when one doesn’t appear to be more damaging than the other and would be treated in the same way? Dokken-Bouchard responds that it is important to make the distinction between A. pisi leaf and pod spot and mycosphaerella blight to watch out for any shift in the pathogen population between the two. Once clearly identified, farmers and researchers can then also keep an eye out for any differences in the susceptibility or resistance of different pea varieties to the two diseases.
“We are still learning about the differences between the two pathogens and if they affect the plants differently, maybe even if they have any effect on each other,” she says. “For example is it possible for one pathogen to be dominant over the other if they were both present at the exact same level? Would one spread farther or faster or would they be equally damaging? These are things we don’t know for sure.”
Distinguishing between the two diseases, although tricky, is worth the extra effort to be proactive with disease control.
“With mycosphaerella blight the lesions are purplish brown or black and indistinct and blotchy,” says Dokken-Bouchard. “Ascochyta leaf and pod spot has a distinct shape and will have a dark border around the lesion, which is an orange or tan colour. You should also be able to see very distinct black spots in the lesion.”
Other common diseases found in the survey included root rot and sclerotinia stem rot, which also seemed to be more common in the northeast section of the province (74 per cent) as opposed to 21 per cent overall.
Several years of surveys will be needed to establish any kind of valid disease pattern and the regional variations will need to be examined to see whether environmental conditions, varieties grown or other factors are playing a part in the incidences seen.
“That’s the aim,” says Dokken-Bouchard. “To keep an eye out, keep monitoring and see if things are developing that farmers should be aware of.”
Angela Lovell is a freelance writer based at Manitou, Man.