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Investigation of an Outbreak of Fusarium Foot and Fruit Rot of Pumpkin Within the United States

Isolates of two biologically and phylogenetically distinct species, referred to as Fusarium solani f. sp. cucurbitae race 1 (Fsc-1 = Nectria haematococca mating population I [MPI]) and F. solani f. sp. cucurbitae race 2 (Fsc-2 = N. haematococca mating population V [MPV]), were suspected of causing an outbreak of Fusarium foot and fruit rot of pumpkin during 2001 to 2003 in Connecticut, New York, Ohio, and Missouri. Both species affect the fruit, but Fsc-1 also affects the crown and causes a stem rot. In this study, 156 isolates from affected plants and from soil under diseased fruit that tentatively were identified morphologically as members of the F. solani species complex were assayed for pathogenicity on pumpkin seedlings and mature fruit. Results of the pathogenicity assay indicated that 81 of the isolates were Fsc-1. The remaining 74 isolates were either nonpathogenic or only weakly pathogenic on the fruit. A subset of 53 test isolates from soil and plants, plus reference isolates of Fsc-1 and Fsc-2 and an isolate from wheat reported to cause a seedling rot on cucurbits, were characterized phylogenetically by sequencing a portion of the translation elongation factor 1-α gene. A BLAST query of the FUSARIUM-ID database at Pennsylvania State University indicated that 42 of the 53 test isolates were Fsc-1, whereas none were typed as Fsc-2. A polymerase chain reaction assay for mating-type (MAT) idiomorph revealed that all of the Fsc-1 isolates were MAT-1-2, suggesting that the pathogen may be strictly clonal in the affected fields. These findings provide convincing evidence that the Fusarium foot and fruit rot outbreaks were incited by Fsc-1.