Interaction of rhizoctonia solani and leuconostoc spp. causing sugar beet root rot and tissue pH changes in Idaho

Rhizoctonia root and crown rot in sugar beet is a serious yield limiting disease problem caused by an interaction between Rhizoctonia solani and Leuconostoc spp. in Idaho. To better understand this synergistic interaction, the two most common Leuconostoc haplotypes were paired with 17 R. solani strains representing the range of genetic diversity established previously in Idaho. The interaction study was conducted by inoculating sugar beet roots in the field using a plug assay in 2017 and repeated in 2018. The study had 54 treatments established by inoculating with the Leuconostoc and R. solani strains individually and in combination and comparing them versus a water check by measuring subsequent root rot and pH changes. The rot data for both years was evaluated together since years did not differ (P <0.0001). With the bacterial and 13 of 17 fungal strains inoculated individually, rot (8 mm or less) was minor enough that it did not differ from the water check (0 mm). In the top 20 ranking treatments with the most rot, there was only one non-combination treatment, R. solani strain F514. When comparing the bacterial strains in combination with the fungal strains, L. mesenteroides strain L12311 (17 mm of rot) had significantly (P < 0.0001) more rot than L. pseudomesenteroides strain L12487 (13 mm). The R. solani anastomosis group (AG), 2-2 IIIB strains (16 mm of rot) led to significantly (P <0.0001 to 0.0073) more rot than strains associated with other AG: 2-2 IV (11 mm), 4 HG-I (5 mm), and 4 HG-II (3 mm). With strains from the three phylogenic groups within AG-2-2 IIIB, rot ranged from 15 to 17 mm and did not differ (P = 0.1275 to 0.5565). The pH for sugar beet root tissue associated with at least 30 mm of rot was lower (4.2 ± 0.45 in 2017 and 4.0 ± 0.25 in 2018) than tissue with < 2 mm of rot (6.2 ± 0.17 in 2017 and 6.4 ± 0.14 in 2018). Both the isolations and pH changes suggest Leuconostoc spp. along with bacterial and yeast contaminants dominate in rotted root tissue but without R. solani AG-2-2 IIIB strains being present when rot initiates, Leuconostoc strains caused very little damage