Phakopsora pachyrhizi growth in tissue of leaves from resistant and susceptible soybean germplasm

Soybean rust, caused by Phakopsora pachyrhizi, is one of the most potentially devastating foliar diseases of soybean, particularly in areas with mild winters. Tan lesions with two or more sporulating uredinia indicate a susceptible reaction. Resistance to soybean rust is characterized by the development of reddish-brown (RB) lesions or a symptomless immune response, though considerable variation in lesion diameter, color and sporulation has been observed among resistant soybean germplasm accessions that develop RB lesions. The objective of this study is to compare the reactions of accessions with different resistance symptoms to P. pachyrhizi infection. Ten soybean plant accessions were rated two weeks after inoculated with two P. pachyrhzi isolates for lesion type and appearance, relative lesion density and average lesion diameter. Quantitative real-time polymerase chain reaction was performed to measure fungal growth following inoculation of detached leaves of different accessions that exhibited different types of resistance reactions in the greenhouse. From the experiments, we confirmed that some SBR-resistant germplasm accessions developed distinctly different reaction phenotypes when challenged with certain isolates of P. pachyrhizi. We also concluded that quantitative PCR assays to quantify P. pachyrhizi DNA within infected soybean plant tissue can show the variation in the amounts of P. pachrrhizi DNA on infected soybean tissue between susceptible cultivars and reistant cultivars, as well as among different resistant cultivars