Variation in the Resistance of Japanese Soybean Cultivars to Phytophthora Root and Stem Rot during the Early Plant Growth Stages and the Effects of a Fungicide Seed Treatment

Abstract

Soybean cultivars susceptible to Phytophthora root and stem rot are vulnerable to seed rot and damping-off of seedlings and young plants following an infection by Phytophthora sojae. In this study, the disease responses of Japanese soybean cultivars including currently grown main cultivars during the early growth stages were investigated following infections by multiple P. sojae isolates from Japanese fields. The extent of the resistance to 17 P. sojae isolates after inoculations at 14, 21, and 28 days after seeding varied significantly among 18 Japanese and two US soybean cultivars. Moreover, the disease responses of each cultivar differed significantly depending on the P. sojae isolate and the plant age at inoculation. Additionally, the treatment of 'Nattosyo-ryu' seeds with three fungicidal agrochemicals provided significant protection from P. sojae when plants were inoculated at 14-28 days after seeding. These results indicate that none of the Japanese soybean cultivars are completely resistant to all tested P. sojae isolates during the first month after sowing. However, the severity of the disease was limited when plants were inoculated during the later growth stages. Furthermore, the protective effects of the tested agrochemicals were maintained for at least 28 days after the seed treatment. Japanese soybean cultivars susceptible to Phytophthora root and stem rot that are grown under environmental conditions favorable for P. sojae infections require the implementation of certain practices, such as seed treatments with appropriate agrochemicals, to ensure they are protected from P. sojae during the early part of the soybean growing season.

Fig. 1. Soybean plants inoculated with Phytophthora sojae cultured on V8 juice agar medium (P. sojae–V8 culture flooding method). The photo shows ‘Nattosyoryu’ plants inoculated with Ps060626-4-1 isolate (the three pots in the front row) and treated with V8 agar as a control (the three pots in the back row).

Fig. 2. Temporal changes in the rates of dead soybean plants (%) following Phytophthora sojae inoculations of plants at different ages. Plants were inoculated with P. sojae isolate 13-B-RO-1 at 16-40 days after seeding. Mean values ± standard errors for three inoculated pots are plotted in two graphs (top: ‘Himeshirazu’; bottom: ‘Nattosyoryu’). Mean values with the same letters in each graph are not significantly different according to Tukey’s honestly significant difference test (p < 0.05).

Fig. 3. Comparison of the resistance to Phytophthora root and stem rot among 20 soybean cultivars at different plant ages. Soybean plants were grown for 14, 21, or 28 days and then inoculated with one of 17 Phytophthora sojae isolates. The boxplots were drawn based on the rates of dead plants (%) for the inoculation of plants grown for 14 days (A), 21 days (B), or 28 days (C). The open diamond in the plots indicates the average rate of dead plants for each cultivar. The mean values for each growth period were analyzed with Tukey’s honestly significant difference test. The same letters in boxplots indicate the mean values are not significantly different among the three growth periods (p < 0.01).

Fig. 4. Effects of a seed treatment with pesticides on the development of Phytophthora root and stem rot. ‘Nattosyoryu’ seeds treated with one of three pesticides containing fungicidal ingredients were sown, and plants grown for specific periods (14-40 days) were inoculated with Phytophthora sojae isolate 13-B-RO-1. The graph represents a plot of the mean rates of dead plants ± standard errors (%) for plants derived from pesticide-treated seeds (white square: amisulbrom; black circle: thiamethoxam, fludioxonil, and metalaxyl-M; grey triangle: cyazofamid) and non-treated controls (white circle). The mean values were significantly different between pesticide-treated samples and non-treated controls following inoculations at 14, 16, 20, 24, and 28 days after seeding according to Dunnett’s multiple comparisons test (p < 0.001).

Fig. 5. Effect of a seed treatment with pesticides on the inoculation with different Phytophthora sojae isolates. ‘Nattosyoryu’ seeds were treated with thiamethoxam, fludioxonil, and metalaxyl-M (A), or cyazofamid (B), and plants grown for 14 days after sown were inoculated with P. sojae isolates. The boxplots were drawn based on the rates of dead plants (%) caused by the inoculation. The open diamond in the plots indicates the average rate of dead plants for each treatment.

Table 1. Phytophthora sojae isolates used in this study

Table 2. Soybean cultivars used in this study and their planted area in Japan in 2012-2015^a

Table 3. Pesticides used in this study