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Root rot races revealed

The knowledge will help farmers and researchers plan 
their response to the soybean disease

Soybean stems displaying the typical dark-brown lesions extending upwards from the soil.

Agroundbreaking study has determined what are the most prevalent races of phytophthora root rot found in Manitoba soybean fields.

The most prominent are Races 4, 25, 28 and 3. The most common is Race 4, according to the study, completed through the Pulse Science Cluster and funded by the Manitoba Pulse and Soybean Growers and the federal and provincial governments.

With this new information, farmers and agronomists can be strategic not only in choosing a variety with resistance, but they can choose what type of resistance genes may be most effective.

“This information is critical for our industry. We wanted to get this knowledge out so that pathologists, breeders and seed companies can work together to strategically develop varieties that provide genetic resistance to the Phytophthora races specifically found in Manitoba,” said Kristin Podolsky, MPSG production specialist.

Phytophthora is one of four pathogens that cause seedling blight and root rot in Manitoba soybeans. It was especially prevalent in the 2016 growing season with wet soil conditions and increased frequency of soybean in rotations.

From 2014 to 2016, approximately 184 soybean fields were surveyed for plants that were symptomatic for phytophthora root rot caused by P. sojae. This in-depth study of the race structure of phytophthora is the first of its kind in Manitoba and has been led by Dr. Debra McLaren, plant pathologist at Agriculture and Agri-Food Canada in Brandon.

“The most effective method to reduce economic losses caused by phytophthora root rot is to use resistant cultivars, and therefore it is critical to know the races of P. sojae that are present in soybean-producing regions,” says Dr. McLaren. “Based on the research that we have completed to date, four races have been identified and this is very exciting because it provides farmers with an additional tool for variety selection. Knowledge of the race diversity in Manitoba is essential for breeding phytophthora-resistant cultivars and for screening soybean cultivars and germplasm accessions against this pathogen.”

Variety selection is the top management practice to prevent and reduce phytophthora, in addition to good crop rotation practices and fungicide seed treatment. Over half the soybean varieties tested in Manitoba Pulse & Soybean Growers variety evaluation trials contain genetic resistance to phytophthora.

“We don’t have the technology yet to measure which races are present in individual fields, but I would suggest that choosing a variety with resistance to the most prevalent races is an excellent start at this point. For example, soybean varieties that contain resistance genes 1k or 6 will be resistant to Race 4, the most predominant race found in Manitoba thus far,” Podolsky said.

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