Variation in Lesion Characteristics, Densities and Sporulation Levels among Soybean Germplasm Accessions Resistant to Soybean Rust (Phakopsora pachyrhizi).

The development of reddish-brown (RB) lesions instead of tan lesions on soybean leaves infected by the soybean rust (SBR) fungus, Phakopsora pachyrhizi, is generally considered to indicate at least partial resistance. Lower disease severity or lesion density relative to that of susceptible hosts with a similar maturity is also considered indicative of resistance. However, recent observations suggest that different plant introductions (PIs) from the USDA’s soybean germplasm collection manifest resistance to SBR through a range of reactions, and that evaluations limited to criteria previously used to rate resistance to SBR may be inadequate. The objective of this study was to investigate the variety and reproducibility of the symptoms developed by a set of PIs challenged with SBR isolates. Data were collected for lesion type, lesion density, uredinia per lesion, and level of sporulation relative to susceptible controls. The results suggest that microscopic inspections of infected leaves may sometimes be necessary to obtain a comprehensive evaluation of resistance, particularly as manifested by reduced sporulation and uredinia development.