Identify Plant Diseases

Plant Diseases:

Disease (definitions):
  • dis - malfunction, not sufficient, poor
  • ease - well-being, health
  • disease - poor health, lack of well-being

Plant disease is an impairment of the normal state of a plant that interrupting or modifing its vital functions.

All species of plants are subject to disease. The occurrence and prevalence of plant diseases vary from season to season and usually depend on the presence of a pathogen, or environmental conditions.

Infectious diseases: caused by a pathogenic organism.

  • fungus
  • bacteria
  • virus

Cultural diseases: caused by poor growing conditions.

  • over-watering
  • poor drainage
  • drought
  • temperature extremes, weather
  • chemical damage
  • physical damage
  • poor nutrition, incorrect pH level in soil for that plant
  • incorrect light and temperature conditions
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Diagnosing Diseases

Define the problem

  • What would a normal plant look like?
    how is this plant different?

Look for patterns

  • non-uniform patterns indicate living factors (e.g. insects, fungus, bacteria)
  • uniform patterns indicate non-living factors (e.g. chemical, physical, weather)

Identify time of development

  • progressive spreading indicates living organism.
  • damage occurring but not spreading to other parts of the plant indicates
  • non-living factor.

Determine cause

  • distinguish between living organisms.
  • distinguish between non-living organisms.
  • the most likely thing for it to be is the most likely thing it will be.

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Look around

  • what types of plants are affected; all the same or different, how many, same age or some new, some old, damage on same side on different plants.
  • wind and sun patterns, driveways soil sterilizer.
  • plants in appropriate growing conditions for that plant.

Ask questions

  • planted by the same person; some by plant owner, some by developer.
  • changes in growing conditions; soil added, construction, watering system installed.
  • any spraying done or pesticides used.


Does it make sense to the plant owner?

Don't be afraid of common sense and trusting your sub-conscious to tell you an answer you already know. If an answer immediately comes to mind, but you wonder if it is right, pay attention to it trust your own judgment. As you learn more, it is stored in retrievable memory, but you have to be able to access it and consider it.

With experience comes a knowledge of specific symptoms you can't possible know unless you have seen them (e.g. honeysuckle aphid, telial galls)

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there is no law that you can't have more than one problem at the same time

Cultural Disease

Plants are considered to be diseased when they do not grow or develop normally. The cause of this condition may be a result of infection by disease-causing organisms, or from environmental factors. When disease is caused by environmental factors they are considered cultural diseases. Examples of environmental disorders that may cause cultural disease are drought, temperature extremes, chemical damage, physical damage, nutrition and soil pH, and incorrect light conditions.

Chemical Damage

Cat urine is another cause of chemical soil contamination in flowerbeds on the warm side of a house in the winter. You can use lime to help neutralize the acid that is created.

Chemical damagecan be contact or systemic. It can come from sprays onto leaves, from soil into roots or occasionally from wounds in bark. Chemicals can be contact, killing the part of the plant they touch, such as weed killers that top-kill only, or systemic, moving within the plant with the sap, such as soil sterilizers.

While one expects herbicides to kill plants, insecticides can also kill when used at the wrong time, temperature, or on a plant that cannot tolerate it. This why it is always important to read the label before using any chemical.

Chemical damage can often be determined by leaf damage patterns. Spray damage will leave a pattern of spots equally spaced on the leaves. It can be all over the tree or only on one side of a tree, for example as a result of spray drift from lawn weed killer being sprayed on a windy day.

Chemical damage from roots usually causes black or dark brown blotches in the leaf material between the veins and around the edges - the most sensitive areas for leaf damage. This can also be overall, from a chemical in the soil around that plant's roots, or one-sided, from the root of a tree reaching an area where the chemical was used.

Cat urine is another cause of chemical soil contamination in flowerbeds on the warm side of a house in the winter. You can use lime to help neutralize the acid that is created.

Chemicals can also cause specific damage which must be known to be recognized, such as the very pale, almost white, needles or leaves, caused by the sterilant Atropel, or the typical twisting and distortion of new growth on tomatoes and potatoes caused by 2,4-D, which may have no affect on any other vegetables in the garden.

Incorrect Light Conditions

The sunlight available in an older garden can change dramatically over many years - the sunny flowerbed planted with roses, yucca, and herbs can become a shade garden when the Mayday tree grows up.

