Novel Powdery Mildew and Leaf Rust Resistance

Powdery mildew bears significant impact on wheat production due to its worldwide distribution, frequent occurrence and the annual yield losses it causes. Powdery mildew resistance genes that can be easily transferred to locally adapted breeding lines are urgently needed. 

 

The Oklahoma State University Wheat Improvement Team identified a new powdery mildew resistance gene, designated PmBN418, on a 1BL.1RS translocation segment in wheat cultivar Bainong 418 by analyzing a population of inbred lines derived from CI 17884 × Bainong 418 (Figure 1). The population was genotyped using single nucleotide polymorphism (SNP) markers and evaluated for responses to powdery mildew isolates OKS(14)-B-3-1 and NEI-1-3. 

 

PmBN418 confers resistance to nine U.S. mildew isolates, eight of which are virulent to Pm8 that resides on the chromosome arm 1RS translocated from rye (Secale cereale L.) variety Petkus. Therefore, the 1RS translocation segment in Bainong 418 is different from that in Petkus, and PmBN418 is either a new gene or a new allele of Pm8.

 

WIT previously identified an incomplete leaf rust resistance gene designated QLr.stars-1RS on 1RS in Bainong 418. Leaf rust is one of the most impactful wheat diseases worldwide, and breeding leaf rust resistant varieties is paramount to the leaf rust-prone U.S. Great Plains, where most known leaf rust resistance genes have lost effectiveness due to the frequent emergence of new Puccinia triticina races. The introgression of the 1BL.1RS translocation from Bainong 418 into locally adapted varieties will simultaneously enhance both leaf rust resistance via QLr.stars-1RS and powdery mildew resistance via PmBN418. In addition, previous studies indicated that the 1RS chromosome arm carries genes for yield, wide adaptation and stress resistance. WIT developed four kompetitive allele specific PCR (KASP) markers for the 1BL.1RS translocation segment in Bainong 418, and one of them, KASP-1RS-1, can distinguish a unique allele of rye from that of wheat. Therefore, KASP-1RS-1 can be used to tag all valuable genes on the 1RS chromosome arm in Bainong 418 in wheat breeding populations.

 

 

Figure 1. Part of a linkage map of chromosome 1B containing PmBN418, a new gene for resistance to powdery mildew, on a wheat–rye 1BL.1RS translocation.

 

Novel Greenbug Resistance

Greenbug, Schizaphis graminum (Rondani), is one of the major wheat pests. Novel greenbug resistance genes are needed to diversify resistance sources. Aegilops tauschii, one of the diploid progenitors of Oklahoma bread wheat, harbors abundant greenbug resistance genes. WIT screened a large set of Aegilops tauschii accessions and synthetic hexaploid wheat lines derived from Aegilops tauschii and emmer wheat, which led to the identification of a few synthetic hexaploid wheat lines that exhibit excellent resistance to greenbug biotypes (Figure 2). Breeding populations are being developed to transfer greenbug resistance in these accessions into Oklahoma wheat varieties, and mapping of the greenbug resistance genes in these lines will commence in 2022. 

 

Figure 2. Greenbug-resistant (left) and greenbug-susceptible accessions after three weeks infestation with greenbug biotype E.

 

Pyramiding Novel Resistance Genes 

Transfer of resistance genes into WIT breeding lines continued in the 2020-21 season via marker-assisted selection. These included leaf rust resistance genes Lr470121, Lr622111, Lr47 and Qlr.stars-1RS; powdery mildew resistance genes Pm59, Pm63 and Pm65; and greenbug resistance gene Gb8. Two recombinant inbred lines (RIL) combining Lr470121, Lr34, Lr37 and Lr39 were selected from an F7 population derived from PI 470121 x Stardust for further tests. In addition, a set of over 30 RIL populations are being developed to find potential new biotic stress resistance genes. 

 

New Invasive Aphid in the U.S.

The hedgehog grain aphid (HGA), a new invasive pest first identified in the U.S. in 2007, has spread to many of the western Great Plains states and poses potential threats to wheat production in Oklahoma. A recent study indicated that all wheat breeding lines currently tested in the USDA-ARS regional hard winter wheat performance nurseries – including those submitted by OSU WIT – are highly susceptible to HGA. Therefore, identification and introgression of HGA resistance into Oklahoma wheat varieties is of economic importance. 

 

With Oklahoma Wheat Research Foundation support, WIT determined HGA responses of more than 1,700 accessions from the National Small Grains Collection core set (Figure 3). The first set of resistant or tolerant accessions was identified, including two landraces collected from Russia and one variety developed in the Czech Republic. These resistance sources will be further tested and used to develop HGA-resistant lines in Oklahoma-adapted varieties.

 

Figure 3. Chlorosis, a typical damage symptom caused by HGA infestation, is observed in susceptible accessions after two weeks of HGA infestation, while a resistant accession (red arrow) grew normally. All susceptible accessions completely died after three weeks of infestation.