Effect of low temperature on the aggressiveness of Rhizoctonia solani AG 2-2 isolates on sugar beet (Beta vulgaris) seedlings

Authors: MINIER, DOUGLAS - Michigan State University; Hanson, Linda

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2021
Publication Date: 11/9/2021
Citation: Minier, D.H., Hanson, L.E. 2021. Effect of low temperature on the aggressiveness of Rhizoctonia solani AG 2-2 isolates on sugar beet (Beta vulgaris) seedlings. Plant Disease. 105(10):3111-3117. https://doi.org/10.1094/PDIS-09-20-1990-RE.
DOI: https://doi.org/10.1094/PDIS-09-20-1990-RE

Interpretive Summary: The fungus Rhizoctonia solani is classified into several groups, called anastomosis groups (AG). Among these, R. solani AG2-2 can cause root rot and seedling death, often called “damping-off”, to several important crops, including all types of beet. The recommendation for sugar beet is to plant early when soil temperatures are low. This allows for a long growing season and can reduce the risk of damping-off from several pathogens, including R. solani. It has been suggested that there is no risk of Rhizoctonia damping-off at temperatures below 15°C, but studies in Ireland indicated this might not be the case. To investigate this further, we exposed sugar beet seedings to a variety of R. solani isolates at 11°C and 21°C. While disease progress and growth rate were greatly reduced at 11°, disease developed in plants at both temperatures. Three weeks after inoculation, 16% of the plants were dead and 77% of the 35 isolates tested had average disease severity scores that were significantly greater than the mock inoculated control at the lower temperature. This is lower than the 34% of the plants dead or dying, and 97% of the tested isolates causing more disease than the controls at 21°C but confirms that infection could occur, and disease develop, at the lower temperature. This indicates that waiting until the soil warms up to above 15°C to apply management, such as fungicides, could leave the crop at risk. Virulence does not appear to be related to fungal growth rate but rather depends on the response of the specific isolate to low temperature.

Technical Abstract: Rhizoctonia solani anastomosis group (AG)2 2 can cause seedling damping-off in sugar beet and substantial losses may occur in all regions where beets are grown. Sugar beets are planted early in the season when soil temperatures are low in order to maximize the length of the growing season and minimize the risk of damping-off. However, the common perception that there is little to no risk of Rhizoctonia damping-off at temperatures below 15°C may not be entirely reliable. We tested this possibility by inoculating sugar beet seedlings in a growth chamber at 11°C with 35 R. solani AG2 2 isolates that were representative of the genetic diversity present in AG2 2. While disease progress and growth rate were greatly reduced at 11°, considerable disease symptoms did develop in inoculated plants. Three weeks after inoculation, 16% of the plants were dead and 77% of the isolates tested had average disease severity scores that were significantly greater than the mock inoculated control. This confirms our concern about the possibility for low temperature infection of sugar beet and indicates that waiting until the soil warms up to above 15°C to apply management, such as fungicides, could leave the crop at risk. Virulence does not appear to be related to phylogenetic group or growth rate but rather depends on the response of the specific isolate to low temperature.