Resistance to fludioxonil and inconsistent fitness penalties in a predominant potato dry rot pathogen, Fusarium sambucinum

Fludioxonil is a fungicide commonly used as a seed treatment in potato production. Due to documented fitness penalties and few reports of naturally occurring resistance in fungi, the risk of fludioxonil fungicide resistance development is characterized as low to medium by the Fungicide Resistance Action Committee. However, recent findings of fludioxonil resistance in populations of Fusarium sambucinum call into question current assumptions. Fifty-nine isolates from the Pacific Northwest were characterized for their response to fludioxonil using a spiral gradient plate method. Thirty-five of the isolates were resistant to fludioxonil (EC50 = 98 to >507 ppm). A subset of 31 isolates (18 resistant and 13 sensitive) were evaluated for their ability to 1) grow on potato dextrose agar, and 2) cause Fusarium dry rot lesions on whole potato tubers. Growth was assessed at ambient temperatures 3 times per week for 21 days to generate areas under the growth curves (AUGC). Surface sanitized whole tubers were inoculated at both stem and bud ends by removing a 5-mm core of potato tissue, adding a 10-day-old 5-mm disc of fungal mycelia grown on potato dextrose agar, then returning the potato tissue plugs. After incubating inoculated tubers for 4 weeks at ambient temperatures and high humidity, tubers were sliced longitudinally to measure length and width of lesions for calculating total lesion area. AUGC was similar among isolates except for two resistant isolates that grew more slowly, with 20-22% lower AUGC compared to the fastest growing isolates. These same two isolates produced smaller lesions ( < 11 cm²) compared to sensitive isolates. Mean lesion sizes among resistant isolates were more variable (9-23 cm²) compared to sensitive isolates (18-25 cm²). Given that few isolates from our data demonstrated reduced fitness and aggressiveness, risk of the development of fludioxonil resistance in F. sambucinum may be higher than previously thought.