There are three different rust diseases that affect wheat—leaf rust (also known as brown rust or orange rust), stripe rust (commonly known as yellow rust), and stem rust (commonly referred to as black rust of black stem rust). Of these, leaf rust is the most frequently occurring in Ohio, but in any given year, any of these diseases can infect and cause substantial yield losses if not adequately managed. Rusts are notorious for their ability to spread rapidly and reduce wheat yield and quality. It all depends on the susceptibility of the variety, race of the pathogen present, timing of infection, and the weather conditions. Damage to wheat depends on the growth stage at the time of infection and the overall level of rust severity. High levels of disease before or during flowering usually have the greatest impact on yield. Rust causes losses by reducing the number of kernels per head and the size of the kernels, and by lowering test weight and the protein content of the grain. In the case of stem rust, additional losses may result from girdling of the stems which cause plants to lodge. However, for years, the widespread use of rust-resistant varieties has substantially reduced losses caused by leaf, stripe and stem rust. In addition, since none of the rust fungi typically overwinter in Ohio and other parts of the Midwest, spores have to be blown up from the south in order for these diseases to develop, and in most years, this usually occur very late in the season, towards the end of grain development. Some leaf rust can be found on volunteer plants in the fall, but these fall infections appear to be of limited importance for the occurrence and spread of the disease in the spring. In Ohio, late May and early June are times when rust infection becomes critical and rust is more damaging on late-maturing varieties in years when cool, moist weather persists into mid-summer, extending the growing season.
The Rust Fungi
Leaf, stripe, and stem rust are caused by Puccinia recondita f. sp. tritici, Puccinia striiformis f. sp. tritici, and Puccinia graminis f. sp. tritici, respectively. These pathogens are specialized into numerous physiologic races that are identified by their reactions on an established set of differential wheat varieties. A complex system has been developed to keep track of the hundreds of known races. Any given variety may be immune, resistant or susceptible to a race of rust, but no variety is resistant to all races of any of the three rusts. Every few years new races of these fungi arise, causing previously resistant varieties to become infected and diseased. The life span of a rust resistant variety is usually from 2 to 4 years.
All rusts are typified by the presence of rusty-colored pustules erupting through the plant surface. They can be distinguished from other leaf diseases by rubbing or smearing the rust spores on the leaf surface with your finger. On wheat, leaf, stripe, and stem rusts are distinguished from each other based on the color, size, and arrangement of the pustules on the plant surface and the plant part typically affected.
Pustules of leaf rust, found predominantly on the leaf blade and sheath, are small, up to 1/16 inch long, round to oval fruiting bodies (uredinia) of the rust fungus (Fig. 1). Reddish-orange urediospores develop within the uredinia and rupture the epidermis of the leaf surface as the spores mature. Pustules can be either scattered or clustered on the leaves and leaf sheaths of infected plants. Each pustule contains thousands of spores. Leaf rust developing from fall infections usually appears first on the lower leaves and progress up the plant to the upper leaves by mid-June. However, infections usually occur first on the upper leaves due to the fact that wind-blown spores are deposited out of the air during spore showers. Under severe epidemics, pustules may develop on the awns and glumes of the heads or occasionally on the stem below the head.
Strip rust pustules are yellowish-orange, much smaller than those of leaf rust, and are neatly arranged in groups forming distinct stripes on the leaf surface (Fig. 2). For stem rust, on the other hand, pustules are much larger, orange-red, oval to elongated, and develop predominantly on the stem, leaf blade and sheath, and occasionally on parts of the spike. One of the most characteristic features of stem rust that helps to separate it from the other two rusts is the fact that the uredinia tear the plant tissue, giving the affects stem and leaf a distinctly tattered appearance (Fig. 3).
|Fig. 1. Leaf rust.
Image courtesy of E DeWolf
|Fig. 2. Stripe rust||Fig. 3. Stem rust.
Image courtesy of E DeWolf
Disease Cycle and Epidemiology
Wheat rusts have very complex life cycles that include two hosts (wheat, the primary host, and an alternate host) and five different spore stages. However, the in-season rust cycle in Ohio is fairly simple, since the alternate host and three of the five spore stages are of little importance for in-season rust development. In Ohio and other parts of the Midwest, the urediospore stage is the spore type responsible for dispersal and infection of the wheat crop. Urediospores overwinter on infected wheat in the more moderate climate of the southern states and Mexico, and are carried northward by the wind. Under favorable temperature and moisture conditions, urediospores germinate and infect leaves within 6 to 8 hours after landing on the plant surface. Once established, a new crop of urediospores may be produced every 7 to 14 days if environmental conditions are favorable. The earlier rust develops, the more spore and disease cycles are likely to occur during the season and the greater the risk of severe epidemics and yield loss. Frequent heavy dew, light rain, or high humidity and temperatures of 77 to 86°F are ideal for leaf rust development. Stem rust has a similar optimum temperature range, but stripe rust develops best under much cooler conditions (50 to 64°F). All three diseases are spread by wind blow urediospores from plant to plant and from field to field until the crop matures. As the plant matures, black, submerged pustules develop on the leaves, leaf sheaths, stems, and spikes, depending on the rust. These pustules (telia) contain the winter spores (teliospores). Teliospores do not infect wheat. Telia may not develop when plants become infected very late in the season (close to maturity). In the fall urediospores are blown southward and infect wheat and overwinter as urediospores or mycelium on volunteer wheat plants.
- Plant rust-resistant varieties adapted to Ohio conditions. Be willing to change varieties when rust epidemics occur.
- Plant after the Hessian fly-safe date recommended for your area. Early planting increases the chance of fall infections. Severely infected seedlings are stressed and are more prone to winter injury.
- Adequate, balanced fertilization based on a soil test should reduce the possibility of severe yield loss due to rust. High N rates without sufficient P and K may increase rust severity, reducing grain yields.
- Fungicides are available for control of leaf rust on wheat. Application of a fungicide is often recommended when the variety is susceptible, the disease started early and the flag leaf is in danger of becoming infected.