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.

Investigation of Fungi Causing Twig Blight Diseases on Peach Trees in South Carolina.

A survey of fungal pathogens causing twig blight on commercial peach trees was conducted in South Carolina in the fall of 2016. Shoots with cankers, pycnidia, and dieback were collected from six locations around the state. Isolates obtained from these samples were identified as Botryosphaeria obtusa, Phomopsis amygdali, Leucostoma persoonii, and Cytospora sp., based on colony morphology, conidia size and shape, and ribosomal DNA sequence analysis. L. persoonii was the most prevalent species and was isolated in five of the six locations, followed by P. amygdali and B. obtusa. The sensitivity of representative isolates of B. obtusa, P. amygdali, and L. persoonii to fungicides of different FRAC codes was evaluated. All species tested were sensitive to thiophanate-methyl (FRAC 1) and pyraclostrobin and azoxystrobin (both FRAC 11), whereas all species were resistant to boscalid and fluopyram (both FRAC 7). P. amygdali and B. obtusa were sensitive to difenoconazole and propiconazole (both FRAC 3), whereas L. persoonii was moderately resistant. L. persoonii was the most virulent species based on expansion of mycelium in the cambium layer of 2-year-old, detached twig pieces. Bacterial spot (BS)-sensitive cultivar 'O'Henry' was most susceptible to B. obtusa compared with BS-sensitive 'Summerprince', brown rot (BR)-resistant 'Contender', and BR-sensitive 'Coronet' but was least susceptible to L. persoonii. Coronet was most susceptible to L. persoonii. There were no significant differences between susceptibility of the cultivars to P. amygdali. This study established that L. persoonii is currently the most frequent twig blight pathogen in South Carolina, perhaps owing to its superior fitness. Some fungicides were effective in controlling all twig blight pathogens and may therefore be useful for chemical management strategies. Our study also provides the first evidence that the genetic basis of resistance to BS and BR in peach trees is not necessarily linked to tolerance to wood pathogens.