This article was written by Horst Bohner, OMAFRA Soybean Specialist, and Albert Tenuta, OMAFRA FIeld Crop Pathologist
Phytophthora root rot (PRR) is one of the most destructive soybean diseases in Ontario. PRR thrives in wet soils, so the incidence of this disease is more prevalent in fields that received excess rainfall in 2021. Clay soils, poorly drained areas, and no-till fields were also impacted more severely. PRR can harm or even kill plants any time throughout the growing season, right from seeding to maturity. Because of early dry conditions this year, symptoms were often not evident until mid-season. Fields that turned yellow after heavy rainfall but did not recover as the soil dried should be checked for PRR (Figure 1). It’s important to know if a field has suffered from this disease so future management decisions can be made to minimize its impact. PRR is largely managed by selecting varieties with resistant genes and the use of fungicide seed treatments.
Seeds may fail to emerge or infected seedlings die shortly after emergence, displaying typical “damping off” symptoms. Infected areas of the stem are water-soaked or “bruised” and soft. Early in the season symptoms are similar to other root rots such as Pythium, so it can be difficult to identify Phytophthora without a lab analysis. Plant samples can be submitted to the University of Guelph Pest Diagnostic Clinic to confirm the disease. On mature plants, yellow leaves or wilting is often noticed first. Examination of the roots reveals a browning and rotting of both the tap root and secondary roots. Plants are easily pulled from the ground since they are not well anchored. The brown or purple discolouration may extend up the stem from the roots into the lower parts of the plant. A few dead plants may appear in a row or as patches, especially in low areas of the field. One distinguishing feature of PRR is that the leaves remain attached even after plant death (Figure 2). Plants that are partially resistant may just be stunted but not die from the disease. PRR is most severe when soil temperatures are above 15°C; plants are at higher risk of infection and symptom development.
Other diseases such as Stem Canker and White Mould can be misidentified for PRR from the field edge (Figure 3) therefore it is important to examine affected areas, especially late in the season.
Disease Life Cycle
Phytophthora sojae is an oomycete, which is a fungus-like microorganism. Soybeans are the only known crop host of this species. PRR is found in most soybean growing regions. The thick-walled overwintering spores of the pathogen are called oospores. They overwinter in the soil and plant residue. Under wet conditions they will germinate and directly infect roots or produce mobile spores (zoospores) that will “swim” in the water film between soil particles to infect soybean roots when soil is saturated. PRR colonizes the root tissue and will plug the water-conducting tissue of the plant, causing wilting. Oospores can be moved by machinery or animals and can survive in the soil for many years.
Genetic resistance is the primary method to control PRR, but control requires an integrated combination of variety selection, seed treatments, and good management practices. There are various forms of genetic resistance (Rps gene) and partial resistance (field tolerance) available, so it’s important to check the specific variety before planting a field with a history of PRR. Table 1 of the Ontario Soybean and Canola Committee performance trials lists the source of PRR resistance (Rps genes), as well as the 2-year percent stand loss (field tolerance) observed in a high PRR environment (see gosoy.ca).
The benefit of using Rps genetic resistance is they provide complete resistance to certain Phytophthora pathotypes/strains (previously called races) but are susceptible to others since only a singe gene confers resistance (Figure 4). In Ontario, the Rps1a and Rps7 provide no control since all fields contain Phytopthora pathotypes which can bypass these resistance sources. Rps1c and 1k varieties are still effective but their effectiveness is declining as recent surveys conducted by Dr. Owen Wally (AAFC Harrow) and Albert Tenuta (OMAFRA Ridgetown) have shown. The Rps3a gene is being included in more commercial soybean varieties and is extremely effective in the province.
Field tolerance or partial resistance on the other hand is not dependent on pathotype in the field. Although PRR will still develop under favourable weather conditions, the symptoms will be less severe. It is important to select varieties which have effective Rps genes (3a, 1k, 1c, 8) along with good field tolerance.
There are various fungicide seed treatments containing metalaxyl, metalaxyl-M, ethoboxam or oxathiapiprolin which suppress the disease and should be used in conjunction with resistant/tolerant varieties. See Table 2-9 of the Field Crop Protection Guide for a complete list (Publication 812, Field Crop Protection Guide). For additional control In fields with significant PRR history, consider adding oxathiapiprolin or ethaboxam to your base fungicide seed treatment containing Metalaxyl or Metalaxyl-M. Field drainage and tillage can also have benefits in minimizing the damage caused by PRR. Also, keep in mind that there is no substitute for a good crop rotation since this disease will not impact other field crops.