Chapter 22
Diseases

INTRODUCTION

Diseases are a constant threat to farmers' crops. Historically, crop disease epidemics have been responsible for human famines, widespread suffering, and death. Outbreaks of crop diseases have destroyed the hopes and dreams of many farmers and they have created hardships for those dependent upon farmers for their daily food supply.

The large increase in the human population of this planet has been possible because of the success that modern day agriculture has achieved in controlling crop diseases. However, crop diseases still persist and losses will increase if our efforts to control them are relaxed.

Our crops are under threat of attack by diseases from the day they are seeded to the day they are harvested. Even after the seed is harvested, there is a risk of contamination with the spores of seed-borne diseases like common bunt smut. Consequently, it is important that farmers have a basic understanding of the diseases that damage their crops and the control options available to minimize or prevent economic loses.

Winter wheat is a relatively new crop to most of the Canadian prairie region and the disease problems associated with its production still haven't been completely sorted out. It has also been a low acreage crop in this region, which may have limited the spread of some diseases. However, winter wheat is surrounded by a large expanse of spring wheat in western Canada and most of the diseases that attack spring wheat have the potential to damage winter wheat. For this reason, it is likely that any major winter wheat disease problems should have quickly made their presence known. However, with the notable exception of one outbreak of rust that was aggravated by poor crop management, the expansion of winter wheat outside of its traditional production area in southern Alberta has been remarkably free of major disease problems.

These observations are not meant to suggest that western Canadian winter wheat growers are immune to the threat of crop diseases. They do, however, imply that disease problems are manageable. In addition, given the fact that there has been a minimal effort expended on the development of disease resistant winter wheat cultivars that are adapted to the Canadian prairies, there has to be considerable optimism that an acceptable disease control picture can be made even better.

DISEASE CONTROL

Farmers have many weapons available for use in the war against crop diseases. However, as in all wars, weapons can be put to most effective use when intelligence has provided a good understanding of the enemies weaknesses and the soldiers understand how to make the best use of their weapons.

Crop diseases are a fact of life. The best control that we can hope to achieve will minimize losses, not eliminate the disease entirely. Nearly complete control may be the objective, but the resources available, the expected crop losses, and the cost of employing a control measure must all be factored into disease control decisions.

Three conditions must be met before diseases caused by biological pathogens (disease causing organisms) will develop and spread:

As a minimum, control measures must be designed to reduce or eliminate one of these conditions. However, control is most effective when action is taken on all three fronts.

When designing a control strategy, it is important to remember that crops are attacked by more than one disease, some diseases attack more than one crop, and diseases are often spread by more than one method. Control measures must be practical and based on an understanding of the pathogen, cropping system in place, cost of implementation, and the environmental conditions that prevail. In addition, there is little to be gained if the disease control measures are so drastic that they create problems of equal or greater significance, e.g., routinely deep plowing to bury disease infected crop residues on highly erodible land.

Disease problems can be expected to change each year. Therefore, field scouting combined with close attention to the weather and rate of crop development provide the best means of anticipating disease problems.

Effective control requires knowledge of the diseases that are present in the field and an understanding of their potential to cause economic losses. Regular field scouting trips should be planned to assess the condition of the crop. The frequency of these visits should be increased during potential problem periods and when weather conditions favor disease development. The potential for crop damage is normally greatest in high yield environments where good moisture distribution favors disease infection and development. Periods of hot, dry weather stall the development of many diseases. However, common root rot, which is usually more severe under dry conditions, is a notable exception to this generalization.

When scouting, the most disease prone regions of the field, such as low lying areas and along shelter belts, should be checked first to insure that the early stages of a disease epidemic are not missed. This should be followed by a zig-zag walk across the field to establish the extent that diseases have spread and to obtain an estimate of potential disease losses.

Early identification of potential disease problems is vital in the planning of effective control strategies so that the returns on crop investments can be maximized. However, the presence of disease symptoms is not reason for panic. Make sure the disease is properly identified before taking corrective action. Remember, diseases vary in their abilities to develop into epidemics and symptoms on the lower leaves of a maturing crop do not necessarily mean that there will be measurable yield losses.

