Disorders
Abiotic disorders are those not caused by a living organism or virus, but by soil and weather conditions, or man-made physical and chemical disturbances. Abiotic injury can also weaken the plant and predispose it to attack by insect pests and diseases. To control abiotic disorders, improve overall growing conditions, avoid damaging factors and, whenever possible, select plants that are resistant to abiotic damage.
Air Pollution
Air pollution - particularly ozone, nitrogen oxides and sulphur dioxide - damages some plants more than others and is more severe in some areas of Ontario than in others. Ozone and nitrogen oxides are found in smog. Sulphur dioxide is formed primarily during the combustion of coal and oil. The type of damage seen varies with pollution levels, as well as with the sensitivity of the plant. Symptoms include leaf stippling or chlorosis and premature aging of the leaves.
Chimera
Chimera is a genetic abnormality that can be confused with other diseases or disorders. Chimera results in leaf variegations - patterns of white to yellow intermixed with green. Because this is a genetic abnormality, all leaves usually have a similar pattern; however, occasionally one or two leaves may be affected.
Compacted Soil
Compacted soil around plants prevents air and water from penetrating the soil. For more information see the section Compaction.
Edema
Edema is associated with extreme fluctuations in soil moisture. Tiny water-soaked spots first appear on lower leaf surfaces, and later develop into corky ridges. Older leaves with severe edema turn yellow and die. It is especially an issue on cucurbits. To prevent edema, avoid fluctuating soil moisture conditions, especially in times of high humidity or low temperature. Damage to cucurbit fruit can be minimized by covering soil with plastic or straw mulch.
Nutrient Disorders and Soil pH
A deficiency or excess of a nutrient can cause a range of symptoms including leaf chlorosis, stunting, tipburn, flower and fruit abortion, poor germination, and poor overwintering. For more information on symptoms caused by specific nutrient disorders see the sections Macronutrients and Micronutrients.
Acidic or alkaline soils can also lead to nutrient deficiency or toxicity. For more information see the section pH.
Frost, Freeze, and Chilling Injury
The most common symptom of frost injury is blackening of leaves, especially at the top of plants in exposed areas. Cold temperatures above freezing can cause discolouration of young transplants if not properly hardened off, or can directly damage or kill some sensitive tropical or sub-tropical plants. In perennial plants, freezing temperatures can kill flowers in the spring or burn off young developing shoots. Damage will be worse in low areas where cold air pools on calm nights. Overnight irrigation, wind machines, and row covers are some of the strategies used to reduce frost and freeze injury to specialty crops.
Herbicide Injury
Plants can be seriously harmed by accidental exposure to herbicides. Careless application, spray drift or leaching into the root zone are common culprits. The severity and type of damage depends upon the amount and type of herbicide involved. Slight exposure may have no long-term effects at all. Exposure to soil-borne herbicides that are active over a period of months or even years, however, is far more damaging. Common symptoms of herbicide injury include yellowing, stunting, twisting, bleaching, spotting and burning of leaves and shoots.
Once accidental exposure has occurred, there is little you can do to correct the damage beyond watering and fertilizing to relieve some of the stress to the plant. Prevention is the better practice. Read the herbicide label carefully before using, and follow all instructions. To avoid problems caused by spray drift, do not apply sprays on windy days.
One of the most common herbicides used in agriculture is glyphosate, which is sold under numerous trade names. Glyphosate is absorbed by the green tissues of plants, and is then transferred to the root system. Symptoms of damage include yellowing and browning of leaves as well as deformities of new growth.
Injury Caused by Pest Control Products
Spraying pest control products, whether synthetic or natural, can cause injury to plants if used at the wrong rate, applied at the wrong time of day, applied at the wrong growth stage, or if not registered on a crop and tested for crop tolerance. Applying products at midday during hot weather may injure the leaves of plants because the plants are already under stress and because the pest control products dry too quickly. The younger, softer growth is most prone to injury. Symptoms appear as marginal leaf burn or as small brown spots throughout the leaf where small, beaded chemical droplets remained. Applying the wrong product at bloom can cause flower abortion or poor fruit set. Always use pest control products registered on the crop and follow the label carefully to avoid injury.
