Abstract
Severe blight and necrotic symptoms were observed on a 3-year-old alfalfa crop cultivar Blue Jay' at Indian Head, Saskatchewan during the summer of 2017. Symptoms appeared on blossoms, leaves and stems, and most of the infected plants were severely defoliated. The objective of this study was to isolate and identify the causal agent of the disease. Isolations from diseased alfalfa plants showing visible blight and necrotic symptoms collected from the infected field were made on potato dextrose agar (PDA). Morphological observations of the developing fungal colonies on PDA showed the presence of dark grey to dark green mycelium and conidia borne in chains with vertical and transverse septa, indicating the presence of the fungus Alternaria alternata. Molecular analysis of the ITS-5.8S region of rDNA (541 bp) and the TEF-1 gene region (606 bp) of the fungal isolates from alfalfa confirmed their identity as A. alternata. The pathogenicity of representative isolates was confirmed on alfalfa Blue Jay' and Spredor 4' plants in growth chamber tests. Necrotic and blight symptoms like those observed in the field appeared on leaves and stems followed by defoliation. To our knowledge, this is the first report of A. alternata causing a foliar disease on alfalfa in Canada.