Genetic Diversity and Fungicide Sensitivity of Cytospora plurivora on Peach

Cytospora plurivora D.P. Lawr., L.A. Holland & Trouillas has been associated with recent premature peach tree decline in South Carolina, but very little is known about the pathogen or chemical control options. Ninety-three C. plurivora isolates were collected in 2016 and 2017 from 1-year-old peach wood and symptomatic scaffold limbs, respectively, from orchards in six towns in South Carolina. Six unique genotypes were identified based on substantial ITS1-5.8S-ITS2 sequence variability and classified G1 to G6. Three of the genotypes (G2, G3, and G6) were isolated in high frequency in multiple locations of both years. In addition to the genotypic variation, multiple phenotypes were observed between and within genotype groups. Species identity was determined using additional gene loci: ACT, TUB, and EF, and isolates were found to belong to C. plurivora for all genotype groups. All tested genotypes were sensitive to thiophanate-methyl (FRAC 1) but exhibited slightly lower sensitivity to propiconazole and difenoconazole (both FRAC 3). Boscalid, fluopyram (both FRAC 7s), azoxystrobin, and pyraclostrobin (both FRAC 11s) were ineffective in vitro at inhibiting mycelial growth of C. plurivora genotypes. Field inoculation of peach and nectarine trees revealed that all genotypes developed twig cankers with differences in virulence. G1 was most virulent, and G6 was least virulent. This study provides a link between the C. plurivora genetic variability and virulence and provides fungicide sensitivity information that could be used to improve disease management practices.

Automated summary

Cytospora plurivora has been identified as the causal agent of premature peach tree decline in South Carolina, and this study characterized its genetic variability, virulence, and fungicide sensitivity. Six unique genotypes were identified based on ITS1-5.8S-ITS2 sequences, with three genotypes being isolated in high frequency in multiple locations. All genotypes belonged to C. plurivora based on additional gene loci. Sensitivity to different fungicides varied among genotypes, with all genotypes being sensitive to thiophanate-methyl but showing lower sensitivity to propiconazole and difenoconazole. In vitro testing revealed that several fungicides were ineffective in inhibiting mycelial growth. Field inoculation experiments showed differences in virulence among genotypes, with G1 being the most virulent and G6 the least virulent. This study provides valuable information for disease management strategies.