New races of wheat leaf rust with virulence to Lr21 detected for first time in North America.
The leaf rust resistance gene Lr21 was originally derived from the wild wheat relative Triticum tauschii. Virulence to this gene had not been detected in isolates of Puccinia triticina (wheat leaf rust fungus) in North America in previous years. Lr21 is present in hard red spring wheat cultivars such as Glenn, Faller and RB07, which account for about 50% of the wheat acreage in North Dakota and Minnesota. Spring wheat cultivars with Lr21 have been highly resistant to leaf rust. In 2010 in research plots in North Dakota and Minnesota the wheat cultivars Faller, Glenn, and RB07 with Lr21 had higher levels of leaf rust than had been observed in previous years. Wheat leaf rust races TFBGQ and TFBJQ - both with virulence to seedling wheat plants with Lr21, were identified in the leaf rust collections from Minnesota and North Dakota. Seedlings of the cultivars Glenn, Steele, Faller, RB07, Amidon, and AC Cora all with Lr21, were susceptible to both of these races. McKenzie, which has Lr16 in addition to Lr21 had an intermediate infection type to both races. Both races are well adapted to the spring wheat germplasm as they are virulent to a number of other leaf rust resistance genes that are common in these wheat cultivars. In addition both races are virulent to genes Lr24 and Lr26 that are present in the winter wheats. The overall high levels of virulence in the races with Lr21 virulence may allow these races to become established in the winter and spring wheat region of the Great Plains. Based on the virulence, it appears that both races were derived by mutation and selection for Lr21 virulence from the existing P. triticina population in North America. If races with Lr21 virulence increase in the next few years then leaf rust severities will begin to increase on wheat cultivars that have relied on this gene for effective leaf rust resistance.
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