Online Gardener's Handbook 2010
Chapter 1: Causes of Plant Injury
- Abiotic Injury
Abiotic diseases are those not caused by a living organism or virus,
but by soil and weather conditions, or man-made physical and chemical disturbances.
Leaf and root systems are usually affected and of the two, root problems are the
Unless root conditions are improved, the plant may die.
plant's response to abiotic injury depends upon the severity of the problem and
the plant species itself. Oak, maple, ash and spruce, for example, are very sensitive
to soil compaction, drought, root damage or sudden changes in weather conditions.
Abiotic injury can also weaken the plant and predispose it to attack by insect
pests and diseases.
To control abiotic diseases, improve overall growing
conditions, avoid damaging factors and, whenever possible, select plants that
are resistant to abiotic damage.
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.
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 ageing
of the leaves.
Autumn Shed of Conifers
natural phenomenon is not a disease. All conifers undergo annual needle shed to
varying degrees. Beginning in early autumn, needles toward the inside of the tree
(closest to the trunk) turn yellow or brown and fall. Cedars and many junipers
have scale like needles covering tiny branchlets. These trees also shed the oldest
branchlets in the fall.
Trees of low vigour, or under stress from factors
such as drought, transplant shock, poor drainage or soil compaction may lose more
than one year's quota of foliage. In such cases, identifying the problem and alleviating
the stress will minimize autumn shed.
Pavement or compacted soil around trees prevents air and water from
penetrating the soil. Heavy equipment working on a landscape site or considerable
foot traffic can also result in soil compaction, especially in clay soils, resulting
in the slow decline of both young and old trees.
affects shade trees, and is found throughout Ontario. The gradual loss of vigour
and dying back of branches are classic symptoms. The presence of short internodes,
small, discoloured leaves and premature autumn colour may also occur. Maple, ash,
oak and beech are particularly susceptible.
In Norway maples over 30
years, for example, dieback of individual branches, or smaller leaves overall,
is a common sight. Other trees may begin to prematurely develop their fall colours.
Dieback may appear very suddenly or so gradually you may not know it is happening.
The causes of dieback are poorly understood. Root stresses such as
soil compaction, poor drainage, site alteration, and high soil temperature are
contributing factors, as are viral diseases. Girdling wires and ropes left on
after transplanting can also be the culprit.
To prevent dieback, institute
a proper watering and fertilization program, as this will promote healthy tree
growth. With older trees, avoid changes to drainage patterns or the use of heavy
equipment on top of the root systems.
All evergreens, particularly eastern white cedar, are sensitive
to urine from male dogs. Symptoms include rapid browning and blackening of lower
leaves, and a distinctive greasy appearance. Damaged growth does not recover.
To avoid injury, protect the lower portion of the shrubs.
to lawns results from dogs urinating on the lawn killing the above-ground portion
of the turf. Symptoms usually include circular or oblong patches of dead turf
surrounded by a dark green, more rapidly growing ring of turf. Recovery usually
occurs within two to three weeks under favourable moisture conditions. Thorough
watering of the damaged spots will help leach salts from the urine. However, severely
damaged lawns may have to be renovated.
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 most common with ivy geranium
and other waxy-leaved annuals.
To prevent edema, avoid fluctuating
soil moisture conditions, especially in times of high humidity or low temperature.
Replant large plants that dry rapidly into larger containers to minimize this
much fertilizer, or fertilizer that is improperly applied, can cause burning.
Not enough fertilizer can result in deficiency symptoms, often seen as an overall
yellowing of the leaves or weak, spindly growth.
or Chilling Injury
Frost injury is caused by freezing temperatures after
growth has begun. Symptoms are different for different types of plants:
- Annuals: blackened leaves and stems. Poorly hardened off
transplants are particularly susceptible
- Deciduous plants:
leaf crinkling and blackened leaf margins
- Early flowering shrubs:
flower buds fail to open or appear tattered and brown along the petal edges.
