|Harris, Donna - UNIVERSITY OF GEORGIA|
|Sikora, Edward - AUBURN UNIVERSITY|
|Weaver, David - AUBURN UNIVERSITY|
|Mueller, John - CLEMSON UNIVERSITY|
|Kemerait, Robert - UNIVERSITY OF GEORGIA|
|Buck, James - UNIVERSITY OF GEORGIA|
|Phillips, Daniel - UNIVERSITY OF GEORGIA|
|Li, Zenglu - UNIVERSITY OF GEORGIA|
|Boerma, H - UNIVERSITY OF GEORGIA|
Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2017
Publication Date: N/A
Interpretive Summary: Soybean rust is a destructive foliar disease that can cause severe yield losses. Susceptible soybean plants develop "TAN" lesions with abundant urediniospore production, whereas resistant plants either have immune reactions (rare) or develop reddish-brown (RB) lesions with low levels of spore production, which nevertheless result in a reduction in photosynthetic leaf area. We evaluated the effects of lesion type (RB vs. TAN) and canopy severity (low vs. high densities of lesions) on yield using eight related soybean lines derived from the resistant cultivar Hyuuga. Each combination of lesion type and canopy severity rating was represented by two lines, and all of the lines were grown in six environments in the southeastern United States where natural infection by the soybean rust fungus (Phakopsora pachyrhizi) was likely to occur. One set of plots was sprayed with fungicide and used as a rust-free control to allow assessment of rust-related yield loss in adjacent unprotected plots. Yields of infected plants from the four lines that developed RB lesions (indicating partial resistance) were not significantly lower than those of plants protected by fungicide, whereas unprotected plants that developed TAN lesions suffered significant yield losses. These results indicated that even though the development of RB lesions reduced the area of photosynthetic leaf tissue, this did not cause a significant loss of yield, whereas yield losses for the four lines with a TAN (susceptible) reaction were considerably higher. Since most rust-resistant soybean germplasm accessions develop an RB-type of reaction to the fungus, and use of multiple resistance genes is considered essential for prolonging the efficacy of each gene, soybean breeders should not hesitate to use the RB type of resistance for developing resistant cultivars, even though such plants are not immune to the soybean rust fungus.
Technical Abstract: Soybean rust, caused by the fungal pathogen Phakopsora pachyrhizi, occurs in many areas of the world and is a destructive foliar disease. Susceptible soybean leaves exhibit a “TAN” reaction characterized by lesions with abundant urediniospores, while resistant reactions involve either an immune response or development of reddish-brown (RB) lesions with little or no sporulation. Disease severity on hosts that develop an RB response, such as the cultivar Hyuuga, is more extensive than on hosts with an immune response. Although sporulation is generally suppressed, the RB lesions reduce photosynthetic area. In this study the effects of lesion type (RB vs. TAN) and canopy severity (low vs. high lesion densities) on yield loss were investigated using eight Hyuuga-derived recombinant inbred lines (RILs) chosen for their reactions to P. pachyrhizi. When challenged with the pathogen, two RILs developed RB lesions and had high canopy severity ratings, two had RB lesions and low canopy severity ratings, two developed TAN lesions and had high canopy severity ratings, and two had TAN lesions and low canopy severity ratings. The treatment design was a split plot design with fungicide treatment as the main plot, and the RILs as the subplots. Across six environments, the TAN/low severity and TAN/high severity lines averaged 431 and 417 kg ha-1 more in fungicide treated plots, while the RB/low severity and RB/high severity lines averaged 7 and 142 kg ha-1 more in the fungicide treated plots, respectively, with an LSD(0.05) of 299 kg ha-1. For both the RB/low canopy severity and RB/high canopy severity lines, there were no significant yield differences between fungicide-protected plots and unprotected plots. These results indicate that despite the reduction in photosynthetic leaf area resulting from the RB-type reaction of Hyuuga, this type of resistance can reduce soybean yield losses caused by soybean rust.