Sugar beet root rot at harvest in the US Intermountain West

Root rot in sugar beet (Beta vulgaris) causes significant losses worldwide. To assess the distribution of root rot fungi and their relationship to bacterial root rot, commercial sugar beet roots with rot symptoms were collected at harvest time in the Intermountain West. Isolations for both fungi and bacteria were conducted using standard microbiological techniques, and the root area rotted was assessed. A subset of fungal isolates was tested for pathogenicity to sugar beet in greenhouse assays and field trials with and without manure. In the field survey of rotting beets, the fungi most frequently associated with root rot included Fusarium spp. (Fusarium oxysporum and Fusarium acuminatum with 24% and 15% of isolates, respectively), Geotrichum spp. (16% of isolates), Rhizoctonia solani (15% of isolates), and Mucor spp. (14% of isolates). However, only R. solani isolate F321 (AG-2-2IIIB) consistently caused rot in greenhouse pathogenicity tests. In the field survey, a mean of 6% of the root tissue had rotted for individual roots when fungi were isolated individually, whereas mean root rot was 71% and 68% when bacteria were isolated individually or in combination with other organisms, respectively. In field trials, roots inoculated with F321 averaged 3%-5% fungal rot, whereas the percentage of root tissue with bacterial rot was 6%-78%, which supports survey observations. Manure did not lead to root rots in the field. Traditionally, fungal root rots have been the main focus of breeding programs; however, because of the root area rotted by lactic acid bacteria, especially Leuconostoc, these bacteria should not be ignored in breeding efforts.