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Inheritance of Resistance to Rhizoctonia Root Rot in Sugarbeet¹

Segregating populations from crosses of rhizoctoniasusceptible and resistant strains of sugarbeet (Beta vulgalls L.) were studied for 3 years to determine the inheritance of resistance to root rot caused by Rhizoctonia solani Kuehn. Broad-sense heritabilities were increased by intensity of infection and ranged from 0.07 to 0.65 in the F₂'s. The degree of infection in inoculated field experiments was continuous, but was rated into eight disease index classes for purposes of measurement. Groupings into discrete classes (e.g., susceptible, intermediate, or resistant) were not possible. The partitioning method of genetic analysis was used to study the genetic control of resistance. A somewhat different gentic model was determined to be most descriptive in each of the 3 years. The majority of the variation in the segregating populations could not be assigned to one gene, but two genes did account for most of the variation. All three models involved two loci and two or three alleles. We concluded: i) two loci accounted for the majority of the expression of resistance; ii) resistance was partially dominant at the major loci in all but one instance; iii) year X genotype interactions affected resistance, but their magnitude was not practically important; iv) there was evidence that the two resistance parents did not have exactly the same genotypes for resistance; v) epistatic interactions appeared to affect resistance; and vi) minor or modifying genes probably were involved in resistance.

Key Words: Beta vulgaris L. • Rhizoctonia solani L. • Sugarbeet root rot • Genetic analysis • Disease resistance

² Research geneticist and research plant pathologist, USDA Crops Res. Lab., Colo. State U., Fort Collins, CO 80523.

Received for publication December 5, 1974.