| | Introduction | The cause of stripe rust | Disease cycle | Environmental conditions and disease development | Stripe rust management | Concluding remarks
Introduction
Stripe rust, also known as yellow rust, caused by the fungus Puccinia striiformis, has devastated cereal production worldwide due to rapid systemic infection of affected plants resulting in defoliation and shrived kernels. This is primarily a disease of cool climates and has been found in southern Alberta and British Columbia for many years. Recently, stripe rust prevalence has increased in central Alberta in wheat, barley and triticale. It has also frequently been observed in western Saskatchewan and occasionally in the eastern Prairies, causing substantial losses in some wheat fields. Over the last year or so one of the most common cereal disease inquiries received by the Alberta Agriculture Call Centre from Alberta cereal producers have been about stripe rust identification and management advice. 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. An early investigation showed that wheat yield reduction in the Lethbridge area was up to 75% (Conner & Kuzyk 1988) and reached 45% at Lacombe based on recent observations. Yield reductions in some wheat plots in 2004 and 2005 were undoubtedly high as disease severity reached 100% at Lacombe, Olds and Trochu. A susceptible cultivar could sustain a total loss when environmental conditions are favorable for disease (Chen 2005). Monetary losses due to the reduction of wheat yield caused by stripe rust in the central great plains of the US were estimated to be $27M, $119M, $24M and $267M US for 2000, 2001, 2002 and 2003, respectively (Chen et al. 2004). The highest grain yield loss of a susceptible barley cultivar was found to be 72% in the US (Marshall & Sutton 1995) and stripe rust greatly reduces malting quality (Line 2002).
The Cause of Stripe Rust
Stripe rust of cereals and grasses are 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. Formae speciales means “special form" and differentiates pathogen isolates based on their pathogenicity for one or more specific hosts. Among several formae speciales, 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 cultivars or genotypes of a crop. A total of 109 races of P. striiformis f. sp. tritici in the US (Chen 2005) and 36 races in Canada (Su et al. 2003) have been identified. Chen and Penman (2004) identified 15 races of P. striiformis f. sp. hordei from barley grown in California, Idaho, Oregon and Washington. There is a constant change in the population of fungal races, depending on the climate and the use of resistance cultivars.
Disease Cycle Epidemics may result from the spread of fungal spores carried on travellers clothing. Air currents are the main agent responsible for spreading this disease. During most summers, air currents carry summer spores known as urediniospores into the northern United States and Canada, from plant to plant and from field to field. Stripe rust epidemics in the Netherlands can be generated by urediniospores in a single pustule (uredium) that survives the winter if the spring season is favourable for rust development (Zadoks & Bouwman 1985). Spores need several hours of moisture on plant leaves to germinate and infect the host. After penetration, fungal mycelia from the spores elongate in parallel with the leaf veins, causing a yellow stripe appearance that extends the entire length of the leaf blade. The stem can also be infected, causing similar symptoms. Symptoms appear about one week after infection under conducive conditions. Sporulation (the production of spores) takes place between eight days and two weeks after infection. The yellow stripes consist of rust pustules that contain numerous urediniospores that are ready to be dispersed for further infections. On crop heads, uredia occur on the surface of glumes and seeds, but whether seed can be infected is uncertain. During the growing season, the fungus can reproduce several crops of urediniospores and the newly produced spores continue to infect green tissue or plant. Consequently, severe infections can progress from the middle to upper parts of the plant canopy resulting in yield reductions. Grain yield reduction is caused by the loss of green leaf area used for photosynthesis and water loss through evaporation due to the destruction of the leaf epidermal layer. Later in the season, the yellow unrediospores in the pustules are replaced by dark brown telispores that are considered to play no role in infection, since teliospores cannot infect any known alternate hosts. The fungus overwinters primarily as mycelia in green tissue of plants; urediniospores production resumes when mild temperature and suitable moisture occur.
Environmental Conditions and Disease Development
Wind direction and a specific range of temperatures are essential for the onset and development of stripe rust. Relative low temperatures and moisture are needed for spore germination, initial infection and subsequent sporulation and secondary infections during a growing season. Infection may occur in the early spring, since mycelium remains viable to –5 C. Urediniospores germinate optimally between 5 and 15 C with limits near 0 and 20 C. Disease development is most rapid between 10 and 15 C with intermittent rain. Early reports indicated that urediniospores produced in the Pacific North West and carried by wind are considered to be the major source of inoculum in Alberta (from Line 2002). However, Conner et al. (1988) observed that overwintering of P. striiformis in winter wheat in southern Alberta resulted in stripe rust epidemics on soft white spring wheat. Based on a predictive model developed by Coakley and Line (1981), Conner et al. (1988) showed an inverse relationship between stripe rust on wheat and negative degree days in the winter. Warm, wet winters with mild temperatures or plenty of snow cover favor survival of the rust fungus as mycelium in the leaves and tissues of winter cereals, volunteer plants or other hosts. A cool wet spring and summer then favor subsequent disease development. The latent period for stripe rust during the winter can be up to 118 days and is suspected to be as long as 150 days under a snow cover (Zadoks 1961).
