Soybean root rots more prevalent on saline soils

Twenty short season soybean cultivars were evaluated for root rot on saline soils. While there were differences among cultivars in tolerance to Fusarium avenaceum and Rhizoctonia solani and different salinity levels, all cultivars were still impacted to varying degrees.

The objectives of the research were to determine the effects of F. avenaceum and R. solani on Fusarium root rot and Rhizoctonia root rot in soybean at two levels of salinity, and to evaluate soybean cultivars against these root rots in saline soils under greenhouse and mini-plot trials.

In the first greenhouse experiment, 20 short-season soybean cultivars (provided by Monsanto Canada, Inc. and Pioneer Hi-Bred Ltd.) were evaluated under highly saline conditions (EC 21.1 dS/m, pH 8.2) to determine the reactions to root rot caused by F. avenaceum and R. solani under greenhouse conditions. Each cultivar was grown in soils inoculated with either F. avenaceum or R. solani, and were also evaluated in a non-inoculated control.

Seedling emergence was recorded 20 days after seeding. A Disease Severity Index (DSI) was used to rate the root rots at 28 days after seeding.

Differences in cultivar tolerance

High seedling emergence was observed for cultivars 900Y61, P002T04R, 900Y01, TH27005RR, P001T34R, and 900Y81 in the non-inoculated, highly saline soil control. In the treatments inoculated with F. avenaceum, P002T04R and 900Y61 had high seedling emergence. The cultivars 900Y61, 900Y81, and 900Y71 had high seedling emergence in the R. solani treatment.

Root rot severity was low for cultivars NSC Portage and 900Y61 in the non-inoculated control, and was low for P002T04R in the F. avenaceum treatment. There was no difference in the DSI among cultivars when the soil was inoculated with R. solani.

The presence of root rot in the non-inoculated control indicated that the soil had a background population of unspecified root rot pathogens.

Based on the results of this first experiment, the seven cultivars that showed the greatest resistance to root rot of the 20 soybean cultivars tested in the first experiment were separately evaluated against F. avenaceum and R. solani at low (EC 7.0 dS/m, pH 7.5) and high (EC 18.0 dS/m, pH 7.9) soil salinity levels, and two levels of inoculum for each pathogen [F. avenaceum; 5× and 10× inoculum-to-sand dilution; R. solani; 10× and 50× inoculum-to- sand dilution]. A non-inoculated control was also included.

Seedling emergence was reduced and the Fusarium root rot of soybean was more severe under high salinity compared with low salinity. Seedling emergence was also reduced and root rot severity increased when inoculated with F. avenaceum, but there were no differences in seedling emergence or root rot severity with different inoculum concentrations.

Inoculation with R. solani also reduced seedling emergence and increased root rot severity more under high salinity compared with low salinity. The lowest seedling emergence and most severe root rot were observed at the high level of R. solani inoculum.

Seedling emergence and reactions of seven soybean cultivars to levels of inoculation with Fusarium avenaceum (A–C) or Rhizoctonia solani (D–F) in low-saline (7 dS m−1) and highly-saline (18 dS m−1) soil under greenhouse conditions (A and D).

Source: K.F. Chang et al. 2018

The second experiment assessed the response of two soybean cultivars, 900Y61 and 23-60RY to F. avenaceum and R. solani in a low saline soil (EC 9.17 dS/m, pH 6.8). The two cultivars were chosen based on the results of the previous experiment with 900Y61 being relatively tolerant to salinity under non-inoculated conditions, while 23-60RY showed a low level of emergence.

Following inoculation with F. avenaceum, seedling emergence, plant height, and seed yield were greater, and disease severity was lower for 900Y61 compared with 23-60RY, but were the same in the non-inoculated control.

Plant height was greater and disease severity was lower for cultivar 900Y61 compared with 23-60RY when inoculated with R. solani. In the non-inoculated control, seedling emergence was greater for 900Y61 compared with 23-60RY, but plant height, seed yield, and disease severity were the same for both varieties.

In summary, the researchers caution that the trials were not conducted to endorse any soybean variety, but rather to demonstrate that salinity predisposes soybean to root rot and reduces soybean productivity in western Canada. They conclude that growers must avoid salinity stress when growing soybeans.

Funding was provided by the Saskatchewan Pulse Growers and Manitoba Pulse and Soybean Growers, with support from the Government of Canada through the Agri-Flex Program, Growing Forward 2, and Pest Management and Surveillance Implementation (Pulse Science Cluster of Agriculture and Agri-Food Canada), and.

K.F.Chang, S.F.Hwang, R.L.Conner, H.U.Ahmed, Q.Zhou, H.Fu, G.D.Turnbull, R.Nyandoro, S.E.Strelkov, D.L.McLaren, and B.D.Gossen. Effects of Fusarium avenaceum and Rhizoctonia solani on the growth of soybean in saline soils. Canadian Journal of Plant Science. 99(2): 128-137.

Photo by Sheau-Fang Hwang

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