Winter Wheat Production Manual

Written by D. B. Fowler
Crop Development Centre
University of Saskatchewan

© University of Saskatchewan. All rights reserved.  No part of the Winter Wheat Production Manual may be reproduced in any form by any photographic, electronic, mechanical or other means, or used in any information storage and retrieval system without the written permission of the University of Saskatchewan or Ducks Unlimited Canada.

Rusts

Three highly specialized species of fungi are responsible for the rust diseases of wheat.

    • Stem rust (black rust or black stem rust) - Puccinia graminis
    • Leaf rust (brown rust, dwarf rust, or orange rust) - Puccinia recondita
    • Stripe rust (yellow rust or glume rust) - Puccinia striiformis

The main stem and leaf rust hazard area in western Canada is the southeastern part of the prairies. In years of severe epidemics, this region can expand into western Saskatchewan towards the Alberta border. Stripe rust is occasionally found on the prairies, but high summer temperatures prevent the development of epidemics of this disease.

Rusts can only survive on live plant tissue. They have complex life cycles with many spore stages and a requirement for hosts other than wheat before aeciospores (sexual spores) can be produced. However, while they are required for the completion of the rust life cycle, alternate hosts are not necessary for the establishment of epidemics. The ability to produce large numbers of urediospores (asexual summer spores) that can be blown over vast distances gives rusts the potential to rapidly build up to the epidemic levels required to cause severe crop damage.

Each species of rust has numerous distinctive races that are distinguished by their ability to attack different wheat cultivars. Mutations and other mechanisms are responsible for the development of new races of rust. Alternate hosts (primarily the barberry) support the sexual stages of the life cycle where genetic recombination can speed up the production of new virulent strains of stem rust. Alternate hosts and sexual stages do not play a role in the development of new strains of stripe and leaf rust in North America.

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a) Causes and Symptoms:

Rusts can overwinter in infected winter wheat leaves that remain alive under protective snowcover in the southern part of the Canadian prairies. However, spores blown north from Mexico and the southern U.S.A. are responsible for the rust epidemics that occur in the prairie provinces.

Once summer rust spores (urediospores) have made their annual trip north to the Canadian prairies, susceptible cultivars and favorable environmental conditions are required for an epidemic to develop. Temperatures from 15 to 22oC for leaf rust and 20 to 30oC for stem rust plus 6 to 8 hours of leaf wetness caused by light rain or dew favor the infection of wheat plants. Under favorable conditions, a new cycle of urediospores can be produced every 7 to 10 days. Each rust pustule that forms has the capability of producing 1,000 urediospores thereby giving rusts the ability to explode to epidemic levels in a very short time.

Urediospores are easily dislodged from rust pustules and they will often collect as a reddish-orange powder on clothing and equipment that pass through infected crops.

Stem rust pustules can develop on the leaves, leaf sheaths, stems, and heads of the wheat plant. They are raised, oval to elongated, light orange to brick-red in color with remnants of torn epidermal tissue at their edges.

Although leaf rust can develop on leaf sheaths, it is mainly restricted to the leaves of wheat plants. Pustules of leaf rust may be scattered or clustered and they are found primarily on the upper side of the leaf blade. They are raised, nearly round, yellowish to dark red colored and much smaller than those of stem rust. When they break through the epidermis, leaf rust pustules do not produce the ragged edges that are characteristic of stem rust.

Where epidemics are established early, the production of black overwintering spores (teliospores) may cause stem and leaf rust pustules to turn a darker color as the wheat plants approach maturity. These overwintering spores can only infect alternate hosts and not wheat.

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b) Losses:

Rusts have been a constant threat to farmers' crops as long as wheat has been cultivated by mankind. Roman philosophers referred to rusts as the greatest of plant diseases and they have continued to plague crops as the world's wheat acreage has expanded through the centuries.

In Canada, the earliest attempts to grow spring wheat on the prairies were frustrated by rust epidemics. Even today, their potential to cause widespread damage makes stem and leaf rust the number one and two wheat diseases of concern in western Canada.

Conditions favorable for spore production are the most critical factors in the development of rust epidemics. Stem rust has the highest temperature requirements and its annual spore buildup in North America usually starts in Mexico and southern Texas. The temperature requirements of leaf rust are lower and it can successfully overwinter in more northern regions of the southern U.S.A. Damage to wheat on the Canadian prairies is dependent on the speed at which the rust moves north each year and the level of resistance that wheat cultivars have to the prevalent strains of rust.

