#208: Rust Diseases of Wheat

Wheat is affected by three rust fungi, shown above. From left to right, they are: Leaf Rust (Photo: USDA ARS), Stripe Rust (Photo: USDA ARS), and Stem Rust (Photo: USDA ARS).

Wheat is the most widely grown crop in the world and is a staple food for billions of people. Diseases affecting wheat are therefore of utmost importance to food security. Some of the most destructive and difficult wheat diseases are the rusts. Wheat rust comes in three varieties: Leaf Rust, Stem Rust, and Stripe Rust, each caused by a different species of fungus. These all look slightly different but all cause rust-colored blemishes on wheat surfaces. The three species are closely related and have nearly identical life cycles. Despite this, managing the diseases is complicated and requires using resistant wheat strains, proper cultural practices, and fungicides.^1,2

Life cycle

All three wheat rusts are heteroecious macrocyclic rusts, meaning they have the most complex life cycle possible for rusts. The rusts begin each year by infecting an “alternate host” – a plant that is unrelated to wheat. The fungus does minimal damage to its alternate host but steals enough resources to produce spores that will infect wheat. Once a wheat plant is infected, the fungus viciously attacks the wheat plant and produces spores that can infect other wheat plants. At the end of the year, the fungus produces resting spores that will survive in the ground during the winter before germinating in the spring to infect the alternate host once again.^1,3–5

The life cycle of a heteroecious macrocyclic rust, which includes all three types of wheat rust (Photo: USDA ARS).

To read about the rust life cycle in greater detail, see FFF#130 or click here to expand this section.

In the United States, wheat rusts usually do not rely on alternate hosts. Instead, plants are infected by spores (urediniospores) from other wheat plants. The fungi overwinter in the southern states, which don’t get cold enough to kill the fungus. In those states, the growing season for wheat is much longer and wheat plants can survive year-round. Wheat seeds that germinated outside farms or in fields after the harvest can become infected by wheat rust but survive through the winter. When this happens, the fungus can infect more plants next spring.^1–3,6,7 Because the fungi survive in the southern states, wheat rust epidemics in the United States begin in the south and spread northward with the prevailing winds across the Great Plains states.^1,2 Epidemics generally begin around the start of May and reach the Canadian border by the end of June.¹

Wheat Leaf Rust is caused by the fungus Puccinia triticina and features orangish circular uredinia (Photo: USDA ARS).

Leaf Rust

Leaf Rust, also called Brown Rust, is caused by the fungus Puccinia triticina. This disease is the most common rust disease of wheat and can be found at least at low levels wherever farmers grow wheat. Losses due to Leaf Rust are usually less than 10%, but in severe cases yields can be reduced by over 30%.⁵

P. triticina infects bread wheat, durum wheat, and triticale, which are some of the most commercially important grains. The alternate hosts for the fungus come from a variety of genera, making it difficult to remove all possible alternate hosts from the area around a farm. Potential hosts are flowering herbs belonging to: Clematis (clematis plants are widely cultivated for their flowers), Thalictrum (meadow-rue), Anchusa (a genus of the borage or forget-me-not family), and Isopyrum (similar to false rue-anemones).⁵

Leaf Rust uredinia are reddish-orange circular areas that grow up to 1.5mm long. The uredinia appear primarily on leaves and leaf sheaths.^2,4,6,8,9 In severe infections, they might also appear on seed heads.⁴ However, the uredinia never appear on the stem and never tear the plant’s surface.^4,9 P. triticina produces relatively few teliospores, so even in the later stages the disease tends to look more orange-brown than the other wheat rusts. Teliospore production tends to occur on the underside of leaves, further separating it from the other species.⁵

P. triticina performs best in warm to hot weather with frequent precipitation or high humidity. Spores germinate best when there are nightly dews and when temperatures remain between 15 and 25°C. Under these conditions, the fungus grows rapidly and can produce uredinia in only 7-10 days.^2,5,6,8

In the Mediterranean area, the species Puccinia triticiduri also causes leaf rust. This species produces fewer urediniospores, so outbreaks are important only locally. P. triticiduri tends to produce lots of telia in a ring surrounding the first uredinium, which helps distinguish this species from P. triticina.⁵

Wheat Stem Rust is caused by the fungus P. graminis f.sp. tritici and features elongated uredinia that burst through the outer layer of the plant (Photo: USDA ARS).

Stem Rust

Stem Rust, also called Black Rust or Summer Rust and caused by Puccinia graminis f.sp. tritici, is the most devastating of the wheat rusts. Under ideal conditions, it reduces yields by over 50% in less than one month and in severe cases causes complete losses of crops.⁵

P. graminis f.sp. tritici infects bread wheat, durum wheat, and triticale. It uses only common barberry – Berberis vulgaris – as an alternate host, so it is possible to control the disease by eliminating barberry bushes.^3,5 The widespread removal of barberry and the development of resistant wheat cultivars have greatly reduced Stem Rust’s global impact.¹

Stem Rust uredinia are orange-red elongated ovals. They are much larger than Leaf Rust uredinia and burst through the plant’s outer layer, resulting in a rough texture. The uredinia appear on stems, leaf sheaths, spikes, and sometimes on leaves.^2–4 Late in the season or during adverse conditions, Stem Rust produces dark teliospores that turn the lesions distinctly blackish.⁴ These characteristics make it possible to differentiate between Leaf Rust and Stem Rust by sight.

