Stacking Resistance Genes in Multiparental Interspecific Potato Hybrids to Anticipate Late Blight Outbreaks

Stacking (pyramiding) several resistance genes of diverse race specificity in one and the same plant by hybridization provides for high and durable resistance to major diseases, such as potato late blight (LB), especially when breeders combine highly efficient genes for broad-spectrum resistance that are novel to the intruding pathogens. Our collection of potato hybrids manifesting long-lasting LB resistance comprises, as a whole, the germplasm of 26 or 22 Solanum species (as treated by Bukasov and Hawkes, respectively), with up to 8–9 species listed in the pedigree of an individual hybrid. This collection was screened with the markers of ten genes for race-specific resistance to Phytophthora infestans (Rpi genes) initially identified in S. demissum (R1, R2, R3a, R3b, and R8), S. bulbocastanum/S. stoloniferum (Rpi-blb1/ Rpi-sto1, Rpi-blb2, Rpi-blb3) and S. venturii (Rpi-vnt1). The hybrids comprised the markers for up to four-six Rpi genes per plant, and the number of markers was significantly related to LB resistance. Nevertheless, a considerable portion of resistance apparently depended on presently insufficiently characterized resistance genes. Bred from these multiparental hybrids, the advanced lines with the stacks of broad-specificity Rpi genes will help anticipate LB outbreaks caused by rapid pathogen evolution and the arrival of new pathogen strains.

Strategies for RUN1 Deployment Using RUN2 and REN2 to Manage Grapevine Powdery Mildew Informed by Studies of Race Specificity

The Toll/interleukin-1 receptor nucleotide-binding site leucine-rich repeat gene, “resistance to Uncinula necator 1” (RUN1), from Vitis rotundifolia was recently identified and confirmed to confer resistance to the grapevine powdery mildew fungus Erysiphe necator (syn. U. necator) in transgenic V. vinifera cultivars. However, sporulating powdery mildew colonies and cleistothecia of the heterothallic pathogen have been found on introgression lines containing the RUN1 locus growing in New York (NY). Two E. necator isolates collected from RUN1 vines were designated NY1-131 and NY1-137 and were used in this study to inform a strategy for durable RUN1 deployment. In order to achieve this, fitness parameters of NY1-131 and NY1-137 were quantified relative to powdery mildew isolates collected from V. rotundifolia and V. vinifera on vines containing alleles of the powdery mildew resistance genes RUN1, RUN2, or REN2. The results clearly demonstrate the race specificity of RUN1, RUN2, and REN2 resistance alleles, all of which exhibit programmed cell death (PCD)-mediated resistance. The NY1 isolates investigated were found to have an intermediate virulence on RUN1 vines, although this may be allele specific, while the Musc4 isolate collected from V. rotundifolia was virulent on all RUN1 vines. Another powdery mildew resistance locus, RUN2, was previously mapped in different V. rotundifolia genotypes, and two alleles (RUN2.1 and RUN2.2) were identified. The RUN2.1 allele was found to provide PCD-mediated resistance to both an NY1 isolate and Musc4. Importantly, REN2 vines were resistant to the NY1 isolates and RUN1REN2 vines combining both genes displayed additional resistance. Based on these results, RUN1-mediated resistance in grapevine may be enhanced by pyramiding with RUN2.1 or REN2; however, naturally occurring isolates in North America display some virulence on vines with these resistance genes. The characterization of additional resistance sources is needed to identify resistance gene combinations that will further enhance durability. For the resistance gene combinations currently available, we recommend using complementary management strategies, including fungicide application, to reduce populations of virulent isolates.

Characterization of Late Blight Resistance Derived from Solanum pimpinellifolium L3708 against Multiple Isolates of the Pathogen Phytophthora infestans

Sixteen tomato [Solanum lycopersicum L. (syn. Lycopersicon esculentum Mill.)] genotypes (inbred lines or hybrids) were tested against five Phytophthora infestans (Mont.) deBary isolates to characterize race specificity of late blight resistance transferred to tomato from Solanum pimpinellifolium L. [syn. Lycopersicon pimpinellifolium (L.) Mill.] accession L3708. The effects of plant genotype, isolate, genotype × isolate, and isolate × replication interactions were highly significant (P = 0.001). Set of four sister lines fixed for late blight resistance (CU-R lines) exhibited full and equal resistance to the five pathogen isolates tested. In contrast, the heterozygous F1 hybrids, created by crossing the resistant CU-R lines with a susceptible parent, were resistant to US-11; partially resistant to US-17, NC-1, and DR4B; and susceptible to US-7. Differential responses were also observed across pathogen isolates on a set of resistant sister lines (CLN-R lines), which also were bred from L3708. The CLN-R lines were resistant to the DR4B, NC-1, and US-11 isolates, but showed significant disease-affected areas and sporangium numbers following inoculation with either US-7 or US-17. Restriction fragment length polymorphism (RFLP) analysis confirms that both CU-R and CLN-R are homozygous for the Ph-3 gene derived from L3708. Since progeny tests also confirmed that the CLN-R lines are fixed for their level of resistance, these results suggest that late blight resistance in the CU-R lines is not controlled by Ph-3 alone, and that at least one additional gene conferring late blight resistance is missing from the CLN-R lines. Results of genetic tests of the (CU-R × CLN-R) F1 and a (CU-R × CLN-R) F2 population with the pathogen isolate US-17 strongly support a model in which resistance of the CU-R lines requires genes in addition to Ph-3. The implications of this information in breeding for late blight resistance and using of the resulting resistant lines or hybrids are discussed.