Leaf rust usually starts its northward movement in April and May. In an average year, leaf rust appears on susceptible winter wheat cultivars in Nebraska by mid-May, North Dakota near the first of June, and central Saskatchewan by the third week in July. Stem rust usually arrives in central Saskatchewan later than leaf rust (Table 1).

Table 1. Date that leaf and stem rust were first reported on susceptible cultivars growing in central Saskatchewan.
Year Leaf rust Stem rust

1992
1993
1994
1995
1996

July 27
August 4
July 19
August 6
July 21

August 1
August 5
None
August 10
None

Under normal conditions, there is usually a race between rust epidemic development and winter wheat maturity in the high rust hazard region of the Canadian prairies. Crop maturity usually wins the race and yield losses due to rust are minimal for properly managed winter wheat, even when susceptible cultivars are grown in this region. In contrast, later maturing spring wheat cannot be successfully grown in the rust area of the Canadian prairies without a high level of cultivar resistance or other effective methods of rust control.

Occasionally, the right combination of early arrival of rust inoculum and suitable environmental conditions results in economic damage to susceptible winter wheat cultivars grown in the rust area of western Canada. In 1986, stem rust was reported on winter wheat in central Saskatchewan on June 11. This early arrival was accompanied by weather conditions that favored rust infection and development. Significant damage occurred to the rust susceptible cultivar Norstar, which accounted for essentially all of the winter wheat acreage in western Canada at that time. Yield losses as high as 85 percent were reported for a few winter wheat fields. Although two thirds of the rust damage was a result of suboptimal management practices that delayed crop maturity, 1986 clearly demonstrated the need for effective methods to control rust in winter wheat grown in the rust area of the Canadian prairies.

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c) Control:

1. Resistant cultivars. Resistant cultivars provide the most cost effective means of controlling rust. However, rust races are constantly changing and, once cultivar resistance is overcome, new races of rust can quickly increase to epidemic levels. For example, stem rust race 15B, which became prevalent in 1950, built up to epidemic levels by 1954 and caused severe damage to formerly resistant spring wheat cultivars like Thatcher. The regular appearance of new races of rust means that plant breeders must constantly search for new sources of resistance that can be incorporated into adapted cultivars.

Single source (gene) resistance may give a cultivar complete rust immunity, but it is the form of resistance that is the easiest for the rust to overcome. Cultivars with multiple sources of resistance usually provide the most effective long-term rust control. A more general resistance is found in "slow rusting" cultivars. Rust infection and growth is retarded on slow rusting cultivars allowing the plant sufficient time to mature normally before economic damage is inflicted. Identification of slow rusting varieties requires regular crop monitoring and many plant pathologists do not recognize slow rusting as a legitimate option for rust control.

In the past, winter wheat cultivars that were widely grown in western Canada were rated as susceptible or highly susceptible to stem and leaf rust. It is only recently that breeding programs have started to release rust resistant winter wheat cultivars adapted to the Canadian prairies.

2. Foliar fungicides. In epidemic situations, registered foliar fungicides can be used to control rust on susceptible cultivars. Fungicides are usually only considered when there is an early infection of the lower leaves of susceptible cultivars growing in high yield potential environments that favor rust development. When fungicides are applied, the main objective is to prevent the rust from infecting the upper leaves of the plant, especially the flag leaf.

3. Crop management. Any management practice that delays crop maturity increases the risk of rust damage to susceptible cultivars. For example, Norstar winter wheat seeded two weeks later than the recommended seeding date in southeastern Saskatchewan suffered a 67 percent yield reduction in field trials conducted during the severe rust epidemic of 1986. Phosphorus deficiencies that go uncorrected also delay maturity and increase the risk of rust damage to susceptible cultivars.

4. Crop rotation and tillage practices. Under favorable conditions, rust summer spores (urediospores) are produced in great numbers and they can be blown vast distances by the wind. Consequently, crop rotation and tillage practices do not affect the build up of rust epidemics in western Canada.

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rusts rusts
Figure 12. Stem rust pustules can develop on the leaves, leaf sheats, stems, and heads of the wheat plant. (Image Size = 44k)
Figure 13. Leaf rust is mainly restricted to the leaves of the wheat plant. Pustules of leaf rust are much smaller thatn those of stem rust. (Image Size = 26k)