Phenotyping Leaf rust and yellow rust severity, measured as percentage of leaf area infected, were evaluated in field experiments conducted over three years. Spreader rows inoculated with one selected pathotype of each rust were used to infect both leaf rust and yellow rust in the populations. The distribution of yellow rust severity in Avocet x Pavon 76 population is shown in Figure 1.

Marker Development

Bulked segregant analysis (BSA) with AFLPs and linkage mapping with a set of microsatellites and RFLPs were used. In each of the two populations, bulks were made by taking equal amounts of DNA from 7-12 entries that were most resistant to both leaf rust and yellow rust as well as most susceptible to the two diseases jointly. In addition to the bulks made that accounted for the two diseases jointly, bulks were also made individually for leaf rust and yellow rust. Forty-eight Pst1 / Mse1 primer combinations were used in BSA. Polymorphic AFLPs selected from BSA were screened across the full populations.

The MAPMAKER program was used for linkage analysis and qGene 2.27 was used for QTL analysis.

Results

Avocet x Pavon 76 Population

Bulked segregant analysis enabled the identification of markers linked to Lr46 and it was also observed that there is another gene in close association with Lr46 that confers resistance to yellow rust (newly designated as Yr29). By mapping the AFLP markers in the ITMI population, the genomic location of Lr46/Yr29 was established in the distal end of chromosome 1BL (Fig. 2, 3). Table 1 summarizes the loci identified in Avocet X Pavon 76 that condition resistance to leaf rust and yellow rust. In this population, we have been able to identify three loci located on chromosomes 1BL, 4B, and 6A that confer resistance to both leaf rust and yellow rust. There are two other loci, located on chromosomes 6B and 3BS that have significant effects only on yellow rust (Table 1).

 

Avocet x Parula Population

Linkage mapping in this population enabled the identification of a microsatellite marker that is linked to leaf tip necrosis (Ltn) at 4.5cM, which is known to be tightly linked to Lr34/Yr18 (Table 2). In addition, BSA enabled identification of three other loci that have significant effects on leaf rust. Three out of the four loci found to be associated with resistance to leaf rust also have some effect on yellow rust (Table 2).

Conclusions

• In the two populations studied, we identified a total of 7 and 9 QTLs for leaf rust and yellow rust resistance, respectively.
• Six of the above QTLs, including the Lr34/Yr18 and Lr46/Yr29 regions, conferred resistance to both leaf and yellow rusts.
• Two slow rusting resistance genes for yellow rust resistance have been designated as Yr29 and Yr30.
• Gene Yr30 is in the same 3BS chromosomal region where slow rusting stem rust resistance gene Sr2 is located.