Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2011
Publication Date: 8/1/2011
Citation: Paul, C., Hill, C., Hartman, G.L. 2011. Comparisons of visual rust assessments and DNA levels of Phakopsora pachyrhizi in soybean genotypes varying in rust resistance. Plant Disease. 95:1007-1012. Interpretive Summary: Soybean resistance to the fungus that causes of soybean rust has been characterized by three infection types (i) immune response (complete resistance) with no visible lesions, (ii) resistant reaction with reddish brown lesions (incomplete resistance), and (iii) susceptible reaction with tan-colored lesions. Based on visual assessments of these phenotypes, single gene resistance in soybean to the rust fungus has been documented, but colonization within infected tissues based on fungal DNA levels in different soybean genotypes had not been analyzed. The research examined a quantitative molecular assay (polymerase chain reaction) to compare visual disease assessment to fungal DNA in infected tissue. The results of this molecular assay distinguished different types of resistance where precise quantification of incomplete and/or partial resistance is needed. This information will be useful for plant geneticists and plant pathologists interested in quantifying pathogens on plant tissue.
Technical Abstract: Soybean resistance to Phakopsora pachyrhizi, the cause of soybean rust, has been characterized by the following three infection types (i) immune response (IM; complete resistance) with no visible lesions, (ii) resistant reaction with reddish brown (RB) lesions (incomplete resistance), and (iii) susceptible reaction with tan-colored (TAN) lesions. Based on visual assessments of these phenotypes, single gene resistance in soybean to P. pachyrhizi has been documented, but colonization within infected tissues based on fungal DNA (FDNA) levels in different soybean genotypes, had not been analyzed. The research examined a quantitative polymerase chain reaction (Q-PCR) assay to compare visual disease assessment to FDNA from Q-PCR assays based on controlled inoculation experiments using two isolates of P. pachyrhizi. The objective of the first experiment was to compare P. pachyrhizi DNA from Q-PCR assays with digital visual disease assessment using five resistant soybean genotypes (one IM and four RB) and five susceptible genotypes (TAN). The objective of the second experiment was to determine if fungal growth could differentiate five soybean genotypes with different levels of resistance (one IM, two RB, and two TAN) by quantifying FDNA at different time points. For Experiment one, the number of uredinia and uredinia per lesion on four of the five resistant soybean genotypes were lower (P < 0.05) than the other six genotypes.Significant differences (P < 0.05) in FDNA concentrations were found among soybean genotypes with TAN lesions and among soybean genotypes with RB lesions. Soybean cultivar UG5 (IM phenotype) had significantly less (P < 0.05) FDNA than all of the other genotypes. Some genotypes that produced TAN lesions had significantly lower (P < 0.05) or non-significantly different FDNA concentrations compared to those genotypes that produced RB lesions. For Experiment two, the regression of FDNA on days after inoculation was significant (P < 0.01) with positive slopes for all genotypes except for UG5 in which FDNA declined over time, indicating a reduction of fungal colonization. The results of this Q-PCR FDNA screening technique demonstrates its use to distinguish different types of resistance and could be used to facilitate the evaluation of soybean breeding populations, where precise quantification of incomplete and/or partial resistance is needed to identify and map quantitative trait loci.