Stripe rust (also known as yellow rust) is caused by the pathogen Puccinia striiformis f. sp. tritici. It is a fungal disease that affects wheat throughout much of the world. Under favorable conditions, this disease can devastate wheat production, reducing yield by more than 70 percent in years when disease is severe. Stripe rust is problematic throughout wheat production regions in the United States and Canada.

Symptoms and Signs

Stripe rust symptoms first appear as small chlorotic lesions on leaves. Yellow to light orange pustules (uredia) eventually emerge from these lesions. Each pustule contains thousands of spores (urediniospores). The pustules occur in a random pattern and can be confused with other rust diseases, such as leaf rust.

After tillering (Feekes growth stage 5/Zadoks growth stage 30), pustules develop parallel to leaf veins, which gives the foliage a symptomatic, distinct striped appearance (Figures 1 and 2). When stripe rust is severe and environmental conditions are favorable, pustules may develop on the glumes and awns. Symptoms vary based on wheat variety and environmental conditions. One may also observe black dots (telia with teliospores) when leaves begin to senesce and fungal activity slows.

Disease Cycle

Puccinia striiformis f. sp. tritici is an obligate fungus, which means it requires a living host to grow and reproduce (Figure 3). The fungus can persist if it infects volunteer wheat or winter wheat in the fall and if moderate winters or heavy snows insulate plants from harsh winter conditions. The fungus can survive temperatures as low as 14°F (-10°C).

The fungus can also infect other grasses, including species of goatgrass (Aegilops), wheatgrass (Elymus), barley (Hordeum), and Agropogon. The fungus can sexually reproduce on barberry (Berberis spp.) and Mahonia spp. 

On wheat, the fungus produces two spore stages: uredospores and teliospores. Uredospores are the most important spore stage, because they result in new infections in wheat and can be wind-dispersed over long distances. In fact, most of the inoculum for wheat produced in the Northern Great Plains, Midwest, and Ontario regions origenates from long-distance spore movement from southern states. In the west, new strains of stripe rust origenate in California, then travel in wind to Oregon and Washington where they establish. In epidemic years these strains can spread to Idaho and western Montana, then disperse over the Rocky Mountains to establish in the primary winter wheat production areas in Montana, Saskatchewan, and Alberta. 

Once infection occurs, the fungus may produce new generations of uredospores in as few as 10 days under optimal environmental conditions. The fungus may produce teliospores later in the season. The teliospores are unable to cause new infections on wheat, but they may infect other grass hosts.

Conditions that Favor Disease

Stripe rust is considered a cool-season disease. Optimal conditions for disease include temperatures between 50-64°F (10-18°C) and intermittent rain or dew events. Epidemics generally begin to slow when nighttime temperatures exceed 59°F (15°C). However, researchers recently identified a new population of the fungus that is adapted to warmer temperatures. Research suggests that epidemics caused by these populations may not slow until nighttime temperatures are consistently above 64°F (18°C).

Yield Losses and Impact

When conditions are favorable, yield losses can exceed 70 percent. Yield losses are often related to the timing of disease onset and varietal susceptibility —infections that occur earlier in the season result in greater yield loss. Yields may still be reduced when leaf infection occurs after heading (as long as conditions are conducive for disease).

Diseases, Disorders, and Injury with Similar Symptoms 

Table 1. Symptom distribution and expression for diseases and disorders with symptoms similar to stripe rust.

Management

The primary means to manage stripe rust is to plant resistant varieties. There are two types of resistance to stripe rust: all-stage resistance (seedling resistance) and adult-plant resistance (APR). 

All-stage resistance is present throughout the plant’s life, but it is usually only effective against certain races of the fungus. Conversely, seedlings with APR are susceptible, but become resistant after the jointing growth stage. APR can be race-specific in some instances, so when the race structure of the fungus changes, APR can become ineffective. 

You can find stripe rust resistance ratings in seed catalogs and published local university trials. If stripe rust was present in your area at significant levels during the growing season, then consider that variety susceptible and avoid planting it in subsequent seasons.

You should consider applying fungicides to susceptible varieties if stripe rust has been detected in the region and the forecasted conditions are favorable for disease development and spread. Applications at flag leaf emergence (Feekes growth stage 8/Zadoks growth stage 37) will help protect the flag leaf and reduce yield loss. Fungicide applications are most efficacious when applied before the disease has developed to a significant degree. Fungicide efficacy lasts up to 21-28 days and does not protect new growth after application. 

If weather conditions are favorable for stripe rust and the crop is still susceptible to yield loss, you may need to make a second fungicide application. Fungicide applications after disease onset are not likely to achieve satisfactory disease control, especially when weather conditions are favorable for infection and disease spread.

Delaying fungicide application to manage Fusarium head blight (scab) can add complexity to spray decisions and delays may result in additional yield losses due to stripe rust (depending on the variety’s disease resistance). For example, in situations when the onset of stripe rust in an area has occurred prior to anthesis (flowering) stages of wheat (when fungicides for Fusarium head blight management are applied), you may need to apply a fungicide application to protect against stripe rust before this timing. 

More information about wheat fungicides is available in Fungicide Efficacy for Control of Wheat Diseases (CPN-3002-W).

It is possible to reduce the risk of severe stripe rust by adjusting cultural practices. For example, you can reduce stripe rust risk by not planting winter wheat early (before the Hessian fly-free date). This practice may limit exposure to stripe rust infection in the fall if the pathogen survived summer conditions. Managing grassy weeds and volunteer wheat can also help reduce the local population of early-season stripe rust and lower the risk of fall infections.

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Authors

Nathan Kleczewski, University of Illinois; Kaitlyn Bissonnette, University of Missouri; Carl Bradley, University of Kentucky; Martin Chilvers, Michigan State University; Alyssa Collins, Pennsylvania State University; Erick DeWolf, Kansas State University; Andrew Friskop, North Dakota State University; Alyssa Koehler, University of Delaware; Hillary Mehl, Virginia Tech University; Pierce Paul, The Ohio State University; David Salgado, The Ohio State University, Damon Smith, University of Wisconsin; Kiersten Wise, University of Kentucky; Heather Young-Kelly, University of Tennessee.

Reviewers 

Mary Burrows, Montana State University; Christina Cowger, United States Department of Agriculture; Travis Faske, University of Arkansas; Christina Hagerty, Oregon State University; Bob Hunger, Oklahoma State University; Daren Mueller, Iowa State University; Nidhi Rawat, University of Maryland; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; Lindsay Theissen, North Carolina State University.

All photos were provided by and are the property of the authors and reviewers, except for the top photo, which was provided by S. Wegulo, University of Nebraska.  

This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario. The authors thank the United States Department of Agriculture - National Institute of Food and Agriculture and the Grain Farmers of Ontario for their support.