Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2022
Publication Date: 3/1/2022
Citation: Paul, C.P., Walker, D.R. 2022. Aggressiveness of isolates from five Pythium species on seeds and seedlings of six North American soybean cultivars. Canadian Journal of Plant Pathology. 44(4):596-614. https://doi.org/10.1080/07060661.2022.2036814.
Interpretive Summary: Numerous species of Pythium, a soilborne pathogen, cause seed rot, seedling death (damping-off), and root rot of soybean. Fourteen isolates (collected strains) of five different Pythium species were studied in seed and seedling experiments for their ability to cause disease or mortality in five soybean cultivars. Data were collected to assess the effects of infection with the different isolates on seed germination, seedling emergence and root health. Variation was found among isolates of the same species as well as among the five Pythium species for their ability to cause disease and death. Specific isolates from three different species (P. aphanidermatum, P. spinosum and P. ultimum var. ultimum) caused the most severe disease reactions of the isolates used in the experiments, but other isolates from these species did not cause much disease. Overall, 'Archer' and the Canadian cultivar 'Maple Glen' had the least disease of the soybean lines used. The results indicate that an aggressive isolate from each species should be used to screen soybean breeding lines for resistance and that Canadian cultivars like Maple Glen that germinate well in cool, damp soils could be a useful source of Pythium resistance genes.
Technical Abstract: Pythium aphanidermatum, P. irregulare, P. spinosum, P. sylvaticum and P. ultimum var. ultimum are among the species of Pythium that cause seed rot and damping-off of soybean grown in the United States. A study was conducted to assess the relative aggressiveness of 14 isolates representing these five species on six soybean cultivars expected to have varying levels of resistance. Disease was assessed based on seed infection in Petri plate assays and on seedling emergence, plant weight, root weight and root rot severity in temperature-controlled greenhouse assays. Inoculated seeds and seedlings were compared with noninoculated seeds and seedlings of the same cultivar. Relative aggressiveness varied among Pythium species and isolates within some species, and isolate × cultivar interactions were also significant (P < 0.0001). Pythium ultimum var. ultimum isolate PU 350 was more aggressive than most of the other isolates, whereas P. aphanidermatum isolate M17-419 caused only low levels of disease. Most of the Pythium species reduced seed germination and the Arkansas isolates KS17-166B (P. aphanidermatum), KS17-63 (P. spinosum) and KN17-155 (P. ultimum var. ultimum) caused severe seed rot on ‘Conrad’, ‘Sloan’ and ‘Williams’. In contrast, moderately resistant cultivars like ‘Maple Glen’ and ‘Maple Isle’ had either no symptoms or only slight root discoloration in the Petri plate assays. There was a significant negative correlation between seed rot severity and seedling emergence (r = -0.93, P < 0.001), and the slope of the regression also showed a strong relationship (R2 = 0.88). ‘Archer’ and Maple Glen were the most resistant cultivars overall in the greenhouse assays. This study revealed variation in the relative ability of the five Pythium species to cause seed rot, damping-off and severe root rot on the seedlings, in addition to differences in the aggressiveness of P. aphanidermatum and P. ultimum var. ultimum isolates. The results also indicated that some Canadian soybean cultivars like Maple Glen carry genes that could be useful for developing cultivars with broader or more effective Pythium resistance.