Phytophthora capsici in Tennessee: Fungicide resistance, population genetics and cultural control

Phytophthora capsici, a plant pathogenic oomycete, is the causal agent of the vegetable disease Phytophthora blight of pepper and cucurbits. Since the identification of P. capsici in 1922, a significant amount of research has been conducted to understand its biology and disease management. Despite this, little research had been conducted on this species in the state of Tennessee. Three studies were done from 2018 to 2020, focusing on fungicide resistance, population genetics, and testing management strategies in the field. In 2018 and 2019 a total of 248 isolate of P. capsici were collected from five counties in Tennessee. These isolates were used in an in vitro fungicide sensitivity assay to test six fungicides labeled to control Phytophthora blight: mefenoxam, fluopicolide, oxathiapiprolin, mandipropamid, dimethomorph, and cyazofamid. Resistance to mefenoxam, cyazofamid, oxathiapiprolin, and fluopicolide was identified. The 248 isolates and an additional 74 isolates from 2004 and 2007 were genotyped using 39 single nucleotide polymorphism (SNP) markers. Genotypes were used to investigate the population genetics of P. capsici in Tennessee. Using various statistics and analyses such as AMOVA and DAPC, it was discovered that populations of P. capsici are structed by geographic location. Each sampling location had high genotypic diversity, and there were no shared genotypes among sampling locations. Lastly, a field trial was conducted in 2019 and 2020 to test the effectiveness of ground cover combined with fungicides in reducing Phytophthora fruit rot while growing pumpkin. In 2019, the untreated control plots had significant loss of plants at 14 days after transplant compared to the plots treated with fungicides, additionally the untreated plots had significantly less fruit at harvest than plots treated with fungicides and ground cover, but no other significant differences among any treatments in either year, most likely due to low disease incidence.