Field Phenotyping of Wheat Stripe Rust and Leaf Tip Necrosis Associated with Rust Resistance Genes

The three fungal diseases-leaf rust, stem rust, and stripe rust-are considerable challenges to wheat production, causing up to 20%, 50%, and 70% yield losses, respectively, in North America and across the world. Control strategies include regular introduction of resistant varieties as well as fungicidal applications. Stripe rust, caused by Puccinia striiformis f. sp. tritici, is best controlled by utilizing genetic resistance and regularly introducing resistant varieties to combat the rapid breakdown of host resistance by the pathogen. Genetic resistance to stripe rust can be characterized as adult plant resistance (APR) or all-stage resistance (ASR), the former being largely preferred due to the durable mechanisms governing its resistance. Breeding programs utilize large germplasm collections to screen for resistance. Field phenotyping is a critical component for breeding programs, helping in selection of resistant breeding lines to artificial epidemics of relevant pathotypes. This chapter summarizes the methods and protocols for the field phenotyping of stripe rust at the adult-plant stage, including the steps for inoculation, phenotyping, and an overview of the favorable environmental conditions for optimal results. We also summarize methods for phenotyping leaf tip necrosis (LTN), a morphological trait expressed via slow-rusting APR genes. We demonstrated the process of screening over 30,000 lines of wheat-including the organization of entries using, ideal stage for inoculum application to screen for APR, and the ideal stage for phenotyping 10-18 days postinoculation (DPI). In conclusion, field phenotyping is critical in the assessment of stripe rust resistance and development of resistant varieties and is a major component in combatting global stripe rust epidemics in a sustainable and environmentally considerate way.