Potato early blight caused by Alternaria solani and potato brown spot caused by Alternaria alternata are two common potato foliar diseases. Fungicide applications in Wisconsin target early blight with the expectation of managing brown spot simultaneously. However, distributions of the two pathogens over time and space have not been previously reported in Wisconsin. In this study, six potato fields in Wisconsin were systematically sampled during 2014 to 2017 to investigate the incidences of both pathogens; incidences of the pathogens were compared by location, timing, and year. Incidence of A. alternata had been recorded starting in late June in every field every year, whereas incidence of A. solani started later in some of the fields. At the end of each growing season throughout the four years, the highest incidence (100% most of the years) of A. solani was observed in a field in Plover, located in a concentrated potato production area of central Wisconsin, and a field in Arena, located in an isolated potato production area in southern Wisconsin. The highest incidence (100% every year) of A. alternata was observed in a field in Madison, an isolated potato field that was managed organically. Such distribution patterns could be a result of the different host ranges of the pathogens and abundance of inoculum in different geographic locations. In 2016, disease severity was correlated with the incidence of both pathogens (P < 0.0001 to P = 0.0299), but in 2017, disease severity was correlated with the incidence of A. solani in most of the fields (P < 0.0001 to P = 0.0799). These findings suggest that both Alternaria spp. are important for considering disease management strategies in central and southern Wisconsin.
Keywords: disease severity; pathogen incidence; potato brown spot; potato early blight.