| ||Living (biotic) causes of disease | Non-living (abiotic) causes of disease
Spot diseases of leaves and stems are usually caused by fungi, bacteria and environmental conditions. Generally, the size, shape and color of the spot is constant for the disease. Stem-eye spot of fescue is an example of a disease that causes distinctive spots on the flowering stems of fescue.
Blotch diseases appear as irregular-shaped leaf lesions. Net blotch is one of the most common foliar diseases of barley in Canada.
Blight fungi typically kill young stems and roots. Seedling blights are common throughout the prairies. In some springs, large numbers of canola fields must be reseeded because of seedling blight. The disease is very active in dry, cold soil conditions.
Scorch occurs during very hot weather. The sun causes a browning or bleaching of leaves. On cereal seedlings, this takes the form of distinct bleached bands called heat banding or rugby stocking.
Wilt results from a deficiency of water in the leaves and stems and is caused by drought or root and stem diseases. Flax wilt is caused by a soil-borne fungus that attacks through the roots.
Damping-off fungi most frequently attack seedlings and kill the stems and roots near the soil line. The disease is active under wet or water-logged soil conditions and causes infected plants to rot and fall over. Alfalfa seedlings are particularly susceptible.
Rots are caused by various organisms. Root rot of canola is a common disease usually caused by Rhizoctonia.
Smuts appear as black pustules that break out on the heads of cereals. Loose and covered smuts of barley, wheat and oats are common fungal diseases.
Living (Biotic) Causes of Disease
Infectious crop diseases on the prairies are caused by fungi, bacteria, viruses and occasionally nematodes. Other organisms that cause disease are usually unimportant to northern prairie agriculture.
Fungi that feed directly on living plants and cause diseases are called pathogens.
Actively growing fungi in an active stage of growth are composed of very fine hairlike threads growing together in a mat called mycelium. They are familiar as the fluffy growth on rotting food. Fungi reproduce by spores of various shapes, sizes and colors. Spores can be produced directly from the fluffy threads as in the case with bunt balls in wheat. These bunt balls are actually masses of spores. Spores may be formed in or on specialized structures called fruiting bodies. For example, sclerotinia white stem rot of canola produces its spores in tiny mushroom-like structures that grow from black resting structures called sclerotes.
Parasitic fungi are usually limited in the species and even cultivars of plants they attack. Covered smut is a disease of most cereals, but the smut fungi that infect wheat cannot infect barley and vice-versa.
Disease develops when a fungal spore or other infectious agent is able to infect and grow on a healthy plant. The reaction of the plant to the infection is the symptom of the disease.
Spores are the "seeds" in the life cycle of the fungus.
- They are the reproductive structures of the fungus.
- They spread fungi to new locations.
- They survive over winter or other periods of adverse weather.
- Spores are dispersed by air currents, running water, splashing rain, insects, and on or in seeds.
Some fungi, such as Sclerotinia and Claviceps, produce special resting bodies called sclerotes or ergots. Sclerotes about the size of cereal grains can survive in or on the soil for years. Sclerotia or ergots germinate during moist summer weather and produce tiny mushrooms that release infectious spores. Sclerotinia spores infect canola, sunflowers and beans, whereas ergot spores only infect the flowers of cereals and grasses.
Bacteria are invisible, single-celled organisms. They invade plants through natural openings or wounds. They need free water to enter natural breathing pores on plant leaves and stems. Bacteria can multiply quickly and are most destructive under moist warm conditions. They spread by splashing rain, plant to plant contact, on seeds or by insects.
They cause leaf and head blights, wilts, scabs, cankers and soft rots and overwinter on or in seeds of living plant parts (shoots, buds, crowns) and plant residue.
Viruses are visible only under an electron microscope. They are found in all parts of infected plants but usually not in the seed. Many viruses are transmitted by insects such as aphids, leafhoppers and mites. Viruses cannot survive outside the host plant or insect vector. They overwinter in perennial or biennial weeds and in the case of a few viruses, inside seed embryos.
Nematodes are tiny worm-like creatures. They may exist on organic matter in the soil or as parasites of plants. The life cycle of nematodes may take from weeks to months to complete with six growth stages: egg, four larval forms and the adult.
Non-parasitic nematodes are common in prairie soils, but only a few parasitic species are problems in some crops. Stem nematodes may cause yield losses in irrigated alfalfa and cyst nematodes may cause problems in sugar beets.
