Virulence, distribution and diversity ofRhizoctonia solanifrom sugar beet in Idaho and Oregon

Abstract

Rhizoctonia root rot causes serious losses on sugar beet worldwide. In order to help explain why Rhizoctonia root rot management practices have not performed well in some areas of the Intermountain West (IMW), a survey was conducted. In the IMW from 2004 to 2006, 94 Rhizoctonia solani field isolates were collected from sugar beet roots. These field isolates were compared with 19 reference strains and 46 accessions from GenBank for genetic diversity based on sequencing of the ITS-5.8S rDNA region. Greenhouse pathogenicity tests on sugar beet and silage corn were conducted and plant damage was assessed using a randomized complete block design with at least four replications. The majority of the isolates had sequence identity with the AG-2-2 IIIB (47%) or AG-4 subgroups (44%). Most of the AG-2-2 isolates (87%) were associated with fields in the western portion of the production area, while 71% of the AG-4 isolates came from the eastern portion of the production area. Isolates from AG-2-2 IIIB were frequently more virulent on sugar beet and sequence of the ITS-5.8s region required cloning because of genetic diversity within isolates. Seven (all AG-2-2 IIIB) of 18 isolates tested could attack both sugar beet and corn, while two of the seven virulent isolates caused less root rot on corn. To reduce Rhizoctonia root rot on sugar beet and corn, crop rotations and the isolates utilized for selecting host resistance could be given further consideration.Résumé: Le rhizoctone brun cause de graves pertes chez la betterave à sucre partout dans le monde. Afin d'expliquer pourquoi les pratiques de gestion du rhizoctone brun n'ont pas été très efficaces dans certaines des régions de l'Intermountain West (IMW) américain, une étude a été menée. Dans l'IMW, de 2004 à 2006, 94 isolats de Rhizoctonia solaniont été collectés, en champs, sur des racines de betteraves à sucre. Ces isolats ont été comparés à 19 souches de référence et à 46 obtentions de la GenBank afin d'en établir la diversité génétique basée sur le séquençage de la région de l'ITS-5.8S de l'ADNr. Des tests de pathogénicité ont été effectués en serre sur des betteraves à sucre et sur du maïs à ensilage. Le dommage subi par les plants a été évalué au moyen d'un dispositif en blocs randomisés avec au moins quatre réplications. La majorité des isolats possédait une identité de séquence avec les sous-groupes AG-2-2 IIIB (47 %) ou AG-4 (44 %). La plupart des isolats d'AG-2-2 (87 %) étaient associés à des champs de la partie occidentale de la région de production, tandis que 71 % des isolats d'AG-4 provenaient de la portion orientale. Les isolats d'AG-2-2 IIIB étaient très souvent plus virulents à l'égard de la betterave à sucre et la séquence de la région de l'ITS-5.8s a dû être clonée à cause de la diversité génétique trouvée chez les isolats. Sept (tous d'AG-2-2 IIIB) des 18 isolats testés pouvaient attaquer et la betterave à sucre et le maïs, tandis que deux des sept isolats virulents ont causé moins de pourridié sur le maïs. Il faudrait davantage porter attention aux rota...

Interaction of Sugarbeet Host Resistance and Rhizoctonia Solani Ag-2-2 Iiib Strains

“…A number of germplasm lines with Rhizoctonia crown and root rot resistance have been developed and utilized by commercial seed companies but resistance is quantitatively inherited and associated with yield drag, making cultivar development difficult (13,20,24). Preliminary evidence suggests that the cultivar selection process also may be complicated by strain interactions with resistance in commercial fields and disease screening nurseries (C. A. Strausbaugh, unpublished data).In previous investigations, differences in virulence between R. solani AG-2-2 IIIB strains were observed in both greenhouse and field studies (9,(31)(32)(33)38). Previous studies also suggest that resistance to R. solani should be stable across growing areas (3,15,27) but recent observations in Idaho field studies with R. solani AG-2-2 IIIB strains and commercial cultivars from different seed companies suggested that an interaction between strains and source of resistance might exist, at least in the presence of the bacterial rot complex (C. A. Strausbaugh, unpublished data).…”

“…Because R. solani strains form a species complex, strains have been further classified into subgroups known as anastomosis groups (AGs) and intraspecific groups (ISGs) (8,11,30). In Idaho, R. solani AG-2-2 IIIB is the primary AG and ISG associated with the most damaging rot in mature sugar beet roots (33). Recent Idaho studies have shown that the fungus is primarily limited to damaging the outer 3 to 5% of the root mass, while subsequent bacterial rot led by Leuconostoc mesenteroides subsp.…”

“…In previous investigations, differences in virulence between R. solani AG-2-2 IIIB strains were observed in both greenhouse and field studies (9,(31)(32)(33)38). Previous studies also suggest that resistance to R. solani should be stable across growing areas (3,15,27) but recent observations in Idaho field studies with R. solani AG-2-2 IIIB strains and commercial cultivars from different seed companies suggested that an interaction between strains and source of resistance might exist, at least in the presence of the bacterial rot complex (C. A. Strausbaugh, unpublished data).…”

“…Kühn can lead to yield losses of 50% or more in commercial sugar beet (Beta vulgaris L.) fields, affect sucrose losses in stored roots, and lead to difficulties in factory processing (6,17,33,36,39). Because R. solani strains form a species complex, strains have been further classified into subgroups known as anastomosis groups (AGs) and intraspecific groups (ISGs) (8,11,30).…”

“…Previous studies also suggest that resistance to R. solani should be stable across growing areas (3,15,27) but recent observations in Idaho field studies with R. solani AG-2-2 IIIB strains and commercial cultivars from different seed companies suggested that an interaction between strains and source of resistance might exist, at least in the presence of the bacterial rot complex (C. A. Strausbaugh, unpublished data). To investigate the potential for a resistance by strain interaction in sugar beet, lines with potentially different sources of Rhizoctonia crown and root rot resistance (most likely the sources of resistance for commercial cultivars) were compared versus a diverse set of R. solani AG-2-2 IIIB strains (24,33).…”