Molecular and morphological analyses of Alternaria and Stemphylium from onion

Alternaria porri causes a foliar disease of onion purple blotch reported worldwide. Foliar diseases caused by Stemphylium vesicarium are reported in some countries. The objective of this research was to identify and characterize the fungi causing Alternaria purple blotch and Stemphylium leaf blight on onion at the molecular and morphological levels of resolution. Some physiological characteristics of these fungi were also studied.; Thirty nine Mexican and New York isolates were characterized using three molecular characters: the ribosomal DNA Internal Transcribed Spacer (ITS) region, and portions of the two genes at the Mating Type locus (MAT ). For the latter, the alpha box and the High Mobility Group (HMG) box regions of the MAT-1 and MAT-2 genes, respectively, were used.; Based on morphological criteria, the subset of 39 isolates from diseased onions were classified into five groupings: 19 isolates of A. porri , 13 isolates of S. vesicarium, one isolate of S. lycopersici, and five isolates (two groups) of unknown species of Alternaria.; The molecular data were in general agreement. Using ITS sequences, there were five groups: (1) A. porri; (2) S. vesicarium; (3) three unknown Alternaria spp.; (4) two additional unknown Alternaria spp. and (5) S. lycopersici which was in a group by itself. Within these groups, all sequences were identical. Alpha box sequence data supported these groupings. The HMG data did also, with two exceptions. One isolate of S. vesicarium and the only S. lycopersici isolate examined grouped with A. porri. The MAT sequences were more variable than ITS.; MAT sequences were identified in all isolates, even though some of these species have no known sexual stage (e.g., A. porri). Some isolates have a single MAT gene and thus appear to be heterothallic, while others have both MAT genes and should be considered homothallic. All the isolates (except S. lycopersici and A. spp. 113 and 114) were pathogenic when reinoculated on onion. Most of the isolates produced pigments in culture, and increased the pH of the growth medium, and the pH of infected tissue. The main species causing foliar onion diseases were A. porri and S. vesicarium.