The incidence of sclerotinia in canola was higher than normal in 1999.
Canola is usually the crop most severely affected by sclerotinia, but the incidence of this fungal disease was higher than usual in lentil and pea crops as well. Sclerotinia also affected yields in chickpeas, sunflowers and flax this year.
Farmers planning next year’s rotations may have questions about seeding broadleaf crops into broadleaf stubble known to have sclerotinia this year.
Experts agree that in wet years, crop rotation has little effect on sclerotinia development. But it is not a straightforward issue.
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“If producers are looking at planting broadleaf crops back-to-back with broadleaf crops, they’re playing a game of Russian roulette with the weather,” said Karen Bailey, a research scientist at Agriculture Canada’s Saskatoon Research Centre.
“The most important factor in 1999 was the weather but over the long-term, crop rotation is very important,” added Saskatchewan Agriculture plant disease specialist Penny Pearse.
“This is because, over time, an increase of broadleaf crops in rotation will increase the number of sclerotia, the fungal body, overwintering in the soil. When wet conditions occur, this inoculum will result in increased levels of sclerotia in the crop.”
Bailey’s views are based on her involvement in two studies of crop rotation. A 1992-95 study at Indian Head, Sask., looked at a cereal-oilseed-cereal-pulse rotation using lentils, flax, mustard, sunola and canaryseed. No broadleaf crops were planted on broadleaf stubble.
“We found that tillage and rotations had minimal effect on disease. Environment overshadowed the impact of cultural practices.”
“We are also compiling data from a 12-year study at Indian Head – a wheat-flax-pea
rotation from 1987 to ’98 – and I suspect the results will be similar: an increase in diseases and subsequent loss of yields during wet years, and less disease and less impact on yield during dry years,” said Bailey.
Randy Kutcher, a plant pathologist at the Agriculture Canada research centre at Melfort, Sask., said rotations that include many broadleaf crops such as peas, lentils, canola and flax, can be expected to increase risk of sclerotinia in subsequent crops because all are susceptible.
Rotation is not as effective at controlling sclerotinia as it is at controlling other diseases. This is due to three factors: the sclerotia can survive in the soil for many years; the disease can be spread by wind-borne spores; and there are a number of sclerotinia-susceptible broadleaf crops that can be included in a rotation.
Citing a study at Melfort that contained more than one broadleaf crop in four years, Kutcher said the sclerotinia stem rot levels in canola did not increase in the first four years of the study, 1994-97, when environmental conditions were not highly conducive to the disease.
In 1998 and 1999, however, environmental conditions were much more conducive to
sclerotinia stem rot.
Canola yields were slightly reduced in rotations with two or three years of broadleaf crops out of four, compared to a rotation with one year of canola and three years of cereals.
“In summary, growers should be aware that canola crops within rotations containing many broadleaf crops may be more at risk than rotations with low numbers of broadleaf crops.
