Increased Virulence of Wheat Rusts and the Threat to Global Crop Production

Abstract

Stem, leaf, and stripe rust diseases have caused devastating yield losses in wheat worldwide since biblical times, and continue to be a major threat to global food production. Rust pathogens produce phenomenal amounts of wind-borne fungal spores, which are rapidly dispersed over hundreds (and sometimes thousands) of kilometers where they can subsequently infect wheat fields. Rust diseases can produce several infection cycles during the crop year over a large geographical area, and can cause total yield losses under epidemic conditions. Although fungicides can control rust diseases, they are expensive and not easily available in some areas of the world. The most economical, efficient, and environmentally sound method of rust control is using resistant cultivars. Major efforts have been made by plant breeders worldwide to develop high-yielding rust-resistant cultivars since the early 1900s. Wheat cultivars produced by Dr. Norman Borlaug (father of the “Green Revolution”) were rust-resistant and grown worldwide starting in the 1950s. Since then, development of rust-resistant wheat cultivars have been accomplished by wheat breeders in many countries, which led to a general reduction in rust epidemics worldwide and apathy towards these dangerous pathogens. However, rusts are “shifty enemies” that can quickly change their ability to attack host resistance genes and acquire new combinations of virulence. Within the past decade there have been several significant outbreaks of new strains of stem rust (e.g., Ug99), leaf rust (Lr21), and stripe rust (Yr27) with increased virulence reported worldwide. This chapter describes the worldwide increase in virulence of stem, leaf, and stripe rust and their threat to wheat production.