Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2008
Publication Date: 2/1/2008
Citation: Castell-Miller, C., Szabo, L.J., Gale, L.R., O'Neill, N.R., Samac, D.A. 2008. Molecular variability of a Minnesota population of Phoma medicaginis var. medicaginis, the causal agent of spring black stem and leaf spot of alfalfa. Canadian Journal of Plant Pathology. 30:85-96. Interpretive Summary: Diseases caused by fungi are the main causes of yield loss in alfalfa and are a major contributor to loss of plants in rainfed alfalfa stands. The fungus causing spring black stem and leaf spot (SBSLS) of alfalfa decreases yield by causing leaves to drop from plants and decreases feed quality by production of anti-nutritional factors. The fungus also causes root rot and crown rot, leading to plant death. Alfalfa cultivars with good levels of resistance to the disease are not available. The genetic and pathogenic diversity in the fungus was evaluated to better understand this important pathogen. The fungus was collected from infected alfalfa plants from two ecologically different areas in northern and sourthern Minnesota and 122 distinct isolates were cultured. The genomic DNA of each isolate was fingerprinted and the diversity among isolates was measured. The majority of isolates were only 55% similar at the DNA level. Fingerprinting did not find differences between isolates from different regions or isolates originating from different parts of the plant. Isolates also varied significantly in the degree of pathogenicity toward alfalfa leaves. The amount of diversity uncovered is surprising because the fungus appears to reproduce asexually. Diversity may develop due to a rapid rate of mutagenesis or from asexual recombination among isolates. The large amount of diversity may also reflect the large amount of diversity in the plant host. The results of this study indicate that in order to develop cultivars with resistance to SBSLS and improve alfalfa productivity, alfalfa breeding programs should use several diverse isolates of the fungus and isolates should be periodically renewed with fresh isolates from nature.
Technical Abstract: Spring black stem and leaf spot of alfalfa caused by Phoma medicaginis Malbr. & Roum. is an important disease of alfalfa (Medicago sativa L.) in temperate regions of the United States, causing yield losses and decreasing forage quality. Isolates of the fungus have been shown previously to vary in morphological and cultural characteristics and in pathogenicity on alfalfa. Sequence of the internal transcribed spacer (ITS) regions of nuclear ribosomal DNA (rDNA) and amplified fragment length polymorphism (AFLP) fingerprinting were used to characterize the diversity of P. medicaginis isolates collected from different organs of alfalfa plants from northern and southern areas of Minnesota. Sequences of the ITS1, ITS2, and 5.8S subunit from 14 isolates were highly conserved and failed to reveal variation among isolates. AFLP analysis of 122 isolates with 9 primer pair combinations revealed a moderate amount of variation among these isolates. However, polymorphisms were not correlated with geographic locations of isolates or plant organ from which the isolate was collected. Thus, AFLP fingerprinting may be useful for isolate identification while the ITS region may be useful for taxonomic and phylogenetic purposes.