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

Production of broccoli (<i>Brassica olearaceae</i> L. var. <i>italica</i>), and other cruciferous vegetables (<i>Brassica</i> spp.), has expanded throughout the United States. As a result, associated diseases have become more important in new locations. <i>Alternaria brassicicola</i>, the primary causal agent of Alternaria leaf blight and head rot of brassicas (ABHR), is a major pathogen ubiquitous in all brassica production operations. To better understand how <i>A. brassicicola</i> 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 (<i>MAT1-1</i>:<i>MAT1-2</i>), skewed toward <i>MAT1-2</i>. A total of 250 isolates were collected from nine broccoli fields from five states, and from one field each of cabbage (<i>B. olearaceae</i> L. var. <i>capitata</i>) and brussels sprouts (<i>B. olearaceae</i> L. var. <i>gemmifera</i>) in Minnesota. A relatively large amount of genetic diversity was identified within each field, though population structure was only apparent regionally within <i>MAT1-2</i> 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 (<i>MAT1-2</i>) also had non-significant linkage disequilibrium, which is 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 <i>A. brassicicola</i>, specifically the structure and potential parasexual populations of <i>MAT1-2</i> isolates within fields.