Pulse and soybean diseases to watch

Soybean cyst nematode (l) and white mould in soybeans. Photo: USDA/Syngenta

While dry conditions during the 2018 growing season didn’t stop all instances of foliar, stem and root rot diseases across Western Canada, pulse growers didn’t see the high levels of pulse or soybean diseases associated with warm, moist conditions. But that doesn’t mean scouting won’t be necessary in 2019.

The main diseases in pulse and soybean crops are foliar diseases such as septoria brown spot, bacteria blight, downy mildew, stem diseases such as sclerotinia (white mould) and phytophthora (exclusive to soybeans), and root rots like fusarium, rhizoctonia and pythium.

Foliar diseases are very conspicuous. “If you’re walking in the field you see yellow spots on the leaves, and it looks bad, but the foliar diseases actually have the lowest impact on yield and quality so we’re encouraging farmers and agronomists to keep an eye out for the root and the stem diseases,” says Cassandra Tkachuk, production specialist with the Manitoba Pulse & Soybean Growers (MPSG). “White mould is a stem disease and we don’t always see it and not in high numbers. We keep track of how prevalent these diseases are year by year and if there are any new ones, we’re trying to identify if they’re here yet. The root rots including phytophthora and fusarium, are our top root rots. There’s also rhizoctonia and pythium but we don’t see those ones very much.”

Root rot issues

Saskatchewan saw some root rot issues in 2018 despite the dry weather, not that it impacted yields that much in the drier areas, says Sherrilyn Phelps, agronomy manager with Saskatchewan Pulse Growers (SPG). “In peas and lentils, we’re seeing a lot of fusarium as well as aphanomyces. With soybeans, we did see some phytophthora stem rot this year, which was confirmed by lab tests. Root rots on some of the other pulses, chickpea and fababean, weren’t much of an issue.”

AFFC pathologists, Debra McLaren in Brandon and Bob Conner in Morden are conducting research into soybean root rot diseases, and samples are sent to them each year for analysis.

In Manitoba, root disease surveys of soybean have been conducted with the assistance of staff from Manitoba Agriculture and the Manitoba Pulse & Soybean Growers. “Our research has shown that root rot of soybean in Western Canada is primarily caused by several species of fusarium,” says McLaren.

Drs. Sheau-Fang Hwang and Kan-Fa Chang at the Crop Development Centre North in Edmonton recently reported on the occurrence of Fusarium proliferatum as a new root rot pathogen of soybean in Canada. The same fusarium species can also cause damping-off or seedling blight of soybeans in which the seedlings die before or shortly after they emerge.

Field studies were conducted at Morden and Brandon to determine the effects of different root rot pathogens on the yields of soybean varieties with different levels of resistance. “It is critically important to quantify the magnitude and nature of damage caused by root disease in order to determine the need for developing and implementing new disease management strategies,” says Conner. “Understanding the disease-yield relationships is a prerequisite for measuring the agronomic efficacy and economic benefits of the management measures.”

Rotation is key

Crop rotations can be helpful in controlling root rot, soybean cyst nematode (SCN) and phytophthora rot. If inoculum levels are high, Conner says, “crop rotations of five years or more with non-host crops like cereals and certain oilseed crops is effective in reducing the inoculum levels of the pathogens in the soil, so there is less disease and yield loss when soybeans are grown.”

Planting soybeans in warm, well-drained soils will reduce the severity of root diseases of soybeans. Agricultural practices that promote plant growth such as adequate soil fertilization and avoiding soil compaction and salinity will also reduce the adverse effects of root diseases on soybeans.

A number of fungicide seed treatments are available to control seedling blight caused by root pathogens, but most products do not persist long enough to effectively control root diseases past the seedling stage. “Many soybean cultivars have been developed that carry resistance to many of the common races of P. sojae or to the SCN, but growing resistant cultivars in short rotations should be avoided in order to prevent the buildup of new races that can attack formerly resistant cultivars,” advises Conner. “In future, resistance to root rot diseases in soybeans will become more common enabling better disease control and reducing losses in seed yield.”

This year McLaren, Conner and their team have expanded the scope of their studies on root rot and phytophthora rot to include all of the soybean-growing regions in Western and Eastern Canada. “We have co-operating scientists and crop specialists in each province, who will be conducting annual surveys of soybean root diseases to determine the incidence and severity of each disease and to identify the prevalent root pathogens and detect the presence of any new root diseases or new races of pre-existing pathogens.”

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