Date of Award

12-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Division of Agriculture (SAFES)

Committee Member

Dr. Guido Schnabel, Committee Chair

Committee Member

Dr. Ksenija Gasic

Committee Member

Dr. Julia Kerrigan

Abstract

In 2016, fungal pathogens causing twig blight disease were isolated from symptomatic one-year-old shoots from peach orchards in 6 locations in South Carolina. Four twig blight pathogens, which included Phomopsis amygdali, Botryosphaeria obtusa, Leucostoma persoonii, and Cytospora sp., were isolated. L. persoonii was isolated in the highest frequency, followed by P. amygdali and B. obtusa. All pathogens were sensitive to thiophanate-methyl (FRAC 1), pyraclostrobin, and azoxystrobin (both FRAC 11). However, they were not sensitive to boscalid and fluopyram (both FRAC 7). L. persoonii exhibited less sensitivity to difenoconazole and propiconazole (both FRAC 3) while P. amygdali and B. obtusa were sensitive. L. persoonii was most virulent of the species based on the average necrotic area of fungal growth on detached, two-year-old wood of 4 peach cultivars exhibiting varying disease resistance. ‘O’Henry’ [bacterial spot (BS)-susceptible] was the most susceptible to B. obtusa when compared to ‘Summerprince’ (BS-resistant), ‘Coronet’ [brown rot (BR)-susceptible], and ‘Contender’ (BR-resistant) but was the least susceptible to L. persoonii. Additionally, ‘Coronet’ was the most susceptible to L. persoonii. There were no significant differences between cultivar susceptibility to P. amygdali. In 2017, L. persoonii isolates were collected from scaffold limbs from 5 locations. High genetic variability of ITS1-5.8S-ITS2 was observed in L. persoonii isolates from both the 2016 and 2017 collections. The isolates were classified into genotypes (G) 1 to 6 based on how they clustered in a phylogenetic tree. Three of the genotypes (G2, G3, and G6) were isolated in the highest frequency in both years. Several morphology types were also observed between and within genotype. All isolates were sensitive to thiophanate-methyl (FRAC 1) but were not sensitive to fluopyram and boscalid (FRAC 7). No significant differences in EC50 were observed between genotypes. This research indicates the presence of three main twig blight pathogens in peach orchards in South Carolina with the genetically diverse L. persoonii being currently most prolific. Fungicide assay information indicates that some fungicidal active ingredients were effective in inhibiting mycelial growth of all twig blight pathogens. These data also provide information about cultivar tolerance in that resistance to twig blight pathogens is not necessarily connected to resistance to other peach pathogens.

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