Optimizing the screening of sugarbeet lines for resistance to Aphanomyces seedling damping-off.
The soilborne oomycete Aphanomyces cochlioides is the causal agent of Aphanomyces seedling damping-off and root rot, important diseases of sugarbeet in Minnesota and North Dakota as well as other sugarbeet growing regions of the world. Various chemical and cultural control measures can reduce disease severity; however, losses due to infection can still be severe under favorable environmental conditions. Adult plant resistance can be effective in limiting losses due to disease, but the genetic mechanism underlying resistance is largely unknown. Screening for resistance is typically done in naturally infested fields or established field disease nurseries. Field-based screening is not effective for phenotyping individual plants and also disease pressure is not uniform across a given field. Information on individual plants will facilitate the identification of regions of the sugarbeet genome associated with resistance to Aphanomyces. Two inoculation types (mycelium and zoospores) were tested for screening sugarbeet lines for resistance to seedling damping-off. Two A. cochlioides isolates, both originated from disease nurseries, were used to determine whether or not the optimal inoculation procedure is different depending on the isolate used. For both isolates, inoculation by mycelial suspension resulted in less disease, as measured by root rot index values, compared to inoculation by zoospores. Additionally, disease severity varied between experiment replications to a greater degree in mycelial inoculations than those inoculated by zoospores. In order to identify the ideal inoculum concentration that allowed for discriminating (phenotypic) resistance rating of sugarbeet lines, four zoospore concentrations were compared. Disease severity showed no significant differences based on inoculum concentration, as the majority of plants were dead by 14 days post-inoculation at all concentrations, whereas disease progress, measured by the change in percentage of living plants compared to initial stand, was statistically different between concentrations. The zoospore concentration that led to sufficient symptom development in susceptible plants was found to be at 5,000 zoospores per pot. Taken together, these experiments suggest that zoospore inoculation (particularly at 5,000 per pot) can be used for phenotyping sugarbeet germplasm for resistance to Aphanomyces seedling damping-off.