Like Leaf Rust, Stem Rust is favored by warm to hot temperatures and high humidity. Frequent rains or nightly dews and temperatures between 15 and 30°C promote spore germination.^1–3 Raindrops efficiently deposit spores on leaves, so rain also facilitates the spread of P. graminis f.sp. tritici and other wheat rust species over long distances.^1,5

Wheat Stripe Rust is caused by the fungus Puccinia striiformis f. tritici and features tiny yellowish uredinia arranged in rows (Photo: USDA ARS).

Stripe Rust

Stripe Rust, caused by Puccinia striiformis f. tritici and also called Yellow Rust, is the least problematic of the wheat rusts. It prefers cooler weather than the other two rusts, so it is only significant in northern climates or at higher altitudes. However, within this limited region it can reduce yields by 50% or more and in severe cases can ruin the entire wheat crop.⁵

P. striiformis f. tritici primarily attacks bread wheat, durum wheat, triticale, and some cultivars of barley. Other variants of P. striiformis infect primarily infect barley, especially in Europe and South America. For a long time, scientists assumed that P. striiformis reproduced without an alternate host, but a recent paper confirmed that the fungus uses Berberis spp. (barberry) as an alternate host.^5,7

Stripe Rust produces very small yellowish-orange uredinia in straight lines along leaves (thus the name “Stripe Rust”), which distinguishes this rust from the other two wheat rusts.^2,4,7 P. striiformis f. tritici grows best in cool or cold weather, so it tends to infect wheat plants early. This often leads to stunting of the plants and can significantly impact crop yields.⁵

Cool mist provides spores of P. striiformis f. tritici with the optimal conditions for germinating. The fungus needs free water and temperatures between 0 and 25°C. This means the fungus can infect plants under snow, although it grows faster at higher temperatures. It may take five to six months before an infected plant shows symptoms if the temperature is around 0°C, but takes only 11 days at around 15°C. Hot summers and dry spells significantly impair the growth of P. striiformis f. tritici.^2,5,7

Impact

Wheat rusts co-evolved with wheat and their alternate hosts in the Fertile Crescent region, so they have been problematic for humans since the inception of farming. The first descriptions of the disease date back to Ancient Greece, but archeological finds from 1300 BCE have included Stem Rust spores. As wheat spread around the world, so too did wheat rusts and their alternate hosts (barberry, for example, was useful as a dye and for its edible berries).¹

Farmers recognized the relationship between Stem Rust and barberry by the mid-1600’s at the latest, but it took scientists until the late 19^th century to figure out the full life cycle of Stem Rust. After that, life cycles of other rusts were elucidated and scientists gradually figured out how the other two wheat rust species survived.¹

Barberry is the alternate host for wheat Stem Rust. The picture on the left shows an infected leaf with pycnidia. The top right picture shows a barberry leaf underside with aecia. The bottom right picture shows barberry leaves and berries as they normally appear in the fall. Photos: USDA ARS.

Barberry eradication programs in the early 1900’s significantly reduced the incidence of Stem Rust in North America, but epidemics still occur because the fungus overwinters in plants grown on the southern end of the continent. However, barberry eradication did prevent P. graminis f.sp. tritici from reproducing sexually, which reduced its genetic variation. This made it easier to develop wheat varieties that were resistant to Stem Rust. These varieties were planted around the globe and greatly reduced the incidence of Stem Rust.¹ Now, Leaf Rust is the most common type of wheat rust.⁵

In 1999, a variety of P. graminis f.sp. tritici was discovered in Uganda that could overcome the most important resistance genes. Since then, this variety has been spreading across Africa and up into Europe. It has not yet reached North America, but it is still one of the biggest threats to global food security.¹

Solutions

The best way to prevent rust infections is by planting resistant wheat varieties. There are hundreds of strains of wheat rust and no varieties of wheat protect against all those strains. Farmers must choose the correct resistant strain for their farm based on which strains of wheat rust are prevalent in their area. Unfortunately, wheat varieties remain resistant for only two to four years, so farmers have to routinely plant new varieties of wheat.^2,4

There are also some cultural practices farmers can follow to reduce the likelihood of infection. First, the remains of any infected plants should be removed and any volunteer plants (plants that seed themselves and grow without having to be planted), which can act as reservoirs of disease. Second, wheat should be planted at the correct time for the local climate to reduce the chances of infection. Finally, farmers should monitor the nitrogen, phosphorous, and potassium levels in their fields and apply the proper amount of fertilizers. An overabundance of nitrogen compared to phosphorous and potassium can make the disease more severe.^2,8

However, resistant cultivars and cultural practices are often not enough to keep wheat rust at bay. Spores from many wheat rust species and strains are continually blowing in the wind, so infections are likely to occur sooner or later. When rust is spotted in a field, farmers should apply fungicides to protect their uninfected crops.^2,8,9 Fungicides are effective, but costly, so farmers shouldn’t apply fungicides unless they need to and it is economical.^1,4

Taxonomy

All species of wheat rust are closely related and belong to the same genus, Puccinia.⁵ This is not very surprising, considering their similar morphologies, life cycles, and primary hosts. Puccinia is the quintessential rust genus, and these fungi show it: they have the most complex life cycle and are aggressive plant pathogens.