Improving the fungicide-based management of Cercospora leaf spot in table beet in New York

Authors: PETHYBRIDGE, SARAH - Cornell University; SHARMA, SANDEEP - Cornell University; HANSON, ZACHARIAH - University Of Tennessee; VAGHEFI, NILOOFAR - University Of Southern Queensland; Hanson, Linda; KIKKERT, JULIE - Cornell University

Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2019
Publication Date: 11/27/2019
Citation: Pethybridge, S.J., Sharma, S., Hanson, Z., Vaghefi, N., Hanson, L.E., Kikkert, J.R. 2019. Improving the fungicide-based management of Cercospora leaf spot in table beet in New York. Canadian Journal of Plant Pathology. https://doi.org/10.1080/07060661.2019.1690048.
DOI: https://doi.org/10.1080/07060661.2019.1690048

Interpretive Summary: Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is the dominant foliar disease affecting profitability of table beet production in New York, USA. The disease is largely controlled using fungicides but management is critically threatened by resistance to one class of fungicides, the strobilurins, within the C. beticola population and a limited suite of alternatives. This study evaluated the temporal dynamics of strobilurin fungicide resistance within the C. beticola population over three years, and examined the sensitivity to two other classes of fungicides, the demethylase inhibitors (DMIs) and carboxamides. Specifically two DMIs, (propiconazole and difenoconazole), and four carboxamides (fluopyram, penthiopyrad, boscalid and benzovindiflupyr), were tested. Resistance to a strobilurin was common and detected in all fields sampled in 2015 and 2016, and 60% of fields in 2017. No significant differences were detected in the frequency of resistant isolates within populations from fields with grown using conventional or organic practices in 2016 and 2017. For the other fungicides tested, reduced sensitivity to the two DMIs was detected in less than 1% of isolates, but all isolates grew on the highest carboxamides levels tested. Two small-plot replicated field trials were conducted over two years to evaluate alternatives to the commercial standards, propiconazole and the strobilurin, azoxystrobin. Two fungicide treatments were found that reduced CLS severity and epidemic progress compared to plots which received the industry standard and the untreated controls. These have shown some potential for disease management, but little evidence for alternatives for use in fungicide resistance management.

Technical Abstract: Cercospora leaf spot (CLS) caused by Cercospora beticola Sacc. is the dominant foliar disease affecting profitability of table beet production in New York, USA. The disease is primarily controlled by fungicides but sustainability is critically threatened by strobilurin resistance within the C. beticola population and a limited suite of alternatives. This study evaluated the temporal dynamics of strobilurin resistance within the C. beticola population over three years, and quantified sensitivity to selected demethylation inhibitors (DMIs; propiconazole and difenoconazole) and carboxamides (fluopyram, penthiopyrad, boscalid and benzovindiflupyr). Resistance to azoxystrobin (EC50 > 0.2 µg ml-1) was prevalent and detected in all fields sampled in 2015 and 2016, and six of 10 fields in 2017. Within fields, the incidence of azoxystrobin-resistant isolates ranged between 4.8% and 70%. No significant differences were detected in the frequency of azoxystrobin-resistant isolates within populations from fields with conventional and organic practices in 2016 and 2017. Reduced sensitivity to propiconazole and difenoconazole was detected in less than 1% of isolates. EC50 values for the carboxamides were higher than the maximum concentrations (= 20 µg ml-1) evaluated. Two small-plot replicated field trials were also conducted over two years to evaluate alternatives to the commercial standards, propiconazole and azoxystrobin. Two applications of benzovindiflupyr + difenoconazole and pydiflumetofen + difenoconazole significantly reduced CLS severity and epidemic progress compared to plots which received propiconazole and nontreated controls. The potential to enhance the durability of fungicide-based management of CLS by broadening rotation options is discussed.