Poorly Drained Soil
Most plants do not grow well in poorly drained, waterlogged soil because it lacks the air necessary for healthy root development. Under these conditions, most plants develop poor, weak root systems or very shallow systems that are prone to root rot.
Salt Injury
Salt injury from runoff and spray drift caused by passing traffic occurs widely along roads where de-icing salt is used extensively in winter. When plant tissues are wet and temperatures are above freezing, salt enters the plant and accumulates. Typically, the injury develops on the side of woody plants facing the road and on the side of the road facing the prevailing wind.
Salt runoff from melting ice and snow from roads and sidewalks will accumulate in the soil until damaging levels are reached. When a plant takes up too much salt through the root system, leaf size can be reduced, there may be marginal leaf scorch, and leaves can fall prematurely.
Walnut Allelopathy
The roots of the butternut and black walnut (though not the English walnut) have a harmful effect on many plants growing within their root zone because of the juglone produced by their roots. This injury is called allelopathy. Plants in the Solanaceae (e.g. pepper, eggplant, tomatillo) and Brassicaceae (e.g. kohlrabi, bok choy) families are particularly susceptible to walnut allelopathy.
If you notice that a plant near one of these trees is growing poorly, avoid planting that species in the future. The best remedy may be to remove the tree, but the harmful effects last for more than one year.
Wind and Sandblasting
Depending on the crop, wind can cause a variety of symptoms. Whipping of tender leaves in windy conditions can lead to tearing and tattering, leaving the leaves susceptible to pests. In the spring, wind-whipped sand particles can cause sandblasting on leaves and stems facing the prevailing winds. On very young seedlings, the damage to the tender stem can cause collapse of the plant. In larger plants, symptoms can appear as speckling, which can eventually lead to collapse of part of the plant and poor growth. Sandblasting can be reduced through the use of a fast growing cover crop planted between the rows of the crop or by establishing windbreaks around the field.
In some crops dry winds can cause the plant to lose moisture faster that it can be replaced by the roots. As a result, the edges of leaves can burn, a condition often referred to as windburn.
Lightning
A lightning strike in the middle of a field can cause a dead patch with completely dead plants in the middle, and partially scorched plants around the perimeter. This can be distinguished from disease because the patch will not grow over time and will appear suddenly.
Winter Injury
Many trees, shrubs and vegetative perennials suffer winter injury because they are only marginally hardy to your area. If the winter is particularly harsh or if there are rapid temperature fluctuations during winter, leaf and flower buds can be killed. The effects are often not evident until late spring, when new growth collapses and inner wood appears black to the core. To guard against this, avoid excess fertilization and do not fertilize late in the summer or in the fall. This will prevent late fall growth and ensure adequate hardening off.
Symptoms of winter injury are varied. For young, thin-barked trees, winter injury takes the form of frost cracks or `south-west injury'. These are long splits in the bark mainly on the south or west side of the trunk. Wrapping the trunk with burlap or painting it with a white latex will reduce this problem.
For marginally hardy wood plants, extreme low temperatures or rapid fluctuations during late winter will result in the death of leaf or flower buds - flower buds are the most sensitive.
For vegetative perennials, winter injury can result in poor or no emergence the following spring. Winter injury can be reduced in these crops through protection from straw or row covers over winter.
Excess Light
Plants adapted to a shade environment, such as native forest medicinal herbs, will be damaged by exposure to full sun for longer than a couple of hours. Affected plants often turn white or yellow and eventually die. Plants adapted to a certain light level can be damaged by exposure to higher light intensity over an extended period, even if they are still under shade. For example, ginseng is adapted to 70-85% shade and growth will be stunted if exposed to 60% shade over a few weeks.
Heat
Cool-season, temperate crops will grow poorly under hot temperatures, especially when combined with low soil moisture. These plants will often appear healthy, but yields will be decreased. Young plants of many crops with a thin leaf canopy can develop heat canker at the crown. Affected plants will have a constricted crown, stunted growth, and occasionally, will collapse. Crops adapted to a temperate forest environment can be directly damaged by hot temperatures when grown under artificial shade, because temperatures under the shade can be higher than the ambient air. Symptoms of heat burn often begin along the main leaf vein and eventually extend to the edges of the leaves.
