Wheat leaf rust in the United States in 2020

Authors: Kolmer, James; Fajolu, Oluseyi

Submitted to: Wheat Newsletter
Publication Type: Other
Publication Acceptance Date: 6/2/2021
Publication Date: 8/28/2021
Citation: Kolmer, J.A., Fajolu, O.L. 2021. Wheat leaf rust in the United States in 2020. Wheat Newsletter. Available: https://www.ars.usda.gov/midwest-area/stpaul/cereal-disease-lab/docs/cereal-rusts.

Interpretive Summary: Wheat is attacked by the rust fungus called Puccinia triticina, which causes the disease wheat leaf rust. There are many different forms or races of the wheat leaf rust fungus that vary in their ability to attack different resistance genes in wheat. Every year the USDA, ARS, Cereal Disease Laboratory makes collections of the wheat leaf rust fungus from the major wheat growing regions of the United States to determine which forms of P. triticina are present. In 2020, 36 different forms of the leaf rust fungus were found in the United States. The forms with virulence to the resistance genes in the commonly grown hard red winter wheats were widespread throughout the eastern states and the Great Plains region. The races with virulence to the genes present in the soft red winter wheats were found in the eastern states and the Ohio Valley region. The most commonly grown hard red winter wheat cultivars and soft red winter wheat cultivars are susceptible to the most common leaf rust races found in the regions where these wheat cultivars are grown. It will be important to develop wheat cultivars with new combinations of leaf rust resistance genes in the winter wheat regions. The results from the annual virulence survey are used by wheat breeders and plant pathologists to help develop wheat cultivars that are very resistant to the leaf rust disease.

Technical Abstract: Leaves of wheat infected with the leaf rust fungus, Puccinia triticina, were obtained from farm fields and breeding plots at experimental stations in the Great Plains, Ohio River Valley, and southeastern states in 2020 in order to identify virulence phenotypes prevalent in the United States in different wheat growing regions. A total of 260 single uredinial isolates derived from the leaf rust collections were tested for virulence to 20 lines of Thatcher wheat that differ for single leaf rust resistance genes. A total of 36 virulence phenotypes were described in the United States in 2020. The three most common virulence phenotypes across the United States were TBBGS, MNPSD, and BBBQB. TBBGS is virulent to genes Lr21 which is in many hard red spring wheat cultivars, and Lr39 which is in many hard red winter wheat cultivars. Phenotype MNPSD is virulent to Lr17, Lr37, and Lr39, and had virulence to seedlings and adult plants of the cultivar SY Monument. BBBQB is virulent to most durum wheat cultivars. This race was found on susceptible wheat cultivars in the Rio Grande Valley of Texas. The hard red winter wheat SY Monument, widely grown in the central Great Plains had increased rust severity in 2020. The P. triticina population in the United States was characterized by two major regional groups of virulence phenotypes in the Great Plains region where hard red winter and spring wheat cultivars are grown, and in the southeastern states and Ohio Valley region where soft red winter wheat cultivars are grown.