An evaluation of year-to-year fungicide efficacy and cultivar resistance combined with fungicide programs to manage cucumber downy mildew

Abstract

Michigan growers have had to annually manage cucurbit downy mildew (CDM) using fungicides since host resistance was overcome in 2005. Fungicide efficacy can vary year-to-year and combining intermediate resistance (IR) pickling cucumber cultivars with fungicides may help improve CDM management. The study objectives were to compare Citadel and Peacemaker (IR) with susceptible cultivars Expedition and Vlaspik for their response to CDM when treated with three fungicide programs or not treated. Also, determining the efficacy of single product fungicides across three field seasons was of interest. Fungicides were applied weekly in 2016 and 2017 to IR and susceptible cultivars and included: (i) chlorothalonil, (ii) cyazofamid + chlorothalonil alternated with propamocarb + chlorothalonil alternated with amectoctradin/dimethomorph + chlorothalonil; and (iii) oxathiapiprolin/chlorothalonil alternated with propamocarb + chlorothalonil, alternated with cyazofamid + chlorothalonil. Protecting IR cultivars with fungicides increased their total marketable yield compared to the untreated. In Peacemaker, chlorothalonil alone was equally effective as the other fungicide treatments at improving yield. Single-product fungicides were applied on a 7-day schedule to field plots from 2018 to 2020. The efficacy of some of the most effective fungicides was influenced by year. Oxathiapiprolin + chlorothalonil had reduced efficacy during the year with the highest disease pressure. There were significant interactions between year and fungicide; propamocarb and ethaboxam did not always correlate with higher disease pressure. IR cultivars should be protected with fungicides. A better understanding of the relationship between yield response and disease severity in IR cultivars and continued monitoring of fungicide sensitivity will help optimize fungicide use.