Characteristics of Stagonosporopsis species causing gummy stem blight and prevention of seedborne diseases in cucurbits

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Seblani, Rewa
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Munkvold, Gary
Leandro, Leonor
Gleason, Mark
Peters, Nicholas
Goggi, Susana
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Plant Pathology
Vegetable crops in the family Cucurbitaceae are economically important around the globe and can be an important source of nutrients in a healthy diet. These crops are affected by a wide range of plant pathogens that affect yield and quality of fruits and seeds. Gummy stem blight (GSB) is a destructive, seed-transmitted fungal disease that affects all commonly cultivated cucurbit crops. The causal agent of GSB was previously placed in the genus Didymella, teleomorph Didymella bryoniae (Auersw.) Rehm (=Mycosphaerella citrullina and Mycosphaerella melonis, and anamorph Phoma cucurbitacearum (Fr.:Fr.) Sacc. (=Ascochyta cucumis). Using a multi-locus sequencing approach for the internal transcribed spacer, beta tubulin, chitin synthase I, and calmodulin genes, Stewart et al. (2015) revealed that GSB is caused by three phylogenetically distinct species: Stagonosporopsis cucurbitacearum (Fr.:Fr.) Aveskamp, Gruyter & Verkley, Stagonosporopsis citrulli M.T. Brewer & J.E. Stewart and Stagonosporopsis caricae (Sydow & P. Sydow) Aveskamp, Gruyter & Verkley. Most of the previous findings related to GSB were considered to be linked to Didymella bryoniae. With this new classification, rises the importance of better understanding characteristics of each of the Stagonosporopsis species. Cucumber green mottle mosaic virus (CGMMV) is another important seed-transmitted pathogen of cucurbit crops. This pathogen was recently introduced into the United States and current efforts are focused on eradication, seed testing, and public education to prevent further introductions. The goal of this research was to better understand pathogenicity of the three Stagonosporopsis species and the effects of fungicide seed treatments on seed transmission. The objectives were: i) conduct a review of GSB literature focusing on molecular aspects; ii) to better understand the degree of susceptibility of various cucurbits to S. cucurbitacearum, S. caricae, and S. citrulli rather than D. bryoniae in general; and iii) compare seed transmission among isolates of S. citrulli, S. caricae, and S. cucurbitacearum from seeds treated with four fungicides as seed treatments. Moreover, a fourth objective was to create a case study for CGMMV to raise visibility of this important pathogen and illustrate the value of planting pathogen-free seeds. For objective one, a comprehensive review about GSB was prepared, covering taxonomy, the GSB species complex, disease symptoms and life cycle, current research tools, disease management and future prospects. The review discussed fungicide resistance as well as genetic resistance sources. For objective two, five cucurbit cultivars susceptible to gummy stem blight were inoculated with 6 GSB isolates of which two were S. caricae, three were S. citrulli, and one was S. cucurbitacearum to explore the host range and relative aggressiveness of each isolate. All three species were pathogenic to the cucurbits tested, but disease severity differed significantly among isolates. Moreover, there also were significant interactions between isolate and crop cultivar where S. citrulli isolates caused the most severe symptoms on Waltham butternut squash, Athena cantaloupe, Jack O’ Lantern pumpkin, and Charleston grey watermelon, and S. caricae caused the most severe on Tyria cucumber. These results help better understand the degree of susceptibility of various cucurbits to S. cucurbitacearum, S. caricae, and S. citrulli rather than D. bryoniae in general. For objective three, the efficacy of four fungicide seed treatments against seed transmission of S. citrulli, S. caricae, and S. cucurbitacearum were tested. All fungicides increased the emergence rate, decreased the infection rate, and had a control effect on all the Stagonosporopsis isolates. S. citrulli, S. caricae, and S. cucurbitacearum, were most effectively controlled by thiabendazole; thiram and fludioxonil had similar control effects, and azoxystrobin was less effective. These results show that seed treatments can effectively reduce GSB seed transmission and illustrate the relative efficacy of the four fungicides against the isolates tested. For objective four, a case study for CGMMV was constructed. This study would help undergraduate students develop critical thinking and problem-solving skills. The objectives of the case study were to introduce key facts about CGMMV a cucurbit seed transmitted plant virus and its importance as well as recognize CGMMV symptoms and illustrate the importance of using certified disease-free seeds.
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