Some diseases are carried over on crop residues. Therefore, it is important that records be maintained for each field so cropping and disease history can be quickly and accurately reviewed.

Diseases can spread from neighbouring fields, so it is a good idea to make a note of the crops that are growing across the fence line or road. For example, late seeded spring wheat crops can harbor wheat streak mosaic virus which can be a threat to winter wheat that is seeded early in adjacent fields.
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Diseases must be correctly identified before effective measures for control can be put in place. Most diseases have characteristic symptoms that can be used for identification. However, some diseases have similar symptoms and the symptoms of the same disease may be different on different wheat cultivars (varieties), e.g., leaf spots.

Biological pathogens (disease causing organisms) are not responsible for all the spots, blotches, and other lesions that are found on plants. This type of damage can also be caused by environmental stresses, nutrient imbalances, and genetic disorders.

Even the experts can be confused by the symptoms seen on plants in the field and special laboratory techniques are often required before some diseases can be accurately identified. Plant disease laboratories located in each of the prairie provinces provide these diagnostic services.
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Resistant cultivars provide the most environmentally safe, reliable, and cost effective means of controlling important diseases. In theory, it should be possible to develop cultivars with resistance to every disease that causes significant crop damage.

Development of cultivars that combine disease resistance with all the other desirable agronomic and quality traits requires the investment of considerable time and money. Maintaining a high level of disease resistance is also a never-ending battle, because diseases eventually find some means to overcome cultivar resistance. As a consequence, plant breeders and other scientists are continually searching for new and improved sources of genetic resistance that can be used to maintain the farmers' advantage in the war against diseases.

Spring wheat breeders have been very successful in producing adapted cultivars with resistance to many of the major diseases that can be a problem in western Canada. Most of the diseases that attack spring wheat also have the potential to damage winter wheat. However, winter wheat is a relatively small acreage crop with an unstable history in this region and there has not been the resources available for breeding programs to give the production of disease resistant cultivars a high priority.

Information on the disease resistance of registered cultivars can be obtained from provincial variety of grain crop publications that are updated annually.
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Crop rotation is one of the oldest methods of disease control. It can be used to effectively control diseases like ergot, which overwinters as sclerotia and can only survive away from a susceptible host for a year. Crop rotation may also help to slow the development of epidemics of diseases that overwinter on crop residues and produce inoculum that only spreads short distances, e.g., tan spot and leaf and glume blotch. It is not an effective method for controlling diseases that are spread long distances by airborne spores, e.g., rusts and powdery mildew, or insect vectors, e.g, barley yellow dwarf.

Many diseases overwinter on infected crop residues. Crop rotation allows these residues to decompose, thereby reducing the amount of disease inoculum available for infection of susceptible crops. Unfortunately, crop residues decompose very slowly in the semi-arid western Canadian prairie climate, especially following high production years when crop residues are very abundant and in direct seeding or no-till production systems where the crop residues are left on the soil surface. Two or more years are often required for wheat residues to decompose to the point where there is a meaningful reduction in disease inoculum. Consequently, crop rotation away from wheat has limited practical value in preventing epidemics of most diseases on the Canadian prairies. Some diseases, such as take-all root rot, can also attack other cereals making it even more difficult to design an effective rotation to control these diseases in wheat.

In some situations, the sequence of crops growing in adjacent fields may be more important than the crop rotation that is followed on a single field. For example, crops sown in neighbouring fields play an important role in the spread of wheat streak mosaic virus, which requires a "green bridge" to survive between crops.
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Tillage is often recommended as a method of controlling diseases that survive on crop residues, e.g., tan spot and leaf and glume blotch. Incorporation of straw into the soil speeds up decay and reduces the risk of carryover and spread of these diseases. However, tillage has not proven to be a very practical method for the prevention of wheat disease epidemics on the semi-arid Canadian prairies where wheat is a large component of rotations and residues are difficult to eliminate as an inoculum source.

