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Fine scale population genetic structure and within tree distribution of mating types of Venturia effusa, cause of pecan scab in the USA

Katherine Stevenson: University of Georgia

<div>Scab (caused by <em>Venturia effusa</em>) is the major disease of pecan in the southeastern USA. There is no information available on the fine scale population genetic diversity. Four cv. Wichita trees (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 30 microsatellite markers and mating type was determined. 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. AMOVA demonstrated 92.1% of variance was explained among isolates within leaflet populations. The vast majority of clones existed on individual leaflets on a terminal (72 isolates) compared to other spatial scales (≤4 isolates at other strata). Using clone corrected data, the <em>MAT1-1-1</em> and <em>MAT1-2-1</em> idiomorphs were at equilibrium (92:73). Both mating types were frequently observed on the same leaflet. The results provide novel information on the characteristics of populations of <em>V. effusa</em> at fine spatial scales, and provide insights into pathogen dispersal within and between trees. The proximity of both mating types on single leaflets is further evidence of the opportunity for sexual reproduction in the field.</div>