Resistance to QoI and DMI fungicides does not reduce virulence of Cercospora beticola isolates in North Central USA

Authors: LIU, YANGXI - North Dakota State University; DEL RIO MENDOZA, LUISE - North Dakota State University; QI, AIMING - University Of Hertfordshire; Lakshman, Dilip; BHUIYAN, ZIAUR - North Dakota State University; Wyatt, Nathan; Neubauer, Jonathan; Bolton, Melvin; KHAN, MOHAMED - North Dakota State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2023
Publication Date: 2/23/2023
Citation: Liu, Y., Del Rio Mendoza, L.E., Qi, A., Lakshman, D.K., Bhuiyan, Z.M., Wyatt, N., Neubauer, J., Bolton, M., Khan, M. 2023. Resistance to QoI and DMI fungicides does not reduce virulence of Cercospora beticola isolates in North Central USA. Plant Disease. https://doi.org/10.1094/PDIS-11-21-2583-RE.
DOI: https://doi.org/10.1094/PDIS-11-21-2583-RE

Interpretive Summary: Cercospora leaf spot (CLS) is a destructive disease which limits sugar beet production and is managed using resistant cultivars, crop rotation, and timely fungicide applications. Since 2016, the fungal pathogen Cercospora beticola has been reported to be resistant to Quinone outside Inhibitors (QoIs) and have reduced sensitivity to Demethylation Inhibitors (DMIs) in sugar beet growing areas in North Dakota and Minnesota. Isolates of the fungus resistant to either QoIs or DMIs as well as to both QoIs and DMIs were collected from Foxhome fields, Minnesota in 2017, and analyzed for growth characteristics and ability to cause disease. The resistant fungal isolates were found to have significantly less mycelial growth and spore production than the sensitive isolates but were still able to cause as high disease severity as those of the QoI/DMI susceptible isolates. This information should be factored into CLS disease management practices. This information will be useful to plant pathologists, extension workers and sugar beet farmers.

Technical Abstract: Cercospora leaf spot (CLS) is a destructive disease which limits sugar beet production and is managed using resistant cultivars, crop rotation, and effective fungicide applications in a timely manner. Since 2016, the fungal pathogen Cercospora beticola has been reported to be resistant to Quinone outside inhibitors (QoIs) and have reduced sensitivity to Demethylation inhibitors (DMIs) in sugar beet growing areas in North Dakota and Minnesota. Isolates of C. beticola resistant to either QoIs or DMIs as well as both QoIs and DMIs were collected from Foxhome fields, Minnesota in 2017. Fitness of these resistant isolates was compared to that of both QoI- and DMI-sensitive isolates. The parameters of mycelial growth, spore production, and spore germination were measured in the laboratory. The results showed that the resistant isolates had significantly less mycelial growth and spore production than the sensitive isolates, while no significant difference in spore germination was detected. The measurement of aggressiveness was conducted in a greenhouse study. Six leaf-stage sugar beets were inoculated with spore suspension made from each resistant group and then incubated in separate humidity chambers. CLS disease severity was evaluated visually at 7, 14, and 21 days after inoculation (DAI) and the area under the disease progress curve (AUDPC) was calculated. The resistant isolates had significantly lower AUDPC but still caused as high disease severity as the sensitive ones at 21 DAI. This research demonstrated that although the QoI and/or DMI resistant isolates had a relatively slower disease development due to their fitness penalty on radial growth and spore production, they still caused high disease severity and need to be factored into disease management practices.