Raymond J. Taylor and
Julie S. Pasche, Department of Plant Pathology, North Dakota State University, Fargo, ND 58105;
H. David Shew and
K. R. Lannon, Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695; and
Neil C. Gudmestad, Department of Plant Pathology, North Dakota State University, Fargo, ND 58105
A study was undertaken in 2008 and 2009 to examine potato (Solanum tuberosum) cultivar susceptibility, the potential of other host species to act as sources of inoculum for potato infections, and other aspects of potato–Phytophthora nicotianae interactions. Twelve isolates of P. nicotianae collected from five leaf, one petiole, and six tuber infections of potato from five states, as well as isolates from a variety of other host species, were evaluated for ability to cause tuber rot of potato via inoculation studies. Additionally, the susceptibility of 27 potato cultivars commonly grown in the United States to tuber infection by P. nicotianae was determined. Eighty-three percent of the isolates recovered from potato were highly aggressive, infecting tubers at nearly four times greater incidences than isolates originating from nonpotato hosts. With the exception of two tobacco isolates, zoospores of all isolates recovered from nonpotato hosts were able to infect potato tubers. Russet cultivars were significantly less susceptible to P. nicotianae than red and white cultivars in 2008, and red cultivars in 2009. Umatilla Russet was the most resistant cultivar in both years, whereas Red Norland and Dakota Rose were the most susceptible in both years. Results of a survey for P. nicotianae conducted in four states from 2008 through 2010 confirmed previous observations of naturally occurring infections of potato in Missouri, Nebraska, and Texas, as well as infections of potato in Michigan (documented for the first time). All isolates recovered in the survey were sensitive to mefenoxam (EC50 < 1.0 μg/ml).