First description of the sexual stage of Venturia effusa, causal agent of pecan scab

Authors: CHARLTON, NIKKI - Noble Research Institute; YI, MLHWA - Noble Research Institute; Bock, Clive; Zhang, Minling; YOUNG, CAROYN - Noble Research Institute

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2020
Publication Date: 5/29/2021
Citation: Charlton, N.D., Yi, M., Bock, C.H., Zhang, M., Young, C.A. 2021. First description of the sexual stage of Venturia effusa, causal agent of pecan scab. Mycologia. 112:711-721. https://doi.org/10.1080/00275514.2020.1759998.
DOI: https://doi.org/10.1080/00275514.2020.1759998

Interpretive Summary: Pecan scab is the most damaging disease of pecan in the southeastern USA. It is caused by the plant pathogenic fungus Venturia effusa. Epidemics of the disease regularly result in economic losses to the pecan industry. We characterized the mating types of the fungus and found them to be at equilibrium, indicative of regular sexual recombination. However, the occurrence of the sexual stage of V. effusa has never been observed, and the pathogen was previously believed to rely entirely on asexual reproduction. We paired opposite mating types on oatmeal culture media at 24 C for 2 mo for hyphal interactions to occur between mating types and then maintained at 4 C for 4 mo. Immature pseudothecia (the sexual stage) were observed, and following exposure to a 12 h photoperiod for 2 weeks at 24 C, asci and ascospores (the sexual spore) developed. Further experiments explored progeny from a cross of an albino and a wild-type isolate. Evaluation of isolate pigmentation, mating type, and genotyping of single ascospore progeny provided evidence that recombination occurred within the sexual crosses. Determining the source of the overwintering sexual stage in the field will aid in management decisions to reduce the severity of scab.

Technical Abstract: Venturia effusa , cause of pecan scab, is the most prevalent disease of pecan in the southeastern USA; epidemics of the disease regularly result in economic losses to the pecan industry. Recent characterization of the mating type distribution revealed the frequency of the MAT idiomorphs are in equilibrium at various spatial scales, indicative of regular sexual recombination. However, the occurrence of the sexual stage of V. effusa has never been observed, and the pathogen was previously believed to rely entirely on asexual reproduction. To explore the existence of a sexual cycle, we paired opposite mating types on oatmeal culture media. In initial experiments, cultures were incubated at 24 C for 2 mo for hyphal interactions to occur between mating types and then maintained at 4 C for 4 mo. Immature pseudothecia were initially observed but following exposure to a 12 h photoperiod for 2 weeks at 24 C, asci and ascospores developed. Further experiments explored the effect of time on pseudothecial development with 4 mo at 4 C as the optimal requirement. The results of this study demonstrate the heterothallic nature of V. effusa . Following experiments investigated progeny from a sexual cross of an albino and a wild-type isolate. Evaluation of isolate pigmentation, mating type, and multilocus genotyping of single ascospore progeny provided evidence that recombination occurred within the sexual crosses. The impact of determining the source of the overwintering ascostroma will aid in management decisions to reduce the primary inoculum in the disease cycle.