Wheat Rust Diseases

Leaf rust, also known as brown rust, is one of the three major types of rust infections in wheat.

Rust diseases represent the most economically significant fungal diseases in wheat and other cereal crops worldwide, and they are widely distributed across wheat growing regions. With the capacity to form new strains of fungus, rusts can attack even previously resistant varieties. Rust diseases possess the ability to spread and travel long distances by dispersal of windborne spores and can rapidly develop under optimal weather conditions.

The three rust diseases affecting wheat are leaf, stem and stripe rust. Leaf rust is the most common of the three diseases. In some states, leaf rust disease occurs every year. Stem rust is not typically as prevalent as other rusts because many varieties are now resistant to the disease. Stripe rust is becoming an increasingly important disease, with recent outbreaks in the Great Plains states.

Leaf Rust
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.

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. According to research, 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. Some areas report 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.

Stem Rust
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. 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 coloured, 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.

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.

Stripe Rust
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 globally. 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 research, 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.

Managing Wheat Rusts
Farmers have several options to manage wheat rusts, including seed treatments, variety selection, scouting, cultural practices and fungicide applications.

Seed treatments
Using the correct fungicide seed treatment and rate is one way to provide effective and economical disease control in wheat production. Seed treatments protect seed and young plants from disease and other threats to plant health and yield. Areas commonly infested with rust also will need a foliar fungicide treatment to provide protection beyond the seedling growth stage.

Variety selection
Where available, use rust-resistant varieties for best protection against leaf rusts. Every commercially available wheat variety has a unique disease package, and excellent disease resistance is not available to manage all disease threats in high-yielding varieties. It’s best to select two or three high-yielding varieties that offer the best resistance to common diseases found on your individual farm. 

Scouting
When scouting for weeds and insects, check for the presence of wheat rusts and other diseases. Monitor reports of wheat rust development occurring in your area. This will allow you to track the progression of rust diseases migrating north from overwintering hosts and will also help you predict the timing and severity of infestations before they might affect your region. Keep a close eye on weather conditions because rust spores spread through wind currents to promote disease infection.

If you suspect leaf rust, stem rust or stripe rust infection, take samples and work with your agronomist to confirm a diagnosis.

Crop identification guides are also helpful. Scouting helps determine levels of infection so you can make the best decision about the necessity and rate of fungicide applications.

Cultural practices
Disease-free seed gives seedlings a good start. Good weed control preplant, at planting and throughout the growing season also helps protect against disease and other pests.

Fungicides
Good control of wheat rusts can be achieved with commercially available fungicides and proper application timing. The decision to use fungicides should be based on scouting for symptoms. It’s important to assess disease severity from the onset of infection through the various growth stages. Application timing should take into consideration that diseases should be managed before infection reaches the upper leaves. Other factors affecting fungicide application are infection levels in the field, the susceptibility of the variety and the market price for wheat grain.

Conclusion
A well-thought-out diseasemanagement program, including best management practices, proper seed protection and selection and fungicide applications using multiple modes of action, should be implemented to sustainably manage diseases.

Before selecting a seed treatment or applying any fungicide, please read the entire label for the best possible results and to confirm that the product is effective on the disease you need to control. Not every product is suitable for every situation, and correct application technique will ensure the best results.