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Abstract
Isolates of Diaporthe phaseolorum varieties caulivora, meridionalis and sojae, and Phomopsis longicolla were grown on various substrates in the laboratory and inoculated into soybean plants in the greenhouse. Isolates varied in their ability to produce pycnidia and/or perithecia in culture. Both α and β conidia were observed in isolates of varieties caulivora and sojae, whereas only α conidia were observed in isolates of variety meridionalis. Stroma morphology and perithecial arrangement in stromata varied considerably in isolates of varieties caulivora and meridionalis, depending on the substrate. Some isolates of variety caulivora formed perithecia in a valsoid arrangement on potato dextrose agar and single perithecia in a well developed, immersed stroma in inoculated soybean stems. Isolates of variety sojae always formed pustulate stromata and clustered perithecia. Nine quantitative morphological characters were examined using two different cluster analysis methods and canonical discriminant analysis. The length and width of ascospores and asci and apical width of the perithecial beaks were stable across different substrates and resolved the isolates into three distinct clusters. Length and width of perithecia did not vary considerably. Conversely, length of perithecial beaks distorted the assessment of relatedness among isolates due to its variability. Morphological characters and cultural characteristics of isolates in these clusters corresponded with previous descriptions of varieties caulivora and sojae. Genomic DNAs of isolates of D. phaseolorum and Phomopsis longicolla from soybean were analyzed using the Random-Amplified Polymorphic DNA (RAPD) technique. Two arbitrary decamer primers yielded reproducible polymorphic amplification products. Amplification products generated with primer 425 revealed distinct groups. Groupings of D. phaseolorum isolates based on similarity of band patterns corresponded with varietal groupings based on morphological characters. Most isolates from the Midwest showed close relatedness to isolates of variety caulivora. Primer 455 showed close relatedness of two isolates of var. sojae to the Phomopsis longicolla group. Since isolates identified as meridionalis exhibited consistent morphological characters and RAPD patterns distinct from varieties previously described in D. phaseolorum, meridionalis is designated a new variety of D. phaseolorum.