Coniferous trees: wilting and possible dying off of young shoots.
constrict some part of the base of a tree are called girdling roots. They have
become a major problem in the urban landscape, causing the decline of many 25-30
year old Norway and red maples. Nursery grown trees are most susceptible, as they
are often cultivated close to the stem or grown in containers, causing small roots
to grow tangentially to the trunk. Years later, once both trunk and roots are
established, constriction by the tangential roots occurs.
can usually be found within 15 cm of the soil line. Their presence is often indicated
by a lack of normal flaring where the trunk meets the roots. Visible symptoms
include slow growth over a period of several years, small leaf size, premature
autumn colour and dieback. These symptoms do not appear until girdling roots have
done irreversible damage. Because the problem is usually restricted to one side
of the tree, some branches may die while others remain healthy.
this problem, remove any spiral-like roots before planting young trees.
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.
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.
Non-selective herbicides such as paraquat kill leaves and green
twigs by direct contact only. Spray droplets cause discrete brown spots similar
to leaf spots caused by fungi.
Glyphosate - commonly known by its trade
name Roundup - 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.
and Root Scorch
Leaf scorch is a typical example of what can occur when
plants have difficulty taking up water. Horse chestnut, oak, harlequin maple,
and many others are susceptible, and the problem is most common along city streets.
Leaf scorch is seen most often in mid-August, and usually develops
on branches in the upper canopy first. It appears as sudden leaf death or browning,
either interveinally or at the margins. A late spring leaf scorch of maple and
beech has been attributed to sudden exposure of leaves to full sunlight following
a wet, cloudy period.
Root scorch is caused by high levels of salt
in the soil, either due to salty irrigation water or salt residues from winter
To treat leaf scorch, water deeply once a week during
dry spells to minimize stress to the tree.
Natural gas leaking from a pipeline seriously harms plants.
Spraying chemicals at midday during hot weather may injure the leaves
of plants because the plants are already under stress and because the pesticides
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.
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. Only certain species of woody plants will tolerate
these types of conditions: silver maple, willow, dogwoods, balsam fir, black spruce
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 the tree facing the road and on the side of the road facing
the prevailing wind.
Salt Spray Injury
- Deciduous plants: failure of flower and leaf buds to open,
small misshapen leaves, tip dieback and shoot tufting.
needles or branchlets turn brown from the tip down and then fall, leaving bare
Salt Injury from Root Uptake
runoff from melting ice and snow from roads and sidewalks will accumulate in the
soil until damaging levels are reaches. 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. Trees known to be sensitive include white pine,
eastern white cedar, sugar maple, linden, red and white oak and crabapple.
pine, blue spruce, Russian olive, sumac and Norway maple are quite tolerant.
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. Tomato is one of the most susceptible
vegetables and will wilt and collapse about the time the fruit begin to size.
Other susceptible vegetables include broccoli, Brussels sprouts, cabbage, cauliflower,
eggplant, peas, pepper, and potato. Susceptible ornamentals include potentilla,
azalea, rhododendron, euonymus and red and white pines.
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. Using raised beds may be helpful. Another alternative
is to plant tolerant plants, including vegetables such as beet, onion, snap beans
or sweet corn or ornamentals such as mock oranges, viburnums, honeysuckle, hosta,
iris, periwinkle and many others.
For further information, see OMAFRA Infosheet,
Black Walnut Toxicity
Wind, Hail or Lightning Injury
or hail injury can cause tattering of leaves on flowers, vegetables, trees and
shrubs. Because strong winds are often seen in the spring, young, immature leaves
are most susceptible to tattering.
A direct lightning strike to trees can
cause serious damage, tearing bark and turning leaves black.
Many trees and shrubs 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, do not over-fertilize.
This will prevent late fall growth and ensure adequate hardening off.
of winter injury are varied. Broad-leaved and coniferous evergreens can dry out
if planted in exposed, windswept areas. An early warning sign in broad-leaved
evergreens is leaf margins that curl down and in. If moisture loss is severe,
buds, leaves, needles and small branches eventually turn brown and die. In foundation
plantings, water well late in the fall, use thick mulch on the soil surface to
insulate the root system, and use protective screens of burlap to reduce exposure.
For young, thin-barked trees such as maples, linden, London plane and flowering
cherry, 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.
Many ornamental trees and shrubs may only be marginally hardy to your area.
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.
guard against winter injury in all plants, do not over-fertilize; this prevents
late fall growth and ensures adequate hardening off. As well, plant any injury
prone trees and shrubs in the most protected locations around your property.