Date of Award

12-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Division of Agriculture (SAFES)

Committee Member

Anthony P. Keinath, Committee Chair

Committee Member

Patrick Gerard

Committee Member

Guido Schnabel

Committee Member

Julia Kerrigan

Abstract

Fourteen cucurbit species of unknown susceptibility representing 12 genera were inoculated with Stagonosporopsis citrulli in Charleston, SC, in spring 2015, spring 2016 and fall 2016 to evaluate their level of susceptibility to gummy stem blight and the ability of the pathogen to reproduce on crown cankers. Coccinia grandis was highly resistant to gummy stem blight. To the best of our knowledge this is the first report of susceptibility to gummy stem blight of 14 species, which expands the host range of S. citrulli to 37 species representing 21 genera in the family Cucurbitaceae. Crown cankers were found to be important reproductive sites for S. citrulli. A survey of foliar pathogens of watermelon based on two-stage cluster sampling was conducted on 60 commercial farms in South Carolina from spring 2015 to fall 2017. The influence of environmental and management factors on the occurrence of foliar pathogens was analyzed. Estimates for the statewide probability of pathogen occurrence on symptomatic leaves were obtained. Six fungal pathogens, an oomycete and three viral pathogens were identified. With the exception of fall 2017, Stagonosporopsis spp. was the most prevalent pathogen in every season followed by P. xanthii. Myrothecium s.l. was the most common pathogen in fall 2017. In fall 2017, the third most common pathogen was Corynespora cassiicola. Eight of 80 isolates of Stagonosporopsis spp. were identified as S. caricae, the rest as S. citrulli. This is the first report of C. cassiicola, S. caricae, and Myrothecium s.l. on watermelon in South Carolina. There was a consistent increased probability of occurrence of Stagonosporopsis spp. in fields with a previous cucurbit crop, increasing probabilities of pathogen occurrence with increasing plant age, a lower probability of occurrence of some pathogens on triploid cultivars compared to diploid cultivars, and a decrease in probability of pathogen occurrence in fields aligned towards southwest or west. Six species of Stachybotriaceae were identified in a collection of 95 isolates obtained from 11 fields in South Carolina using Bayesian inference based on combined sequences of four partial gene regions. Two species, Gregatothecium humicola and Paramyrothecium foliicola were the predominant species found. Within these two species, genetic differences within small spatial scales were detected. In pathogenicity tests the two predominant species were the most virulent on watermelon. Cowpea plants were less susceptible than watermelon and tomato plants, which were equally susceptible. This is the first report of pathogenicity of five of the six identified species of Stachybotriaceae on the three tested vegetable crops and of X. tongaense on watermelon.

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