Nematodes survive in the soil as eggs or cysts that hatch when a suitable host is present. They are spread by humans, wind, water or animals. Nematode distribution may be restricted by quarantine measures as is the case with the golden nematode of potatoes, which occurs in soils on Vancouver Island and Newfoundland. Stringent quarantine measures on the movement of potatoes, other plant parts and soil have prevented this pest from being introduced to the rest of Canada.
Non-Living (Abiotic) Causes of Disease
- Soil moisture
Drought is easily recognized. Plants are wilted or dry and dead. A milder moisture deficiency is more difficult to recognize, especially if the plants are also affected by parasitic disease. Water stress weakens plants and makes them more susceptible to infectious diseases.
Excessive soil moisture deprives plant roots of oxygen causing death. Canola seeds in cold wet soil may rot during germination or produce weak yellow seedlings that are susceptible to infectious diseases. Alfalfa roots are sensitive to water logging and are killed by a week of flooding. Grasses may thrive for many weeks under water.
Fluctuation in soil water can cause irregular tillering in cereals.
- Soil nutrients
Nitrogen is the most universally deficient nutrient for crops. Nitrogen deficiency limits chlorophyll development in canola; older leaves turn light green or yellow and wither and fall off. Plants are stunted and the flowering period and seed production are reduced.
Excessive nitrogen can cause heavy vegetative growth. Dense crop canopies are conducive to many diseases and lodging, and both reduce yield and crop quality.
Sulphur deficiency occurs in canola especially on well-drained, sandy and leached Grey Wooded soils. Affected canola leaves are cupped and purplish, flowers are pale yellow, and pods are poorly filled or missing. A deficiency of manganese in organic or alkaline soils can cause gray speck disease of oats and barley. Phosphorous deficiency in cereals will cause reddish or purple leaf discoloration and may predispose wheat seedlings to browning root rot. Copper deficiency in wheat predisposes plants to head discoloration (melanosis), take-all, ergot and low grain yields. Copper is essential for seed production and development in wheat, barley and oats.
Potassium deficiency in canola frequently occurs during cool weather. Although soil tests may indicate that potassium is abundant, this element is not released in a form that is available to plants at a fast enough rate when the soil is cold. This is particularly true during flowering when potassium demand is high. Potassium deficiency in alfalfa is recognized by white spots on the leaves.
Acidic soils below pH 5 may have abnormally high quantities of minerals that interfere with normal plant growth. In addition, calcium needed for normal growth is often lacking in these soils. Alfalfa will not do well in soils below pH 6 because nitrogen fixing bacteria that produce the nodules on the alfalfa roots cannot survive. High pH (alkaline soils) may also limit the availability of certain micro-nutrients. Most crops do best when the soil pH is between 6 and 8.
- Meteorologic conditions
Light stress can occur around heavily treed areas. Plants that do not tolerate shade grow tall and spindly and are susceptible to root rots.
Sudden changes in temperature can produce symptoms similar to those of an infectious disease. Blast of oats is caused by extreme heat or very cool weather shortly before the head emerges; seed-producing flowers are killed in the boot stage and no grain is formed.
Heat from the sun is greatest at the soil surface where the tender leaves or young stems of cereals and oilseeds are damaged.
Cold temperatures may damage many crops during the growing season (frost) or over the winter (winterkill). When the growing point of sunflowers or peas is killed by frost, new growth will begin from dormant buds below the damaged area. This results in a distorted appearance or production of an excessive number of branches. Soil surface temperatures of -3 to -5 degrees Celsius for several hours can kill canola seedlings in May and June. Summer or early fall frosts at temperatures of -1 to -20 degrees Celsius can reduce the quality and quantity of canola and cereal grain. Alfalfa and winter wheat can be killed when soil surface temperatures fall below -20 degrees Celsius.
Typically, wind, hail and lightning cause mechanical damage and make crops more susceptible to infectious diseases.
Toxic emissions or their by-products in the atmosphere can damage plants. For example, ozone in concentrations of more than 80 parts per billion (ppb), which can be produced by severe thunderstorms, may damage field beans. Sulphur dioxide emissions from gas plants, in concentrations of 5 to 10 parts per million (ppm), may injure crop foliage.
Mechanical injuries from almost every agricultural practice done the wrong way, at the wrong time or with the wrong implements can damage plants. Roots can be damaged by cultivation. Rough or improper handling of grain during combining, cleaning or drying can affect seed germination.