Abstract
Sugarbeet account for over half of all sucrose production nationwide in the United States. Domestic sugarbeet production is challenged by a variety of diseases and pests impacting yield, quality, and storability. Cercospora leaf spot, the most important of these foliar diseases, has had issues with fungicide resistance reported in several sugarbeet producing regions. In addition, reports of increased prevalence and fungicide resistance within the historically minor disease, Alternaria leaf spot, has indicated a need for improved management. This study aims to characterize current Alternaria spp. impacting Michigan sugarbeet crops and, further, to investigate the stability of fungicide resistances within isolates of causal agents, Alternaria spp. and Cercospora beticola. Isolates were collected from symptomatic sugarbeet leaf samples and characterized using a detached-leaf assay for virulence. An initial spiral gradient dilution screen was conducted to assess responses to common fungicide classes, including demethylation inhibitors (DMI), benzimidazoles, quinone outside inhibitors (QoI), and organo-tin fungicides. From two years of sampling, approximately 60-90% of lesion area caused by isolates of Alternaria spp. varied significantly from the controls on an Alternaria-susceptible commercial variety. Characterized cultures of eight virulent Alternaria spp. isolates and seven virulent C. beticola isolates were exposed to cold temperatures (20, 4, and -20°C), mimicking mild to extreme overwintering conditions. Isolates were screened after two weeks and then every subsequent month for seven months to monitor potential between-season shifts in fungicide sensitivity. Following seven months of cold exposure, no biologically significant shifts in sensitivities to two DMIs, difenoconazole, or tetraconazole, or a benzimidazole, thiophanate-methyl, were observed for any isolates. However, tin sensitivities for both pathogens increased significantly over time (P < 0.05), which indicates that a return to tin sensitivity could be observed over several months regardless of temperature. None of the fungicides screened displayed meaningful shifts due to environmental factors, however, second year of data is continuing for both pathogens. These studies will help to inform in- and between-season management considerations for major sugarbeet leaf spot pathogens.