Features Agronomy Diseases
Keep an eye on stripe rust

Stripe rust on wheat has become more prevalent in Alberta.  Photos courtesy of Kequan Xi, Alberta Agriculture and Rural Development.
Barley stripe rust is rarely seen.

Stripe rust has become more prevalent since the late 1990s, and is moving up the list of plant diseases that wheat, barley and triticale growers need to watch. Primarily a disease of cool climates, it has been found in southern Alberta and British Columbia for many years.

Since 2000, though, stripe rust in wheat is also beginning to appear more frequently in the eastern Prairie. For example, in 2006, relatively high levels of stripe rust were observed in southeastern Saskatchewan, while in 2007 stripe rust was found in a few fields at trace levels in both Manitoba and Saskatchewan, although stripe rust in wheat is generally uncommon in those provinces. Recently, stripe rust prevalence has increased in Alberta in wheat and barley. “This disease has been widespread on winter and spring wheat in Alberta, and the practice of disease management is needed to minimize yield reduction caused by stripe rust infection,” says Kequan Xi, a cereal pathologist with Alberta Agriculture and Rural Development (AARD) at Lacombe, Alberta. While the severity of stripe rust is not often enough to cause concern, Xi says that researchers are working to better understand the disease on the Canadian Prairies.
As for all rust diseases, stripe rust spores do not typically overwinter so far north, instead arriving on wind currents from rust-infested cereal regions in the United States. However, information from AARD indicates that “the stripe rust fungus overwintered in the Prairie region during the 2005/06 winter, which is why the disease was observed earlier and had more time to develop in 2006 compared to previous years.”

During the summer of 2009, stripe rust, caused by the fungus Puccinia striiformis, was severe enough around Lethbridge that AAFC released an Alert on the disease: “Infections were first observed in the Lethbridge winter wheat nurseries on June 20, 2009, and conditions favoured secondary infections.

Infections were noticed in the spring wheat nurseries on July 3, 2009. Subsequent rains favored continued sporulation and secondary infections.”  

Information from AARD suggests that recent outbreaks of cereal stripe rust in Western Canada could “primarily be attributed to mild winters and cool, wet summers, in addition to the presence of inoculum and susceptible varieties.” Disease severity reached 100 percent in susceptible wheat varieties at Lacombe in 2004 and 2005, with high yield losses.

Stripe rust and leaf rust have different symptoms and can be distinguished by the colour and distribution pattern of the pustules (blister-like lesions) on infected leaves. When the stripe rust spore lands on a susceptible variety, fungal strands grow beneath the leaf surface. Ten days later, the spores emerge in a raised orange or yellow strip on the leaf surface. Conversely, the pustules of leaf rust are orange to red and occur randomly on the leaves.

The stripe rust fungus can cover the entire leaf under optimum conditions, and the spores can go on to infect the entire field. A severely infected field can look to have an orange colour, and the wind can spread the huge number of spores to other crops. Ultimately, the disease causes yield loss because of shrivelled kernels that result from the lack of photosynthetic capacity. 

Xi says that stripe rust of cereals and grasses is caused by different formae speciales of P.
striiformis that have the same fungal morphology, but differ in their ability to attack cereal and grass species. He says that formae speciales means “special form” and different forms can attack different crops. Xi says that “Puccinia striiformis f. sp. tritici and P. striiformis f. sp.
hordei appear to be the most economically important in causing stripe rust of wheat, triticale and barley, respectively. Generally, wheat is more susceptible compared with triticale and barley. The fungus in each formae speciales can be further divided into races that differ in their ability to attack different varieties of a crop, although a few strains of the pathogen are found to attack both wheat and barley. A total of 109 races of P. striiformis f. sp. tritici in the US and 36 races in Canada have been identified. There is a constant change in the population of fungal races, depending on the climate and the use of resistant cultivars. The changes in the fungal races resulted in breakdown of cultivar resistance.”

Investigating the green bridge
At Lacombe, Xi has been involved in three years of fall surveys from 2007 to 2009, which looked at stripe rust infections on winter wheat fields. During the three years, in commercial winter wheat fields and in plant breeding plots, stripe rust severity was variable, depending on environmental conditions and crop varieties.  In the fall of 2007, the majority of 23 winter wheat fields surveyed showed no stripe rust or below five percent incidence, but several fields had substantial levels with incidence up to 100 percent on susceptible varieties. In the fall of 2008 and 2009, no stripe rust was found on surveyed commercial winter wheat fields, however a few pustules of stripe rust were found at plant breeding sites. “These survey results suggest that winter wheat may harbour stripe rust inoculum at variable levels, depending on the amount of inoculum and possibly environmental conditions such as temperature and the depth of snowcover,” explains Xi. “Symptoms observed and positive detection of latent infections suggest that viable pathogen inoculum was present under winter conditions in Lacombe.”

Another three-year field trial at Lacombe looked at whether the inoculum on winter wheat could infect spring wheat, barley and triticale plants. The spring cereals were planted in either a spring wheat field or a winter wheat field, to compare stripe rust severity caused by natural infections. Significantly higher levels of stripe rust were observed in the spring crops located in the field planted to winter wheat than in spring crops located in the field planted to spring wheat in the 2007 and 2008 seasons. That trend was also found in 2009, although the difference was not significant. “These observations suggest that stripe rust may build up on winter wheat crops, which then act as a source of inoculum for spring-seeded cereals in central Alberta. Consequently, planting spring wheat close to winter wheat fields where significant stripe rust is present may increase exposure to stripe rust inoculum,” says Xi.

Control is possible
The easiest control method is to use resistant varieties of cereal grains whenever possible. Xi explains that stripe rust severity can be substantially reduced and using resistant varieties where epidemics have occurred can minimize yield reduction. In addition, the use of resistant varieties can reduce the inoculum that can potentially cause an outbreak in the next cereal crop. 

The results from the Lethbridge Research Centre showed that the majority of Canadian Extra Strong and durum classes are resistant, but susceptible wheat varieties are common among the Canadian Prairie Spring red and white wheat classes, while only a few of the hard red spring varieties have good resistance. Radiant, a commonly grown winter wheat variety, along with McClintock, has good resistance but the rest of the winter wheat varieties are generally susceptible. 

All varieties of triticale are resistant to stripe rust.

Barley stripe rust is rarely observed in western commercial fields, although most barley varieties were susceptible when screened in the stripe rust nursery. 

A fungicide application timed just before flag leaf emergence can also help prevent disease development and yield loss. The early appearance and increase of disease means more severe damage to the crop. Xi says that experience in Australia showed that spraying should be done before stripe rust reaches five percent of leaf area on the flag leaf. Once this infection level is reached, stripe rust becomes very difficult to control.

Xi says that year-round cropping including winter and spring wheat, barley and grass provides a living host known as the green bridge for the pathogen to survive. Removing volunteer cereals and grass by spraying or cultivation in the fall will eliminate a potential source of inoculum and limit subsequent disease development on the next cereal crop. Rotation out of cereals will help to break down the green bridge effect. “Spores can only survive on green host plant tissue, so breaking the green bridge is a good cultural control method,” says Xi.

However, because inoculum can be blown in on the wind from the Pacific Northwest each year, resistant varieties should remain the cornerstone of the disease management strategy.