In 2022, cool wet conditions followed planting, exacerbating an intermittent problem that has been observed in Minnesota, seedling emergence issues, damping off, and higher incidence of root and stem rots (personal communication with Angie Peltier, Brett Arenz of the Plant Disease Diagnostics Clinic, and David Kee). Compacted soils, such as those in northwestern Minnesota, can become waterlogged and are especially prone to seedling diseases and root rots. When present, yield losses from these diseases can be significant, and symptoms are caused by a variety of pathogens including Phytophthora sojae. Planting varieties with genetic resistance to P. sojae is one of the best management options for P. sojae-associated diseases. However, resistance genes in the variety must align with the specific pathogen pathotype present in a grower’s field.
Phytophthora root and stem rot (PRSR) is caused primarily by the pathogen P. sojae (Dorrance et al., 2007). P. sojae is not a true fungus but is a member of a group of organisms called oomycetes that are more closely related to algae. While they do have a fungal-like growth habit, they produce motile spores (zoospores) that can ‘swim’ in films of water in the soil. Thus, organisms like P. sojae are often referred to as water-molds and are problems generally in water-logged or abnormally wet soils. There is also a second Phytophthora species that can cause PRSR and is known as Phytophthora sansomeana. Dean Malvick has previously identified and surveyed this species in Illinois (Malvick et al., 2004). Although it is present in Minnesota, the current distribution and potential contribution of this species to PRSR in the state is not known.
There are more than 50 pathotype s of Phytophthora sojae known in the U.S. (Dorrance et al. 2008). The main method of managing this pathogen with so many pathotypes is to use either pathotype - specific soybean resistance genes (Rps genes) or soybean varieties with incomplete resistance (partial resistance) or even varieties with both forms of resistance (Matthiesen et al., 2021). Despite the use of Rps genes and/or incomplete resistance, Phytophthora sojae can develop resistance and eventually overcome the soybean-host resistance deployed. Therefore, it is important to understand the pathogen population in a particular field, region, or state to be better able to deploy appropriate Rps genes and varieties with high levels of partial resistance. Interestingly, Rps genes are not effective on other species of Phytophthora such as Phytophthora sansomeana. In addition to understanding the Phytophthora sojae pathotype profile, understanding the Phytophthora species profile is also important in fields where soybean is planted.
In Minnesota, all forms of resistance have been deployed in modern soybean varieties. Phytophthora sojae population profiling has not been done recently. It has been approximately 10 years (2012 and 2013) since surveys were conducted understand the pathotype profiles of Phytophthora sojae in the state. At that time, Anne Dorrance suggested that many fields in Minnesota, were infested with Phytophthora sojae pathotype s that could be managed using the Rps 8 or 3a genes with the Rps 6 gene also being somewhat effective (2016), although commercial varieties with these resistance genes were new or uncommon at the time of previous study.
Research by Anne Dorrance et al. (2016) showed increasing virulence on lines containing Rps 1 genes, and research in Iowa and Nebraska indicated that over 70% of isolates collected during the 2016-2018 field seasons were virulent on soybean varieties deployed with Rps 1c or Rps 1k genes (Matthiesen et al., 2021). Varieties grown and screened in Minnesota most commonly contain genes from the Rps 1 locus (Lorenz et al. 2022 Variety Trials). Many of the soybean varieties being commercially sold, therefore, do not contain Rps genes that match the current Phytophthora sojae pathogen population.
Widely used methods for determining whether P. sojae isolates have overcome the plant resistance (pathotyping) are time consuming. It involves inoculating soybean lines with known Rps genes with an isolate of P. sojae and evaluating the isolate’s ability to infect the host to determine the Avr gene present (Dorrance, 2003 and Dussault-Benoit et al., 2020). In recent research, Avr genes were sequenced and primers were developed to amplify the specific Avr genes that are present in P. sojae isolates (Dussault-Benoit et al., 2020 and Tremblay et al., 2021). This method needs to be validated and can greatly increase the efficiency of pathotyping to predict whether soybean varieties will be resistant to field populations of P. sojae. Recently a private company using this method, AYOS, has started to offer pathotyping and variety recommendations to growers as a private service. However, this service has received few samples from Minnesota a validating the service as a decision making tool for growers may be useful for growers to make field-specific decisions about which resistant variety to deploy in their field.
Therefore, this proposal aims to provide guidance for soybean breeding programs and Minnesota farmers on Rps genes to manage PRSR and information on the potential risk of P. sansomeana as a yield limiting soilborne disease for which we do not have resistance tools.