In Objective 1, we utilized a molecular identification method developed by Dussault-Benoit et al. (2020) to predict the pathotypes of current P. sojae isolates in Minnesota.. Our molecular predictions suggest that the P. sojae pathotypes in Minnesota are complex, with 76% of the 25 isolates having the potential to defeat at least 5 Rps resistance genes in soybean (Table 1). Importantly, our molecular predictions reveal that Rps3a and Rps6 resistance genes may effectively provide immunity to the current isolates (Table 1, E). We have therefore tested these predictions on soybean varieties that carry Rps3a, Rps6, as well as Rps1c and Rps1k (resistance genes that are no longer effective, but still widely deployed) (McCoy et al. 2023). Our initial results indeed show that Rps3a and Rps6 soybeans are immune and Rps1c and Rps1k soybeans are not immune to the current isolates tested (Table 1, F-I), potentially suggesting that Rps3a and Rps6- containing varieties might be good candidates to deploy more widely in the state.

In Objective 2, of the 21 lines, 3 exhibited complete resistance (highlighted dark green, Fig. 3a) and 3 others exhibited near complete resistance (highlighted light green, Fig. 3a) when challenged with a complex or simple pathotype. It is unknown which Rps resistance genes, if any, are present and active in these lines. Because it is unlikely that they carry many of the common Rps genes, the other possible explanation is that they carry quantitative resistance genes that may be contributing to resistance. Experimental repeats are currently being performed.

Figure 3. Tolerance of 21 breeding lines to P. sojae. (a) 21 soybeans with different genetic backgrounds selected from Aaron Lorenz breeding program were inoculated with either a P. sojae isolate with pathotype 1c (vir-1), pathotype 1a, 1b, 1c, 1d, 1k (vir-5), or agar vehicle (negative control). Shown are % diseased. (b-d) Representative images of plants 7 days post inoculation.