Research by Dr. Debra McLaren, Dr. Yong Min Kim, Dr. Robert Conner at Agriculture and Agri-Food Canada and adapted by Laura Schmidt, MPSG – Spring (March) Pulse Beat 2020
SOYBEANS
In 2019, 68 soybean crops were surveyed in Manitoba for root diseases. Root rot was observed in all soybean crops that were surveyed. Root samples from 40 of 68 Manitoba fields were assessed in the laboratory for root pathogens and Fusarium root rot was the most prevalent root disease (Table 1). Rhizoctonia root rot (Rhizoctonia solani) was detected in one of the 40 Manitoba crops surveyed in 2019. Pythium root rot was not detected in any of the soybean crops surveyed.
On a scale of 0–9, root rot severity ratings ranged from 2.7 to 6.2 with a mean of 4.3. Forty-one (60%) soybean crops had average root rot severity ratings above 4 (i.e., symptoms were present on 50% of the root system) and this would have had a detrimental effect on crop yield. Analysis of root tissue for detection of nucleic acid targets of multiple pathogens using droplet digital PCR (ddPCR) is ongoing and will provide additional information on the root rot complex of soybeans.
Five percent of the soybean crops (4/83) tested positive for the presence of Phytophthora root rot. Approximately 168 stems from Manitoba soybean plants, were plated on petri dishes to identify Phytophthora based on their morphological characteristics. Pathotype identification of the Phytophthora isolates will begin shortly.
Review the risk factors and considerations to determine your risk of seedling diseases and root rots in each field (Table 2). Prevention strategies include fungicide seed treatments, crop rotation and genetic resistance of soybean varieties (if available). It is important to remember that seed treatment can only provide protection for two to three weeks beginning at the time of planting (not emergence). These diseases can also cause issues later in the season if conditions are conducive for infection; therefore, seed treatment is not a complete solution. Visit MPSG’s Seed Treatment Risk Assessment and the On-Farm Network (OFN) article, Thinking Spring Inputs, by MPSG OFN Agronomist Megan Bourns, to identify where seed treatments are most likely to be beneficial and provide a return on investment.
FIELD PEAS
A total of 45 pea crops were surveyed in Manitoba for root diseases. Two diseases were identified based on laboratory assessment of the roots collected — Fusarium root rot and Fusarium oxysporum (Table 3). Fusarium root rot was the most prevalent, as in previous years, with F. avenaceum being the most predominant Fusarium species. Of all crops surveyed, root rot severity ratings ranged from 1.2 to 5.7 with a mean of 2.9. Rhizoctonia root rot (Rhizoctonia solani) was not detected in any of the crops sampled. Four (9%) pea crops had average root rot severity ratings above four (i.e., symptoms were present on 50% of the root system) and this would have had a detrimental effect on crop yield. Fusarium oxysporum, an efficient root colonizer known to cause Fusarium wilt of pea, was detected in 21 of the 45 crops sampled for fungal isolation and identification. Assessment of frozen samples for root pathogens using a rapid, molecular test (ddPCR) is planned for the near future.
Root samples collected from a total of 105 pea fields in 2016 (30), 2017 (30) and 2018 (40) indicated that Aphanomyces root rot (A. euteiches) was present in 77%, 47% and 56% of these fields, respectively. Aphanomyces root rot is favoured by wet, poorly drained soils and is most severe under flooded soil conditions. Seasonal precipitation in many of the pea growing regions of Manitoba in 2016 was above normal, which would have contributed to the increased incidence of Aphanomyces root rot. Drier conditions prevailed in 2017 and 2018. The 2019 PCR assessment for A. euteiches is ongoing and results are pending at this time.
DRY BEANS
A total of 40 bean crops were surveyed for root diseases in 2019. Fusarium root rot was detected in all 40 of the dry bean crops surveyed, with severity ratings ranging from 2.3 to 5.8 (average of 3.8) (Table 4). It has remained the most prevalent root disease of dry beans for several years. A number of Fusarium species, including F. redolens, F. oxysporum, F. acuminatum and F. avenaceum, were isolated from symptomatic root tissue. Rhizoctonia root rot and Pythium root rot were not detected in any of the crops surveyed based on microscopic examination and morphological characterization. Use of molecular technology (ddPCR) for the detection of root pathogens from additional root samples collected per crop is ongoing.
Fourteen crops (35%) had average root rot severity ratings above four (i.e., symptoms were present on 50% of the root system and plants were stunted) and this would have had a detrimental effect on yield. There were more surveyed crops with average root rot ratings above four in 2018 (68%) and 2017 (63%). However, in 2016, a much wetter year, 93% of bean crops had severity ratings above four, which represents the highest percentage of bean crops surveyed with yield-robbing root rot severity ratings over the past six years.
SPRING 2020
Saturated soils from last fall increase the expected risk of root rot diseases during spring 2020. It is important to scout for these diseases early in the growing season. To help farmers and agronomists identify which diseases and insects they should be looking for and when, MPSG has created new scouting calendars for field peas and dry beans, and has updated the calendar for soybeans. 
You must be logged in to post a comment.