What is stripe rust?
Stripe rust is a disease of cereals caused by the fungus Puccinia striiformis. Generally, wheat is more susceptible than barley and triticale. Historically, stripe rust has not been of economic significance in most parts of western Canada but incidences of the disease have been increasing over the past few growing seasons.
Stripe rust infections rob nutrients and sugars from the host plant while the infected tissues on the leaf surface reduce the photosynthesizing area of the plant and cause excessive moisture loss. These conditions reduce plant survival, head development, grain development, grain filling, yield, grain grade and quality.
This disease can result in yield losses of 10 to 90 per cent in wheat varieties and 5 to 50 per cent in barley varieties, as demonstrated in experimental plots under natural infection conditions in central Alberta. A total yield loss has been reported when severe epidemics occur in other parts of the world. Like other leaf and stem diseases, yield losses due to stripe rust are roughly proportional to the leaf area infected. Yield losses are generally most severe when the infection occurs prior to heading.
Why has stripe rust become a problem recently?
Although stripe rust is not a new disease, it was limited to cooler climates (e.g. Pacific Northwest region of the USA), and so was previously not a disease of concern for the wheat breeding programs on the Canadian Prairies. However, new races of the fungus appear to have evolved that are better adapted to the moderate summer temperatures as well as becoming more adapted to some of the previously resistant cereal varieties of the Prairies. Widespread production of susceptible stripe rust varieties and delayed seeding of wheat for swath grazing may have heightened stripe rust levels.
As for all rust diseases, stripe rust spores do not typically over-winter this far north, instead arriving on wind currents from rust-infested cereal regions in the United States. However, the stripe rust fungus has been overwintering on winter wheat beneath snow cover in Alberta in recent years. Warmer winters may have led to a greater potential for overwintering. Overwintering is why the disease has been observed earlier in the growing season and why it can be much more damaging to winter and spring-seeded cereal crops. Recent research indicates that a green bridge can form between winter wheat and spring wheat. Tests conducted in central Alberta during 2008 to 2011 showed that there was higher stripe rust severity in spring wheat or barley seeded near winter wheat compared with the same crops seeded near spring wheat1. It appears that spores from infected spring cereals transmit to winter wheat in the fall, and then transfer to spring cereals again in the spring when the spring and winter crops are planted in close proximity.
How do I identify stripe rust in my fields and what fields are most at risk?
There are varying levels of resistance to stripe rust in current wheat varieties. Two row barley is generally more resistant than six row barley. More extensive rating for stripe rust reaction has recently been initiated and ratings are included in variety selection charts. Breeding for stripe rust resistance in both wheat and barley has been underway in western Canada breeding programs. Refer to the Alberta Seed Guide for stripe rust ratings.
If you are growing a susceptible variety, it is important to scout. Know the visual symptoms of stripe rust and monitor fields in the morning when new spores are a distinct yellow colour. Stripe rust can be identified as small orange-yellow coloured pustules forming in vertical lines that run parallel to wheat leaf veins. Spore development is favoured by cool (7 - 20°C) environmental conditions. At higher temperatures, development of the fungus slows. This web document from North Dakota State University has excellent pictures for rust identification, even though some of the information in the text is not relevant to Alberta: Stripe rust ID
Stripe rust attacks all the above-ground parts of the wheat and barley plant. On young plants the pustules occur in blotches covering large areas of affected leaves. On older plants (after the jointing stage of growth) the powdery masses (pustules) of yellow-orange spores are arranged in parallel lines, giving affected leaves a characteristic striped appearance. Infection of leaf sheaths and stems can also occur, but spore production on these parts is less than on leaves. Stripe rust can also attack the heads. The rust fungus infects the glumes and awns at flowering that result in an accumulation of spores in the florets and on the surface of the developing grain. However, stripe rust is not seed-borne. Later in the season, the yellow summer spores in the pustules are replaced by dark brown winter spores that are considered to play no role in infection.
Stripe rust should not be confused with leaf rust or stem rust that have reddish brown spores. Stripe rust should also not be confused with septoria leaf blotch complex, which is characterized by grey leaf blotches that are speckled with small black fruiting bodies of the fungus. Stripe rust can be confirmed by collecting plants suspected of being affected and placing them in a bucket with their base in water in a cool, sheltered position. If stripe rust is present, the rust pustules will produce a fresh crop of spores overnight.
A number of fungicides are similarly effective at controlling these foliar diseases. However, the majority of varieties do not have genetic resistance against multiple diseases. Given that several foliar diseases can be found in the same field and that some of them, such as septoria leaf complex, also cause yield losses, it is important to identify the various diseases so varieties with specific resistance to the most important diseases can be chosen for improved disease management.
How do I decide if I need to take control measures?
Seed treatment may delay the onset of the disease on seedlings. However, windblown spores could still attack the crop later in the season, making a foliar fungicide application necessary.
Experience in Australia showed that spraying should be done before stripe rust covers 5% of the leaf area on the flag leaf. Once this infection level is reached, stripe rust becomes very difficult to control.
The period of infection to the time of spore release can be as short as 8 days, which can result in multiple generations per growing season. However, no foliar applied fungicide is necessary if the 5% of stripe rust severity is first observed on the flag leaf at the late heading stage because a late disease onset will have limited disease development during the remainder of the growing season.
What agronomic practices can I use to limit the impact stripe rust has on my farm?
Stripe rust management practices are:
- Plant resistant varieties.
- Use clean seed and apply a recommended fungicidal seed treatment.
- Avoid overwatering irrigated fields.
- Scout for the disease early and often.
- Avoid "green bridging" by controlling volunteer cereals and avoid planting winter wheat and spring wheat in close proximity.
- Delay seeding winter wheat until later in September to avoid a stripe rust green bridge occurring from spring wheat.
- Consider applying a fungicide:
- The window of opportunity to spray may be short, so spray fields planted with susceptible varieties as soon as symptoms are noticed and before they cover 5% of the flag leaf.
- Susceptible varieties may require more than one fungicide application (watch pre-harvest intervals of fungicide products).
- With resistant varieties, a fungicide application protecting the flag leaf may provide a financial return, but only in cases of extreme disease pressure.
- Plant spring cereal crops destined for swath grazing or green feed early so they ripen prior to major amounts of stripe rust inoculum arriving on wind currents into the province typically in late July and August.