Age-dependent resistance to Rhizoctonia solani in sugar beet

Rhizoctonia crown and root rot of sugar beet (Beta vulgaris L.), caused by Rhizoctonia solani, has become the most important concern for the beet industry in Minnesota and North Dakota. Use of resistant cultivars is an important strategy in the management of R. solani in combination with seed treatment and timely fungicide application during the growing season. The objective of this greenhouse study was to determine how sugar beet plants responded to increasing age in resistance to R. solani. One susceptible, one moderately resistant and one resistant cultivar were obtained from each seed company of Betaseed, SesVanderHave and Syngenta. Seeds were planted at weekly intervals to create different plant age groups from seeds to ten week old plants, with growing degree days (GDD) ranging from 0 to 1519°Cd. They were all simultaneously inoculated by R. solani-infested barley. Twenty-eight days after inoculation, plants were pulled, washed, and roots evaluated for disease severity. All cultivars were highly susceptible to R. solani when inoculated at seeds to three-week-old plants (0-464°Cd). At four or five weeks of plant age (617-766°Cd), resistant cultivars started to show significant resistance to R. solani. Proportion of the harvested roots with disease score ≥ 5 followed a sigmoid response declining with increased GDD in resistant cultivars whereas it continued to decline linearly with increased GDD in susceptible cultivars. This study demonstrated that sugar beet cultivars, irrespective of their assigned level of R. solani resistance, were highly susceptible to the pathogen before they reached the six to eight leaf stage (617-766°Cd). Therefore, additional protection in the form of seed treatment or fungicide application might be required to protect vulnerable sugar beet seedlings in fields with a history of R. solani under favorable environmental conditions.
Abbreviations: AGs, anastomosis groups; QoI, quinone outside inhibitor; S, sugar beet cultivar susceptible to R. solani; MR, sugar beet cultivar moderately resistant R. solani; R, sugar beet cultivar resistant to R. solani; MDR, median disease rank; REDS, relative effects of disease severity; CI, confidence interval; SES, SesVanderHave.