Incorrect light conditions, also related to incorrect air and soil temperature, can have a devastating effect on plants. Those that grow best in hot sun usually need well-drained soil and warm temperatures as well as bright, direct sunlight. The sunlight available in an older garden can change dramatically over many years - the sunny flowerbed planted with roses, yucca, and herbs can become a shade garden when the Mayday tree grows up. Sometimes we don't notice the gradual change. Shade loving plants usually prefer damp, cool soil, which is one of the reasons why ground cover plants don't do well under spruce trees. It is shady, yes, but also very dry, because water runs off the branches like an umbrella and the soil can be bone dry even after a good rain. One way to judge what light and temperature a plant needs is to find out where that plant's native growing conditions are. Plants that don't get enough sun will stretch toward the light; have small leaves and long thin stems. If they are too cool they will fail to grow. In a climate where warmth after cold stimulates plant growth, warm soil is important. Plants in too much sun will wilt and leaves can have brown edges. If a plant loses more moisture from leaves through evaporation than the roots can absorb, it wilts and dies. The temptation is to add more water, but that can create an ideal environment for root rot, and still not get leaves.

Nutrition and Soil pH

Nutrition and soil pH - Plants need nutrients for growth, and the pH level of the soil in which they are growing has some bearing on which nutrients they can utilize. Generally, plants that are native here are well able to tolerate our alkali water and soil. Some even prefer it. It's when we attempt to grow plants native to quite different climates and soils that we have challenges. Saskatoon's are native here, blueberries aren't! Potentillas like it here, rhododendrons don't! A look at lists of plants available for planting and growing here will show that most are cultivars of natives. Most of our hardy perennials have very familiar wildflower cousins, and a large percentage of our landscape trees have native counterparts. Assuming that they are planted in growing conditions that mimic their natural habitat, they should do well. An example is Mountain ash, which grow in the mountains, in rocky, quick-draining, acidic soil with very little nutrition. Plant it in rich, organic soil, fertilize it well, and give it lots of water and it will die! In their native growing conditions, much of their nutrition comes from the decomposition of plant and animal material that returns to the soil nutrients that were the organism while it was alive. This worked fine for millions of years. Now we rarely allow dead stuff in our yards, even if it was decomposed before we saw it - everything we don't want goes to somewhere we can't see. It isn't practical to allow dead birds to decompose on our front lawns, but we can, and must, compost as much as possible. It very well could be necessary to add manufactured fertilizers to enhance the nutrients available to plants, especially since we want them to grow at their optimum rate in our short growing season, but these fertilizers are much more easily utilized by the plants when added to soil that contains added organic matter, fungi, bacteria and enzymes which help break them down into a form that the plants can use.

Plants need fertilizer when they are growing, not to make them grow, so a fertilizing program begins in the spring, as soil warms and new growth begins. It is of no benefit to fertilize a lawn to make it green until the soil warms up - look at roadways where grass on the north side of the road facing the south is green sooner than grass on the south side facing north! Plants absorb most water and dissolved nutrients at the ends of their roots, which is new growth, so fertilizer is added around the drip line, not directly around the trunk of a large tree. There are many appropriate fertilizers for flowers, vegetables, evergreens, etc. - it is not necessary to have a different one for each type of plant, but helpful to know what each type of plant needs for best results, depending on what we want from that plant. We plant carrots so we can eat the roots, so a fertilizer containing more phosphorous would be appropriate for them, and also little transplants that need roots in a hurry, to be able to withstand the strange conditions they are going to be subjected to. Lawns need a high nitrogen fertilizer to grow green leaves quickly and steadily, because we keep cutting off the ones that do grow!

Nutrient deficiencies can exist because we fail to provide the plants with enough fertilizer, use one that doesn't contain specific nutrients that a particular plant needs, or the nutrients can't be absorbed or utilized by the plant. Nutrient deficiencies are not the only reason why plant growth doesn't meet our expectations, however. Often, we assume that we have neglected to meet a plant's needs, when growing conditions that are not being met are beyond our control. If soil is too cold and wet for long periods in the spring, root vegetables do not do well, and extra fertilizer will not help. Seedlings growing in windows in the late winter grow long and spindly, with small leaves and eventually keel over. This isn't lack of fertilizer, it is lack of light - we cannot make the day longer, or the sunshine in a snowstorm! African violets don't bloom as well in the winter because they need a twelve-hour day to do it. They come from an area of Africa near the equator. A different fertilizer won't help, but many people believe it will. Try to convince them that the third fertilizer they used, in April, wasn't the answer to new blooms, the longer day was!

