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Title: Fine scale population genetic structure and within tree distribution of mating types of Venturia effusa, cause of pecan scab in the U.S.A.

Author
item Bock, Clive
item YOUNG, CAROLYN - Noble Research Institute
item STEVENSON, KATHERINE - University Of Georgia
item CHARLTON, NIKKI - Noble Research Institute

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2018
Publication Date: 11/1/2018
Citation: Bock, C.H., Young, C.A., Stevenson, K.L., Charlton, N.D. 2018. Fine scale population genetic structure and within tree distribution of mating types of Venturia effusa, cause of pecan scab in the U.S.A. Phytopathology. 108:1326-1336.

Interpretive Summary: The major disease of pecan in the southeastern U.S.A. is scab, which is caused by the plant pathogenic fungus Venturia effusa. There is no information available on the characteristics of the population genetics in tree canopies at small spatial scales. This information can provide insight into inoculum sources and dispersal mechanisms, and potential opportunity for sexual reproduction. Four trees (cv. Wichita) were sampled hierarchically, taking four approximately evenly spaced terminals, and selecting six leaflets from different compound leaves on each terminal and sampling all lesions on each leaflet (from 1 to 8 lesions). The isolates were screened against a panel of 30 microsatellite markers and the multilocus genotypes (MLGs) analysed and mating types determined. Of 335 isolates, there were 165 MLGs (clonal fraction 49.3%). Genotypes were evenly distributed. Differentiation (P=0.001) occurred between populations on leaflets within individual terminals, and between terminals within trees in the tree canopies with 92.1% of variance explained among isolates within leaflet populations. Other analyses indicated little or no affinity of isolate for source population. The vast majority of shared MLGs (clones) existed on individual leaflets on a terminal (72 isolates) compared to those shared at the other spatial scales (=4 isolates shared at the other strata), indicating a notable component of short distance dispersal. Overall, the MAT1-1-1 and MAT1-2-1 idiomorphs were at equilibrium (73:92) in most populations, subpopulations and sub-subpopulations. Both mating types were frequently observed on the same leaflet. The results provide novel information on the populations of V. effusa at fine spatial scales, and provide insights into the dispersal of the organism within and between trees. The proximity of both mating idiomorphs on single leaflets is further evidence of opportunity for development of a sexual stage in the field.

Technical Abstract: Scab (caused by Venturia effusa) is the major disease of pecan in the southeastern U.S.A. There is no information available on the fine scale population genetic diversity, or the occurrence of clonal types at small spatial scales that provides insight into inoculum sources and dispersal mechanisms, and potential opportunity for sexual reproduction. To investigate fine scale genetic diversity, 4 trees (cv. Wichita = populations) were sampled hierarchically: within each tree canopy, 4 approximately evenly spaced terminals (subpopulations) were selected and up to 6 leaflets (sub-subpopulations) were sampled from different compound leaves on each terminal. All lesions on each leaflet were sampled (from 1 to 8 lesions). The isolates were screened against a panel of 30 microsatellite markers and the resulting multilocus genotypes (MLGs) subject to analysis. Mating type was also determined for each isolate. Of 335 isolates, there were 165 MLGs (clonal fraction 49.3%). Nei’s unbiased measure of gene diversity was moderate to high (0.489), and genotypes were evenly distributed. Pairwise comparisons of tree populations indicated they were closely related (Nei’s unbiased measure of genetic distance = 0.0093 to 0.0303). An AMOVA demonstrated differentiation (P=0.001) between populations on leaflets within individual terminals, and between terminals within trees in the tree canopies with 92.1% of variance explained among isolates within leaflet populations. Other analyses (phylogenetic, minimum spanning network, Bayesian and discriminant analysis) all indicated little or no affinity of isolate for source population. Clone correction at different spatial scales showed that the vast majority of shared MLGs (clones) existed on individual leaflets on a terminal (72 isolates) compared to those shared at the other spatial scales (=4 isolates shared at the other strata). Thus, the majority of the clones occurred at a scale of tens of cm or less, indicating a notable component of short distance dispersal. Overall, the MAT1-1-1 and MAT1-2-1 idiomorphs were at equilibrium (73:92), and in most populations, subpopulations and sub-subpopulations. Both mating types were frequently observed on the same leaflet. The results provide novel information on the characteristics of populations of V. effusa at fine spatial scales, and provide insights into the dispersal of the organism within and between trees. The proximity of both mating idiomorphs on single leaflets is further evidence of opportunity for development of a sexual stage in the field.