Wheat Rust Diseases

Identification and lifecycle

Leaf rust, also known as brown rust, is caused by the fungus Puccinia triticina. This rust disease occurs wherever wheat, barley and other cereal crops are grown. Leaf rust attacks foliage only. Identifying symptoms are dusty, reddish-orange to reddish-brown fruiting bodies that appear on the leaf surface. These lesions produce numerous spores, which can cover nearly the entire upper leaf surface.

Leaf rust spores are spread by wind and splashing water. Optimal environmental conditions for development of infection are temperatures ranging from 59 to 68 degrees F and at least six hours of moisture on the leaf surface. With wet weather and ideal temperatures, lesions are formed within seven to 10 days of infection, and spore production repeats another disease cycle. Leaf rust fungi survive to overwinter in wheat crops in southern states. These overwintering fungi develop spores in the spring, which are blown north to infect wheat in the Great Plains and Midwest states.

Crop Damage

Leaf rust causes the most damage when severe rusting covers the upper leaves before flowering. Early defoliation can occur, reducing time for grain fill and leading to the creation of smaller kernels. Grain shrivels, and any nutrients produced – primarily in the flag leaf – are used by the fungal infection instead of being transported to the grain. Early infection can result in weak plants and poor root and tiller development.

Significant yield losses can occur from leaf rust disease. The University of Nebraska reports that in the Great Plains, severe epidemics have caused yield losses of up to 14 percent. In some areas they have even surpassed 50 percent, depending upon weather conditions, disease development early in the growing season and variety susceptibility.

Grain test weight and yield damage correlates to the level of disease infection and how early in the season wheat leaf rust disease attacks. North Dakota State University research reports 30 to 40 percent yield losses when infection becomes severe prior to flowering, with flag leaf damage exceeding 60 percent and, in some cases, up to 100 percent.

Identification and lifecycle

Stem rust, also known as black rust, is caused by the fungus Puccinia graminis f. sp. tritici. It infects wheat and other cereals across all U.S. growing regions. Stem rust occurs when raised spots (pustules) form on stems and leaf sheaths, although occasionally they may form on awns, glumes and seeds.

Stem rust spots appear elliptical and differ from leaf and stripe rust in that they are more elongated. The spots form on both lower and upper leaf surfaces and look orange to dark-red in color. The margins on stem rust spots are ragged. Young pustules release numerous spores. Later in the growing season, spores transform and become dark colored, hence the common name of black rust.

Stem rust development requires the warmest temperatures of the three wheat rusts –ideally 59 to 84 degrees F and six to eight hours of moisture on the leaf surface. With wet weather and optimal temperatures, new lesions are formed in seven to 10 days. Stem rust disease also spreads spores through wind dispersal and splashing water. Spores are produced in multiple cycles during the growing season.

Overwintering spores that develop in the previous year’s wheat crop late in the season survive to produce additional spores, which spread via wind to infect wheat. Thus, another lifecycle is completed. In most U.S. growing regions, the primary source of stem rust infection in wheat is repeated spore production.

Crop Damage

The disease spreads rapidly and can travel long distances by wind or other means of transportation such as farm equipment or plant materials. With severe infections, the disease can turn a healthy crop into a tangle of black stems only weeks away from harvest, resulting in shriveled grain. According to the United Nations Food and Agriculture Organization (FAO), stem rust can result in 70 percent or more loss in wheat yield.

While stem rust has historically been the most damaging disease of wheat, it’s not as prevalent today thanks to resistant varieties.

Identification and lifecycle

Stripe rust, also known as yellow rust, is caused by the fungus Puccinia striiformis f. sp. tritici. The disease primarily occurs on leaves, although glumes and awns may also be affected. This part of the lifecycle leads to “hot spots” of infection seen in crops in later winter and early spring.

In lower humidity, stripe rust spores disperse more freely into the air and can travel for much greater distances. This may result in a uniform pattern of disease development beginning in mid-spring.

In recent years, stripe rust has become more common in the Great Plains states. It reached high levels in early spring 2015 due an unusually wet spring across most of the region.

Stripe rust appears earlier in the season because development is enhanced by the cool, moist weather early in the growing season, versus leaf rust, which is more prevalent later in the spring when temperatures warm. Stripe rust develops under ideal temperatures of 45 to 54 degrees F with six to eight hours of leaf moisture.

Stripe rust typically produces yellow or orange blister-like pustules that are arranged in stripes. The lesions produce massive amounts of spores that are easily dislodged and dispersed by wind. The spores may leave orange dust on the clothing of individuals walking through heavily infected fields.

When stripe rust spores land on a living wheat leaf, germination and infection can result. Growing inside the leaf, stripe rust produces new lesions containing new spores. Under high-humidity conditions during winter, most spores survive in small clumps that are relatively heavy and fall quickly out of the air when dispersed. As a result, spores spread mostly over short distances and may appear in uniform patterns of disease seen in mid-spring. Several cycles of spore production occur throughout the growing season.

Crop Damage

According to the University of Nebraska, it’s possible for stripe rust to cause 100 percent crop loss in susceptible varieties if the disease begins early in the season under wet, cool weather conditions.