Greenhouse and field techniques for testing sugar beet for resistance to Rhizoctonia root and crown rot

Abstract

Rhizoctonia‐resistant sugar beet varieties are the key to an integrated control strategy for Rhizoctonia root rot. Because of the unpredictable occurrence of Rhizoctonia solani in the field testing of sugar beet for resistance to Rhizoctonia root rot is difficult. The aim of the study was to develop advanced greenhouse and field techniques which allow a reliable assessment of sugar beet for resistance to R. solani. A highly infectious liquid inoculum was used for the first time in this study. It can be produced in large quantities of a standardized quality, sterile, and exactly quantified according to its carbon content. In a greenhouse trial, sugar beet grown in the same way as field grown beet was inoculated by applying a suspension of Rhizoctonia mycelium (equivalent to 10 mg carbon per plant) to the beet crown. After 3 weeks, inoculation had led to uniform and severe root rot. Disease symptoms were similar to those found under natural conditions in the field. No ‘escapes’, i.e. susceptible plants apparently expressing resistance were observed in the test. A new nine‐class disease scale was established and a Rhizoctonia index (RI) was calculated. Reliability of disease assessment was demonstrated on progeny of plants, selected from segregating populations, showing Rhizoctonia resistance more closely related to the resistant parent lines than to the susceptible ones. Sugar beet varieties could be assessed in the greenhouse within only 11 weeks. All varieties were affected by the pathogen but partially resistant varieties could be clearly recognized by a significantly lower Rhizoctonia index. Significant differences in susceptibility were also found within the group of new resistant genotypes. Infection studies performed in the field showed the superiority of the new liquid inoculum compared with a solid form and revealed the influence of inoculation date and inoculum level on the development of Rhizoctonia root rot. In field tests performed at different sites under different environmental conditions, susceptible and partially resistant sugar beet varieties could be reproducibly rated according to their susceptibility to R. solani. On average, susceptible varieties showed a Rhizoctonia index of 8 while resistant genotypes ranged from 5 to 6. The newly developed techniques allow fast and reliable evaluation of sugar beet for resistance to R. solani.

Selection for Resistance to the Rhizoctonia-Bacterial Root Rot Complex in Sugar Beet

“…This observation is important because good resistance to R. solani alone should allow for control of the Rhizoctonia-bacterial root rot complex. Traditionally, Rhizoctonia root rot has been evaluated by placing roots into disease classes based on a 0-to-7 scale (32) or a 0-to-9 scale (9). In the present study, percentage data were utilized so that both fungal and bacterial rot could be assessed simultaneously and in a continuous manner.…”

“…Rhizoctonia root rot caused by Rhizoctonia solani Kühn is of considerable concern in sugar beet production areas in the United States and other areas of the world (9,15,38). Rhizoctonia root rot can lead to root yield losses of 50% or more but also seems to be on the increase and can be associated with losses in storage (9,21,27,38,41,43).…”

“…Rhizoctonia root rot can lead to root yield losses of 50% or more but also seems to be on the increase and can be associated with losses in storage (9,21,27,38,41,43). In Idaho, Rhizoctonia root rot on mature roots tends to be associated with the R. solani anastomosis group (AG) 2-2 IIIB strains and is frequently accompanied by a bacterial root rot caused by Leuconostoc mesenteroides subsp.…”

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

“…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).…”

Study of the interaction mechanism between Heterodera schachtii and Rhizoctonia solani using a split-pot design on sugar beet

“…Rhizoctonia solani Kühn (teleomorph Thanatephorus cucumeris [Frank] Donk) AG 2-2IIIB (Ogoshi, 1996) is one of the most important sugar beet diseases in the world and causes yield losses of up to 50% (Herr, 1996;Büttner et al, 2004). In the USA up to 24 % of the sugar beet cropping area is endangered by RCRR and in the European Union 5% of the sugar beet areas are infested (Harveson, 2008).…”