Evaluation of Rhizoctonia solani - Heterodera glycines interactions on soybean

Rhizoctonia solani and the soybean cyst nematode (Heterodera glycines; SCN) are two soilborne pathogens that co-infest fields in soybean-producing states, contributing to significant yield reduction. Both pathogens infect the soybean radicle early in the growing season causing pre- and post-emergence stand reductions. The pathogens thrive in analogous environments, suggesting the potential for an interaction. Field and greenhouse studies were conducted to evaluate four fungicide seed treatments in multiple combinations to evaluate this interaction as well as early season control of the pathogens. Seed treatments were applied to soybean cultivars that were resistant and susceptible to SCN. Field studies were conducted at two Illinois locations (Urbana and Carmi). In 2009, 2010, and 2011, at Urbana, IL and in 2011 at Carmi, IL, SCN HG type 2.5.7 occurred naturally, and natural R. solani pressure in the soil was augmented with sterilized grain sorghum colonized by R. solani. Soil samples were collected at spring planting and after fall harvest to determine SCN reproduction. Soybean stand was evaluated, and roots from the field were collected for digital scanning and root analysis to measure disease symptom severity at the V1 growth stage. R. solani root lesions were rated on a 0-5 scale at the V1 stage and used to produce a root rot index. At the end of the season, harvest data were collected. An infestation evaluation and a seed treatment evaluation were conducted in the greenhouse. In the infestation evaluation, infestations with both a R. solani mycelial suspension and SCN HG type 2.5.7. population at zero, low, and high levels on resistant (S35-T9) and susceptible cultivars (S36-B6) were evaluated. In the seed treatment evaluation, high levels of the mycelial suspension and SCN population were used to infest plots at planting to evaluate the control of four fungicide seed treatment combinations on resistant and susceptible cultivars. Initial plant emergence and sustained stand were assessed throughout plant growth. At the end of the trial, plant height, dry plant and dry root weights, lesion ratings, root characteristics, and SCN reproduction factor (Rf) data were collected. In the field, seed treatments with the active ingredient sedaxane were effective in reducing stand loss, root rot index, and yield losses in plots artificially infested with R. solani. The effect of seed treatments in field root scan analysis was not clearly defined; however, treatments containing sedaxane generally had greater root length, surface area, average diameter, tips, forks, and volume among treatments in infested plots, though significant differences were not consistent. In the greenhouse seed treatment evaluation, SCN Rf was greatest when seed treatments contained sedaxane, and the mean SCN Rf value in pots containing the mefenoxam + abamectin treated seed was not different from the mefenoxam-only control. Average lesion ratings were lowest for seeds treated with mefenoxam + sedaxane + abamectin. Lesion ratings in the mefenoxam + sedaxane treatment were not different from those of the mefenoxam control, which was not consistent with the field results for root rot indices. The SCN-resistant and SCN-susceptible cultivars in the field showed greater yields in non-infested plots; however, the resistant cultivar generally had greater yields than the SCN-susceptible cultivar among environments, though differences were not always significant. Cultivar had a significant effect on root tips among environments, where S35-T9 had a greater number of root tips compared to S36-B6 at Urbana in 2010, with no differences in numbers of root tips at Urbana or Carmi in 2011. The number of root tips for S36-B6 in infested plots was generally lower than that of S35-T9. There was no cultivar effect in either of the two greenhouse evaluations. There is not sufficient evidence to conclude that an interaction between R. solani and H. glycines occurs. Research using additional evaluations and controlled environments should be pursued further in the field and greenhouse settings before claiming that an interaction does not exist. Furthermore, this research showed sufficient evidence to claim that the active ingredient sedaxane was a highly effective seed treatment for the control of R. solani AG 2-2 under both field and greenhouse conditions