Interaction ofRhizoctonia solaniandLeuconostocspp. causing sugar beet root rot and tissue pH changes in Idaho

Late season Rhizoctonia root 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 interaction, the two most common Leuconostoc haplotypes were co-inoculated with 17 R. solani strains representing the range of genetic diversity established previously. The study was conducted twice by inoculating sugar beet roots in the field using a plug assay and measuring root rot and tissue pH. L. mesenteroides strain L12311 (17 mm of rot) had significantly (P < 0.0001) more rot than L. pseudomesenteroides strain L12487 (13 mm) when combined with the fungal strains. The R. solani anastomosis group (AG) 2–2 IIIB strains (16 mm of rot) led to significantly (P < 0.0001–0.0073) more rot than strains associated with other AGs: 2–2 IV (11 mm), 4 HG-I (5 mm) and 4 HG-II (3 mm). When R. solani AG-2-2 IIIB strains from three phylogenetic groups were compared, rot ranged from 15 to 17 mm and did not differ (P = 0.1275–0.5565). The pH for root tissue with at least 30 mm of rot was lower (4.0–4.2 ± 0.2–0.4) than tissue with < 2 mm of rot and water checks (6.2–6.4 ± 0.1–0.2). Both isolations and tissue pH suggest late season sugar beet root rot is primarily associated with Leuconostoc and secondary organisms. However, damage was minor without both R. solani AG-2-2 and Leuconostoc strains present when internal rot initiates.