Quantitative Trait Loci Underlying Partial Resistance to Cercospora sojina Race 2 Detected in Soybean Seedlings in Greenhouse Assays
Cercospora sojina (Hara), an air-borne pathogen, infects soybean [Glycine max (L.) Merr.] leaves causing frog-eye leaf spot (FLS). Three major genes (Rcs1-3) underlie resistance to the major races of FLS but two were not yet mapped. In addition quantitative trait loci provide partial resistance to many strains. FLS race 2 was an isolate first collected in the 1950’s when damaging FLS first arose. ‘Essex’ was partially resistant while ‘Forrest’ was partially susceptible to mixed races of FLS. The objective here was to identify quantitative trait loci underlying resistance to FLS race 2 in the greenhouse using recombinant inbred lines (RILs) derived from the cross of Essex by Forrest. C. sojina race 2 (ATCC 44531) was used to induce leaf symptoms on one hundred F5:14 RILs derived from the cross of Essex by Forrest. The leaf symptoms were measured at 21 days after manual infestation by wounding (dai) and again at 42 dai to show resistance to reinfestation of new leaves from the primary lesions without wounding. Bags over leaves were not used to better simulate field conditions. However, there was no significant correlation between FLS severity at 21 and 42 dai (r =0.08 and P= 0.005). At 21 dai there was a strongly significant QTL near Satt319 on LG C2 (chromosome 7; LOD 3.8; R2 52%) where the Essex allele reduced leaf symptoms by 0.7 units. At 42 dai there was a strongly significant QTL near Satt632 on LG A2 (chromosome 8; R2 was 15%; LOD was 3.6) where the Essex allele reduced leaf symptoms by 0.4 units. Neither locus mapped to the location of Rcs3. By ANOVA thirteen additional minor loci were detected on LGs A1, B1, F, G, H, I, J, K, L, M and O. At two loci (LG B1 and O) the Forrest allele appeared to reduce FLS at both 21 and 42 dai. Eight loci may have reduced FLS at 21 dai (0.006 < P < 0.049; 4% < R2 > 9%) of which 5 had beneficial alleles from Forrest. Seven loci may have reduced FLS at 42 dai (0.001 < P < 0.04; 4% < R2 < 15%) of which 4 had beneficial alleles from Forrest. Therefore, quantitative resistance to race 2 of FLS was inferred to have major loci contributions from Essex and minor loci contributions from both Forrest and Essex. Resistance was dependent on plant age. Breeding and selection for FLS will be complex and may be more efficient with the markers, germplasm and models of inheritance reported here.