P-636 - Structure and diversity of Alternaria brassicicola within and between fields in Connecticut, Massachusetts, Georgia, Virginia, and Minnesota

Alternaria leaf blight and head rot, caused by Alternaria brassicicola, is a foliar disease of cruciferous vegetables (Brassica spp.) that is ubiquitous throughout brassica production in the United States. As broccoli (B. olearaceae L. var. italica) production has increased in the Eastern US, so has the importance of its associated diseases. We designed and tested five novel microsatellite primers to describe the population structure and mating strategies of A. brassicicola in five states (CT, MA, GA, VA, and MN). We also determined mating type idiomorphs present in each field. In total, we collected 250 A. brassicicola isolates from nine broccoli fields, one cabbage field (B. olearaceae L. var. capitata), and one brussels sprout field (B. olearaceae L. var. gemmifera). The mating type (MAT) ratio was 61:189 (MAT1-1:MAT1-2), with two MN fields not deviating from the 1:1 ratio expected for sexual recombination. We used linkage disequilibrium to infer if these populations were sexual or clonal. Five fields were identified as clonal; one field was inferred as sexually recombined in both analyses; five field were identified as sexually recombined (r_d = -0.127 to 0.134, p = 0.110 to 0.998). In those fields, MAT1-2 was either the predominant or the sole MAT present in those fields, which could indicate a cryptic-sexual cycle or a parasexual cycle. Regional structure was identified among isolates in the MAT1-2 mating type, whereas no regional structure was identified for MAT1-1. Further research is required to substantiate these findings, especially at fine scale, within fields.