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Yellow Soybeans Caused by Phytophthora

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.

SYMPTOMS

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.

MANAGEMENT

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

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