By Bruce Barker, P.Ag.
Phytophthora root rot (PRR) is a significant disease in many areas where soybeans are grown. The fungus Phytophthora sojae causes PRR. Yield losses can be substantial.
In Manitoba, multiple reports of PRR in soybeans were received in the Red River Valley in 2016. A 2016 disease survey in Manitoba on 40 soybean fields found 38% of the fields identified with Phytophthora spp. In at least one case, the incidence was reported as up to 30% of plants affected. A disease survey is also being conducted in 2017, and Saskatchewan Pulse Growers will provide samples to Agriculture and Agri-Food Canada in Brandon and Morden to assess the prevalence of the disease in Saskatchewan. In Saskatchewan, while PRR is a concern, only limited testing has been conducted.
PRR is caused by an oomycete and thrives under waterlogged conditions. It can kill and damage seedlings and plants throughout the growing season, from planting until nearly harvest. This disease is favoured by wet, warm soil conditions, especially when soils are saturated early in the growing season.
Symptoms can show up in patches in the field (low spots) or as scattered plants. Severely infected plants will completely senesce, but the leaves will stay attached. PRR is common in areas of Manitoba with a history of soybean production and in years with waterlogged conditions.
Symptoms can be similar to those of other root rot diseases. For PRR, Sherri Roberts with the Saskatchewan Ministry of Agriculture says to look for areas within the field where plants are yellowed. Plants may be killed in pockets in the field, or remnants of dying seedlings can be scattered throughout a field.
Symptoms of PRR on older plants with low tolerance include yellowing between veins and leaf margins, and chlorosis of upper leaves, followed by wilting. Leaves remain attached after the plant dies. Cortex and vascular tissue are discoloured.
Stem symptoms are long, narrow, sunken brown lesions that progress up the stem from below the soil line. Occasionally, a lesion will occur on one side of the plant only, and plants with unilateral stem lesions may not wilt. Generally, the stem rot phase is easily identified by the dark brown colour on the exterior surface of the stem and lower branches.
Disease Management
More than 40 races of P. sojae have been identified. In North Dakota, a 2002 survey found the most prevalent races in this area were races 3 and 4 (about 80% of the isolates). Races 1, 5, 8, 21, 25, 28, 41, 43, and 44 are also found, but at low frequency. In Manitoba, race 4 is the most common, whereas in Saskatchewan, the pathogen’s races have not yet been identified. There can also be multiple races within a field.
Plant breeders have focused on developing soybean cultivars with major gene resistance. However, in established soybean-growing areas, pathogen races now exist in the population that can overcome the major resistance genes. As a result, growers need to know the races present in their fields, as variety selection that includes hybrids with resistant genes can help reduce the impact of the disease. The 2017 disease survey in Manitoba and Saskatchewan will attempt to identify the predominant races in each province.
The Manitoba Seed Guide provides information on resistant genes in soybean varieties that are effective against certain races of Phytophthora spp.
Agronomic management of the disease includes creating an environment that reduces the pathogen’s survivability. This may include using drainage to help manage soil moisture. Seed treatments are recommended, especially when cool, wet soil conditions develop after planting.
Crop rotation can also help prevent the buildup of inoculum, although it will not eliminate or eradicate PRR, since oospores can survive in the soil for long periods. Avoid a soybean on soybean rotation, as the Phytophthora population can build up and promote the development of new races.
If fields are known to have PRR, biosecurity measures such as cleaning machinery of infected soil before moving to uninfected fields may also help prevent the spread of the pathogen.
The bottom line for Saskatchewan soybean growers: scout fields for symptoms, and utilize agronomic practices to help prevent the development and spread of the disease.
