Breeding, physiology and agronomy to mitigate yield loss caused by root rots of pea

This breeding project addressed the gap in researching pea lines resistant to Aphanomyces euteiches and Fusarium avenaceum, identified as the significant causes of root rot in field peas and lentils in Saskatchewan and Alberta. Such research could then be combined with research on breeding resistance to other root rot-causing pathogens currently being conducted by other breeders to allow for pyramiding resistance to multiple pathogens. Researchers also worked to understand better how field-level management strategies may be used to combat the devastating effects of root rot of peas for Canadian growers. 

To achieve this goal, work began by screening germplasm lines for resistance to Fusarium avenaceum to incorporate sources of resistance into Canadian-adapted material. Despite challenges in finding a reliable and representative screening method for the pathogen, greenhouse screening was completed. Greenhouse and field experiments were conducted using the cultivar CDC Meadow as a control line. Fusarium disease nurseries were established annually in 2019, 2021, and 2022 to allow for field-level research. A couple of candidate lines were found to have slightly but significantly lower disease levels than CDC Meadow in greenhouse experiments. Still, candidate lines did not perform differently from the controls in any disease nursery experiments regarding Fusarium root rot infection levels. 

While it was initially planned to conduct crosses between moderately resistant lines and Canadian parents, this was paused after one cross due to the lack of consistent partial resistance found. As such, additional line screening will occur between 2022 and 2024 to seek partial resistance before crossing will re-commence.  

The research group also studied interactions between physiological defence responses of root rot-infected plants and nodulation. This was addressed through the use of RNA sequencing technology and comparative genomics. A protocol was developed to study interactions between root rot and nodulation by having nodulation occur simultaneously with infection. Rhizobium nodulation and diseases caused by F. avenaceum or Aphanomyces were found to impact separate, distinct root areas and did not interact much during the initial stages of plant growth. Therefore, Rhizobium inoculation did not decrease infection.  

Several lines screened and used for RNAseq contained one of the two significant quantitative trait loci (QTLs) previously identified to show resistance against Aphanomyces root rot. However, these lines failed to show consistently lower disease severity in mixed pathogen greenhouse experiments and field experiments where they were exposed to the pathogen complex. Work continues on the physiological aspect of this research to develop molecular markers for classical breeding and to evaluate the potential for using advanced genome editing.  

Additionally, crop rotation and intercropping field trials were performed at 6 locations across Alberta and Saskatchewan to evaluate the efficacy of field-based practices in combating root rots of pea and to understand their in-field impacts on inoculum potential better. It was determined that faba bean and soybean performed well in fields with known Aphanomyces pressure and appeared to be a safe way of maintaining a pulse crop in rotation. While chickpea did not seem to support Aphanomyces infection in five years out of the 6-year study, there were high levels of root rot and Aphanomyces infection in the crop at one site during an exceptionally wet year. As such, careful consideration should go into rotating chickpea with pea and lentil crops to manage Aphanomyces. In infested fields, the frequency at which pulse crops were planted (1, 2, 3, 4, or 5-year break) did not appear to impact disease severity, which remained constant. Yields appeared to be mainly affected by moisture. Intercropping research found that adding canola or mustard crops did not influence disease severity. However, a pea yield boost was noted in three out of 12 years of the intercropping study compared to a monocrop pea in some sites. Yields were significantly variable throughout the experiment, warranting further research.  

Overall, it was determined that the severity of root rot of peas and subsequent yield loss is most significant when multiple root rot-causing pathogens are present. While field conditions can bring about new challenges compared to the higher level of control possible in greenhouse trials, pea lines must be assessed in outdoor nurseries to consider those natural inoculum complexes. Furthermore, management practices should target the entire complex rather than individual pathogens.