Rhizoctonia Root Rot – Can resistant sugar beet varieties contribute to control the disease?

The breeding of rhizoctonia-resistant sugar beet varieties appears to offer a promising way to control increasingly widespread late root rot. In initial trials, rhizoctonia-resistant genotypes after natural inoculation compared favourably with susceptible standard varieties in terms of lower disease incidence and higher recoverable sugar.

Resistance testing for the purposes of identifying breeding material or assessing varieties in approval proceedings in naturally vulnerable locations is methodologically difficult in the case of rhizoctonia. The unpredictable, frequently clusterwise appearance of the disease reduces the evidentiary value of performance tests. Methodological approaches towards artificial inoculation of sugar beets with the causal organism Rhizoctonia solani (Kühn) (Hecker and Ruppel, 1977) have existed for years and should be further developed with a view to the needs of rhizoctonia resistance breeding and the demands of the variety testing system in Germany. The expression of rhizoctonia resistance in sugar beet genotypes is more easily registered and reproducible following standardised inoculation. Initial field trials with artificial inoculation show, compared with corresponding trials under natural attack, a more uniform incidence and therefore have greater evidentiary value for the assessment of a sugar beet variety. The Institute for Sugar Beet Research in Göttingen is currently test-ing an assessment method which aims to register and quantify resistance to late root rot.

In greenhouse trials and in the field, young sugar beet plants are inoculated with defined amounts of fungus mycelium and the damage expression is evaluated and quantified on the basis of the visual damage and sugar beet yield and quality data. The results so far are encouraging and demonstrate that while all genotypes show an effect after inoculation with Rhizoctonia solani, the degree of attack and the expression of damage vary depending on the genotype.