Virulence of Phytophthora sojae and soybean resistance to phytophthora root rot (PRR) in Ontario

Principal Investigator: Allen XueP

Objectives:

  • Determine the predominant races of Phytophthora sojae in Ontario.
  • Identify sources of major gene resistance to the new and predominant races of P. sojae and sources of partial resistance (multi-gene resistance) and tolerance to PRR.
  • Develop molecular markers for the new and major resistance genes for marker assisted selection (MAS) for introduction of new resistance genes and for gene pyramiding in future soybean cultivars for Ontario.

Scientific Summary:

Phytophthora root rot (PRR), caused by the fungus Phytophthora sojae, is a destructive disease of soybean in Ontario. Although the improvement of PRR resistance has been one of the major priorities of soybean breeding in Ontario, the disease has become widely spread and increased in severity in central and eastern Ontario and western Quebec where most of the short-season soybean (2200-2800 HU) is grown. Several resistance genes that are commonly used in soybean breeding in Ontario are RPS1a, Rps1c, and Rps1k, which provide resistance to Race 1, the predominant P. sojae race in Ontario from 1965-1973. There has not been a survey for P. sojae races in Ontario since 1990. The resistance breeding cannot be successful without knowing the pathogen race structure and population dynamic of the major races of P. sojae in the regions and effective sources of resistance to these new races. Our preliminary research demonstrates that the racial profile of the P. sojae in Ontario has changed since the last survey in the 1980s and new and more virulent races might have developed in response to the release of resistant cultivars, thus shortening the effective life-span of these cultivars. In addition, these new races of P. sojae can have virulence against resistance genes that are not currently present in soybean cultivars or breeding lines, rendering these genes ineffective even before use in a breeding program.

This project aimed to continue a race survey initiated in 2011 to determine the race profile of P. sojae in Ontario and evaluate soybean cultivars for reactions to the common pathogen races identified. To determine the race profile of P. sojae, A total of 359 P. sojae isolates were obtained from plant and soil samples were collected during June and July (2010, 2011 and 2012) from 203 farmers’ fields and the two Phytophthora nurseries in Woodslee and Ottawa. A set of eight differentials, each containing a single resistance Rps gene, and the universal susceptible cultivar Williams (rps) were used to test each of the 359 single-zoospore isolates of P. sojae. The differentials were grown in plastic pots containing a sterilized soil mixture with five plants per pot and four pots per differential in growth chambers. Plants were inoculated using a hypocotyl wound inoculation technique at the first-node stage. The percentage of dead plants was recorded 5 days after inoculation. If a differential exhibited <25% seedling mortality, the reaction was considered resistant. If a differential exhibited >50% seedling mortality, the reaction was considered susceptible. Seedling mortality from 26 to 50% was considered an intermediate reaction type (IRT).

A total of 24 races and two IRT were detected. Race 25 was the predominant race, comprising 51 isolates and representing 14% of the pathogen population, and races 3, 4, 5, 6, 7, 8, 9, 28, and 45 were commonly detected, each of these races represented 5 to 12% of the pathogen population. Of the 26 races and IRTs identified in this study, 18, including the predominant race 25, were identified for the first time in Ontario. Most isolates are virulent against Rps1a and Rps7, the earliest Rps genes known to have been deployed. Various combinations (stacking) of Rps1c, Rps1k, Rps3a, and/or Rps6 still have the potential to control the most prevalent races found in this study. However, the emergence of new races and more complex pathotypes underscores the need for new sources of resistance to P. sojae, such as new Rps genes (major gene resistance) or partial (multi-gene resistance) resistance.

Greenhouse screening trials to evaluate 55 short-season soybean varieties obtained from breeding programs in Ontario for major gene resistance to six common races of P. sojae (races 1, 3, 4, 5, 6, and 25) identified from the present study were conducted in 2013 and 2014. Of the 55 soybean lines evaluated, 18 were resistant to race 1 only; one was resistant to races 1, 3, and 6; 13 were resistant to races 1, 3, 4, and 25; and one (OAC Huron) was resistant to all 6 races. The results indicate that sources of resistance to the new races of P. sojae are available in Ontario soybean and these resistant lines may be used as sources of resistance to PRR for new cultivar development for Ontario.

Twelve partially resistant short-season soybean cultivars were artificially inoculated with a mixture of three common races of P. sojae (races 7, 25, 38) under field conditions in 2012 and 2013. These 12 cultivars had seed rot ranging from 0.4 to 2.5% and mortality ranging from 0.0 to 3.2%, which were significantly lower than of that of the susceptible cultivar OX20-8 that was used as control in these trials and had 25.5% seed rot and 38% plant mortality, based on the average of the two years.Nine of the 12 cultivars had yield reductions ranging from 4.7 to 38.1% and were significantly more tolerant than the susceptible control (73.0%). AC Bravor, Maple Donovan, Nattawa, Crest and AC Hercule were the most tolerant cultivars with yield reduction of less than 25%. These cultivars can be used as sources of partial resistance and tolerance to PRR for future cultivar development.

Project Related Publications

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This project was funded in part through Grain Farmers of Ontario.