Pathogenicity of Diaporthe spp. isolates recovered from soybean (Glycine max) seeds in Paraguay

Phomopsis seed decay (PSD) caused by Diaporthe longicolla has been documented as part of a soybean (Glycine max) fungal disease complex that affects the quality of soybean seed. Soybean-producing countries that have not yet documented the presence of PSD impose soybean seed import restrictions to protect their soybean production. The purpose of this study was to determine the frequency of occurrence of Diaporthe spp. in Paraguay. In 2006, 16 isolates of Diaporthe were recovered for the first time from soybean seed in San Alberto, Paraguay in the south-eastern section of the country. The 16 isolates were used to inoculate mature pods harvested from greenhouse grown PSD-susceptible soybean cultivar 'Maverick'. Among the16 isolates, six isolates (TN 214, TN 218, TN 224, TN 226, TN 227 and TN 229) caused infection on both pods and seeds within pods. Two groups of isolates were identified based on conidial types: isolates that produced only alpha conidia and isolates that produced both alpha and beta conidia. The percentage of beta conidia ranged from 0 to 95% for all the isolates except for TN 218, TN-222 and TN 223, which only produced alpha conidia. These results indicate that considerable variability exists in pathogenicity and composition of alpha and beta spores among the 16 Diaporthe spp. isolates recovered from soybean seeds in Paraguay. In addition, these isolates may cause seed quality losses and may spread undetected to soybean production fields in Paraguay and around the world. Although the Diaporthe spp. isolates had three distinct ITS sequence types, none of the three probable species could be identified using currently available information. It is possible that these isolates represent previously unrecognized species, in addition to D. longicolla or D. phaseolorum, but additional data will be required to determine if this is the case.