Stripe Rust Management
Yield losses can be reduced or prevented by timely fungicide application which requires inspecting the crop regularly for symptoms starting at seedling stage. It is important to plan an inspection just before flag leaf emergence. The early appearance and increase of disease means more severe damage to the crop. Experience in Australia showed that spraying should be done before stripe rust reaches 5% of leaf area on the flag leaf. Once this infection level is reached, stripe rust becomes very difficult to control (Murray et al. 2005). Seed treatment can delay the onset of the disease on seedlings. However, windblown spores could still attack the crop later in the season, making a foliar of fungicide application necessary. Timely foliar application combined with seed treatment has proved to be effective in controlling stripe rust of wheat (Conner & Kuzyk 1988, Line 2002). Fungicide application resulted in a 1:12 cost/benefit ratio for Washington state wheat growers in 2002 (Chen et al. 2003).
Chemical control represents an effective short-term strategy. Increasing concerns regarding input costs and environmental concerns emphasize the need for other strategies. Resistant cultivars are convenient to use, impose no environmental hazard, and add little or no additional input costs for the producer. Recent field evaluations for stripe rust reaction in New Zealand, Pullman Washington and Creston BC showed that many wheat cultivars commonly grown in Manitoba (including some grown in Alberta) were moderately resistant to stripe rust while the most popular cv. AC Barrie was highly susceptible (B. McCallum, personal comm.). Nevertheless constant shifts in the race structure of the stripe rust pathogen and the frequent appearance of new races can overcome genetic resistance, rendering resistant cultivars ineffective. Breeding for durable resistance is needed to combat the inherent variability of stripe rust populations.
Year-round cropping including winter and spring wheat, barley and grass provides living hosts known as a green bridge for the pathogen to survive, with subsequent infection and development of stripe rust. An integrated approach using durable resistance and appropriate cultural practices will allow producers to effectively implement sustainable control strategies for stripe rust management. As a result, removing volunteer cereals by spraying or cultivation will remove a potential source of inoculum and limit subsequent disease development on the next cereal crop. Rotation out of cereal crops will help to break down the green bridge effect, helping to reduce disease development considerably, if not eliminate it. Early planting of spring crops will allow them to ripen before major amounts of stripe rust inoculum typically become available in late July and August. Crop management in terms of a combination of crop choice, timing of seeding, and weed/volunteer management may provide effective control of stripe rust.
Concluding Remarks
Although winter temperatures in central Alberta may be too low for the fungus to survive locally, weather conditions during recent summers appear to have been more favorable for stripe rust development. This is supported by the observation that although stripe rust was not often detected in the field until the middle of July to early August, severity often reached 100% on susceptible cultivars that may sustain substantial yield losses. Information is limited regarding the effectiveness of controlling stripe rust and the economic benefit under central Alberta conditions. Research is needed to answer the following questions that address the management of stripe rust in central Alberta: What are the major environmental and agronomic factors contributing to epidemics in central Alberta? How much yield loss is caused by late infection that are typically observed in central Alberta? Do cropping systems with winter cereals play a role in stripe rust epidemics? Is seed treatment effective in delaying the onset of stripe rust in winter cereals in the fall, thus helping to limit a potential disease source? Is late application of foliar fungicide beneficial? What are the dominant formae speciales and races of stripe rust in central Alberta? Are the current genetics sufficient for resistance to the major races? What is the status of variety resistance in Alberta? Ongoing research will help to provide a strong basis for integrated control of stripe rust in Alberta.
K. Xi, Field Crop Development Centre, AAFRD, 6000 C and E Trail, Lacombe, AB T4L 1W1, (403) 782-8861. kequan.xi@gov.ab.ca
T.K. Turkington, AAFC, Lacombe Research Centre, 6000 C and E Trail, Lacombe, AB T4L 1W1, (403) 782-8138. turkingtonk@agr.gc.ca
D. Salmon, Field Crop Development Centre, AAFRD, 5030-50 Street, Lacombe, T4L 1W8, (403) 782-8694. donald.salmon@gov.ab.ca
B.D. McCallum, AAFC, Cereal Research Centre, 195 Dafoe RD, Winnipeg, MB, R3T 2M9, (204) 983-0711. bmccallum@agr.gc.ca
A. Navabi, AFNS, University of Alberta, 410 Ag/For. Edmonton, AB T6G 2P5. (780) 492-5203.
alireza.navabi@afhe.ualberta.ca
References
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