The need for standing stubble to act as a snow trap to prevent winter damage eliminates tillage as a disease control method for winter wheat production on most of the Canadian prairies. High operating costs and an increased risk of moisture loss and soil erosion also make the burying of crop residues an unattractive disease management option.

Straw burning can be used to dispose of crop residues. However, this management practice is not recommended as a method for disease control in western Canada.
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Crop management practices that promote healthy growth and vigorous root development help the plant to defend against attacks by diseases.

Shallow seeding at the optimum date provides the plant with the opportunity to avoid root diseases and become well established before the onset of winter. Delaying seeding to the end of the recommended period is often suggested as a control measure when there is a high risk of infection by wheat streak mosaic virus.

Fertility programs should be well balanced and based on reliable soil test recommendations. Underfertilization produces an unhealthy crop that is more prone to damage by diseases. Overfertilization may produce a dense crop canopy and an ideal environment for the development and spread of some diseases. For example, excessive nitrogen fertilization promotes root diseases and produces a lush canopy that creates a favorable environment for the growth and spread of many foliar diseases. Chloride deficiencies are the most likely cause of physiological leaf spot and phosphorus deficiencies must be corrected to ensure healthy plant growth and development.
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Volunteer plants and weeds often provide a reservoir of diseases that can infect healthy crops. Elimination of volunteer wheat is one of the most important weapons in the battle against wheat streak mosaic virus. Grassy weeds and volunteer cereals are hosts for take-all root rot and ergot. Grasses and volunteer rye growing in neighbouring fields and along fence lines are also a common source of ergot inoculum.
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A number of diseases can be spread by infected or contaminated seed. For example, almost all smuts are seed-borne and seed contaminated with sclerotia is a method by which ergot is spread. Therefore, when using own-farm seed stocks, it is a good practice to only plant seed from fields which are free of loose smut and to avoid using seed that is contaminated with bunt spores or ergot sclerotia.

Seed certification programs and seed applied fungicides play a very important role in the control of seed-borne diseases. Seed certification is usually based on field inspections that are made before harvest to determine the presence of disease infected heads.

Plant disease laboratories can test suspect seed lots for the presence of seed-borne diseases.
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Resistant cultivars and proper crop management are all that is required to provide adequate disease control in winter cereals grown on the Canadian prairies. Unfortunately, a high level of disease resistance has not yet been bred into winter wheat cultivars registered for production in western Canada and crop management may not provide the required control of some diseases in some years, especially in high moisture regions where farmers are moving to more and more intensive crop management systems. As a consequence, fungicides have an important role to play in the control of diseases in winter wheat.

Regular treatment of winter cereal seed with fungicides is recommended for the control of seed-borne diseases like smut. Seed applied fungicides may also help to reduce early infection by soil-borne diseases like common and take-all root rot.

Fungicides may provide an economic method for the control of foliar diseases such as powdery mildew, Septoria leaf and glume blotch, and tan spot in cropping systems designed for areas where favorable moisture conditions realistically permit winter wheat production targets in excess of 55 bushels per acre. In epidemic situations, rusts can also be controlled by foliar fungicides.

An economic assessment should be made prior to the application of foliar fungicides to ensure that the yield potential of the crop justifies the added cost. When foliar fungicides are applied, the primary objective is to prevent the disease from infecting the upper leaves of the plant, especially the flag leaf. Consequently, fungicides are usually only considered when there is an early infection of the lower leaves of susceptible cultivars growing in high yield environments.

The timing of fungicide applications is important for effective disease control. There also may be grazing restrictions and a pre-harvest interval during which the chemical should not be applied. Label instructions should always be read carefully and followed when applying fungicides.

The proper fungicide treatment must be selected for effective disease control and some fungicides can only be applied by trained commercial applicators. Check your local Crop Protection Guide for the most recent recommendations on the use of fungicides and a current list of registered fungicides.

In many areas of the world, diseases have developed resistance to frequently used fungicides. Therefore, the unnecessary use of fungicides should be avoided.
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