Regional Population Genetics Structured by Mating Type in Alternaria brassicicola in Cole Crops ( Brassica spp.) in the Northeastern, Midwestern, and Southeastern United States

Production of broccoli ( Brassica oleracea var . italica), and other cruciferous vegetables ( Brassica spp.), has expanded throughout the United States. As a result, associated diseases have become more important in new locations. Alternaria brassicicola, the primary causal agent of Alternaria leaf blight and head rot of brassicas, is a major pathogen ubiquitous in all brassica production operations. To better understand how A. brassicicola has adapted to new regions, we developed novel microsatellite primers and identified five polymorphic loci subsequently applied to characterize the population structure and reproductive strategies of field populations from Connecticut, Massachusetts, Virginia, Georgia, and Minnesota. Mating type idiomorphs were also identified 61:189 ( MAT1-1: MAT1-2), skewed toward MAT1-2. A total of 250 isolates were collected from nine broccoli fields from five states and from one field each of cabbage ( B. oleracea var. capitata) and brussels sprouts ( B. oleracea var. gemmifera) in Minnesota. A relatively large amount of genetic diversity was identified within each field, though population structure was only apparent regionally within MAT1-2 isolates. Using a chi-square analysis, two Minnesota fields met the 1:1 ratio expected for sexually recombined populations. Five fields predominated by a single mating type ( MAT1-2) also had nonsignificant linkage disequilibrium, which was unexpected and could be the result of a parasexual cycle that recombines genotypes without carpogenic sexual outcrossing between mating types. Overall, our results provide valuable insight into the population structure and reproductive strategies of A. brassicicola, specifically the structure and potential parasexual populations of MAT1-2 isolates within fields. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .