Leaf, Stem, and Stripe Rust Diseases of Wheat

Introduction Rust diseases of wheat are among the oldest plant diseases known to humans. Early literature on wheat cultivation mentions these devastating diseases and their ability to destroy entire wheat crops. Since rust discovery, numerous studies have been conducted on the life cycles of rust pathogens and their management. The information gained from these studies has enabled us to develop best management practices that reduce the impact of the diseases. Today, worldwide epidemic losses are rare, though the diseases can occur at significant levels in particular fields or throughout a particular growing region. The persistence of rust as a significant disease in wheat can be attributed to specific characteristics of the rust fungi. These characteristics include a capacity to produce a large number of spores—which can be wind-disseminated over long distances and infect wheat under favorable environmental conditions—and the ability to change genetically, thereby producing new races with increased aggressiveness on resistant wheat cultivars. Leaf rust is generally found on leaves but may also infect glumes and awns. Symptoms begin as small circular to oval yellow spots on infected tissue of the upper leaf surface. As the disease progresses, the spots develop into orange-colored pustules that may be surrounded by a yellow halo (Figure 1). The pustules produce a large number of spores that are easily dislodged from the pustule, resulting in an “orange dust” on the leaf surface or on clothes, hands, and equipment. As the disease progresses, black spores may be produced, resulting in a mixture of orange and black lesions on the same leaf. Tiny orange lesions may be present on seed heads, but these lesions do not develop into erumpent pustules. This difference helps to distinguish leaf rust from stem rust. Stripe rust is distinguished by the presence of light yellow, straight-sided pustules that occur in stripes on leaves and heads. These elongate pustules are narrow and vary in length. As the pustules mature, yellow-orange spores are produced (Figure 1). As the disease progresses, tissues around the pustules turn brown and dry, resulting in a scorched appearance. The arrangement of pustules into stripes is an important distinguishing characteristic of this disease. Chlorosis, or yellowing, of leaves can be quite evident with both leaf and stripe rust, and fields with plants displaying severe symptoms may be easily detectable from a distance (Figures 2 and 3). Symptoms There are three rust diseases that occur on wheat: stem rust, leaf rust and stripe rust. These diseases are each caused by a particular species of the “rust” fungus, Puccinia. Rust fungi all produce similar disease symptoms on the host plants and have similar requirements for infection. The diseases get their name from their appearance on the plant (Figure 1). Infection can occur on any above-ground plant part, leading to the production of pustules that contain thousands of dry yellow-orange to reddish-brown or black spores. These pustules give the appearance of “rust” on the plant. Stem rust occurs primarily on stems but can also be found on leaves, sheaths, glumes, awns, and even seed. Symptoms begin as oval to elongate lesions that are generally reddish-brown in color. In the late stages of the disease, erumpent pustules produce numerous black sooty spores. Severe infestations with many stem lesions may weaken plant stems and result in lodging. Causal Organisms Stem rust (also known as black stem rust) is caused by Puccinia graminis f. sp. tritici. It is primarily a disease on wheat, though it can also cause minor infections on certain cultivars of barley and rye. Leaf rust is caused by Puccinia recondita f. sp. tritici (now known as Puccinia triticina). Like the stem rust fungus, this pathogen is primarily a problem on wheat, but it may be weakly pathogenic on some cultivars of barley, triticale, and some species of goatgrass and wheatgrass. The third rust disease, stripe rust, is caused by Puccinia striiformis. This pathogen also affects barley, rye, triticale, and over 18