Dynamics of the ascospore dispersal of Stagonosporopsis citrulli, a. causal agent of gummy stem blight of cucurbits

Authors: Rennberger, Gabriel; Turechek, William; KEINATH, A. - Clemson University

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/8/2021
Publication Date: 7/6/2021
Citation: Rennberger G, Turechek WW, Keinath AP. Dynamics of the ascospore dispersal of Stagonosporopsis citrulli, a causal agent of gummy stem blight of cucurbits. Plant Pathol. 2021;00:1–12 https://doi. org/10.1111/ppa.13424
DOI: https://doi.org/10.1111/ppa.13424

Interpretive Summary: Sexually produced, airborne ascospores (spores) may play a role in the dispersal of Stagonosporopsis citrulli, a causal agent of gummy stem blight of cucurbits. Four studies were conducted to investigate unknown aspects of ascospores in the epidemiology of gummy stem blight (GSB). In the first study, colonized watermelon debris from research fields was sampled biweekly starting in Jan 2017, Dec 2017, and Dec 2018. On average, gummy stem blight spores were still detected for nearly 300 days after symptoms were first detected. In a second test, the duration of time and quantity of actively released spores was investigated. The release of spores decreased up to 90% over the course of the experiments which lasted for seven days. In the third study, trap-plant assays with seedlings of watermelon and cucumber were conducted in the winter months of 2017 and 2018 to identify the dispersal period of gummy stem blight spores. In this study, trap plants were placed at varying distances up to more than 300 m from the spore source and exposed for 48 hours. These assays were conducted a total of 9 and 13 times over the course of six and seven months in 2017 and 2018, respectively, and ran until no spores were detected. Generally, the frequency of pathogen recovery from trap plants as measured by the number of gummy stem blight lesions decreased with increasing distance from the spore source. S. citrulli was detected at maximum distances of 244 m and 366 m from the spore source in 2017 and 2018, respectively. In the final study, two experiments were conducted indoors to quantify spore dispersal under controlled conditions. Disease incidence was more than three times as high on cucumber plants than on watermelon plants. The incidence of GSB decreased with increasing distance from the source of spores. This study confirms that spores of S. citrulli can serve as primary inoculum for epidemics and could easily spread from one cucurbit field to another nearby field. Debris from cucurbit crops can be the source of spores for up to 10 months and therefore should be cleared quickly after the end of harvest. The results of these studies will have immediate application for cucurbit growers as an important component of an integrated pest control strategy for reducing the impact of gummy stem blight.

Technical Abstract: Sexually produced, airborne ascospores may play a role in the dispersal of Stagonosporopsis citrulli, a causal agent of gummy stem blight of cucurbits. Four studies were conducted to investigate unknown aspects of ascospores in the epidemiology of gummy stem blight. In the first study, colonized watermelon debris from research fields was sampled biweekly starting in Jan 2017, Dec 2017, and Dec 2018. A Petri-dish assay was used to confirm the duration of time that ascospores were actively released from pseudothecia on debris. On average, ascospores were detected for a mean of 304 days after the research fields were inoculated and 290 days after symptoms were detected. Secondly, the duration of time and quantity of actively released ascospores from pseudothecia was investigated with a Petri-dish assay. The release of ascospores decreased up to 90% over the course of the experiments which lasted for seven days but ascospores were still released at that time. In the third study, trap plant assays with seedlings of watermelon and cucumber were conducted in the winter months of 2017 and 2018 to identify the dispersal period of ascospores of S. citrulli. In each of the two experiments in that study, trap plants were placed at seven and eight distances up to 366 m from the spore source and exposed for 48 hours. These assays were conducted a total of 9 and 13 times over the course of six and seven months in 2017 and 2018, respectively, and ran until no ascospores were detected. Generally, the frequency of pathogen recovery from trap plants as measured by the number of GSB lesions decreased with increasing distance from the spore source. S. citrulli was detected at maximum distances of 244 m and 366 m from the spore source in 2017 and 2018, respectively. The exponential model fit ascospore dispersal better than the power law model. In the final study, two experiments were conducted indoors to quantify ascospores dispersal under controlled conditions up to 55 m from the spore source. Disease incidence was more than three times as high on cucumber plants than on watermelon plants. The incidence of GSB decreased with increasing distance from the source of ascospores. This study confirms that ascospores of S. citrulli can serve as primary inoculum for epidemics and could easily spread from one cucurbit field to another nearby field. Debris from cucurbit crops can be the source of ascospores for up to 10 months and therefore should be cleared quickly after the end of harvest.