While nutrient deficiency isn't common in annuals, or plants only in our gardens a year or two, it can be in trees, shrubs, lawns and perennials over a period of many years. Generally, a suitable fertilizer used at regular and appropriate intervals, will meet the needs of most plants here. A water-soluble fertilizer containing micronutrients, particularly iron, such as 15-30-15, will provide for them; a single-nutrient fertilizer, such as 33-0-0 won't. Rarely is a lack of one micronutrient solely responsible for plant disease, although over-abundance of one can lead to poor absorption of another. For example many years ago, potassium nitrate was a commonly used fertilizer. Soil potassium levels became so high that plants could not absorb magnesium, so magnesium sulphate, Epsom salts, were used to increase magnesium availability. Now, we do not use potassium nitrate fertilizers, but people hear that Epsom salts are helpful. Our soil can be high in magnesium, so adding more can increase the EC, to the point of dehydrating plants. Sometimes, a lack or overabundance of a nutrient can be determined by looking at the plant. Root vegetables with exceptionally large, green tops and small roots may be growing in soil too high in nitrogen and too low in phosphorous. This is typical when lawns, heavily fertilized with nitrogen, are dug up to make a vegetable garden. Soil tests can be helpful but must be read with the knowledge of what ideal levels are. Many are programmed to indicate levels that are too low, but not those that are too high. Remember - if a little fertilizer is good, a lot isn't better!

The pH level of Calgary soil is a direct result of our high alkali water, coming from those limestone mountains. It is difficult for some plants, particularly those that are native to areas with acidic soil, to absorb iron from alkali soil and they develop iron chlorosis. Organic matter, such as your own compost, added to the soil will help modify this somewhat, and also, has added fibre as well as bacteria, etc. Peat moss is slightly acidic, but the very fine peat moss available now is difficult to get wet when it dries, and makes soil powdery. Gypsum (calcium sulphate), added to heavy clay to help break it up, has a neutral pH, and is helpful for that purpose. Do not, of course, add lime! Changing the pH is difficult and slow, but sulphur will help reduce the pH so that the plant can absorb iron available in the soil or added to it. If a particular plant that develops iron chlorosis quickly in our soil, such as a rhododendron, simply must be here, it is easiest to collect rain water, or add an acid, such as citric acid, to the water every time you water, rather than using water from our river.

Physical Damage

Physical damage includes that done by birds, animals, machinery, hail, ice or heavy snow, wind, lightning and, of course, insects.

Physical damage includes that done by birds, animals, machinery, hail, ice or heavy snow, wind, lightning and, of course, insects. Animals can eat tender growth any time of year, but damage is more common when they are lacking their normal food like in winter when there is heavy snowfall, or during a very hot, dry summer when their food is dying from drought and yours is being watered and so succulent. Sapsuckers make square holes in a square pattern in tree bark, looking for insects and enjoying the sap. Leaf cutter bees remove perfectly circular holes from roses. Deer, rabbits, porcupines, ground squirrels, pocket gophers and squirrels are common in this area. Animals such as cats can also scratch bark, disrupting patterns of water movement up a tree, and leaving open wounds that can become infected with bacteria or fungus. There are various ways to deter animals; chemicals that have specific odours repellent to certain animals and those that taste bad, avoiding plants that are known to be susceptible to animal damage in the area, fences, prickly ground cover such as raspberry canes or juniper branches in the fall over sunny beds where cats go in the winter, gopher traps, gas sticks for gopher tunnels, (remembering poisoning is illegal in the city) soap chunks in nylon stocking toes or onion bags hung at deer nose height in evergreen trees, soap chips scattered in beds where bulbs are emerging in the spring. None are guaranteed.

Machinery, most often lawnmowers or edgers, also damages bark, and cultivating around the roots of trees can cause root damage and create an entry for root rot fungus. Adding more than a small amount of soil over the roots of a tree can reduce the amount of oxygen available to the tree roots, and also mean that the bark at the base of the tree is now underground, causing possible fungal problems from soil moisture. This also happens when a tree is planted too deeply. Hail physically damages or destroys a tree's leaves, which means less ability to photosynthesize. Because leaf reduction, from physical damage or insects, also reduces root growth, it can reduce the production of food storage for winter and result in less winter hardiness. Heavy snow and ice can weigh down branches and cause breakage. Proper pruning to avoid long, thin branches that would be easily weighed down, can control this. Wrapping with open netting, not burlap, or using rope to keep plants in a compact shape to avoid snow shelves. Lightning can cause a tree to explode or cause only slight damage. It can also cause root damage where the current entered the ground.

Incorrect Watering

Probably the single most important component of soil is oxygen!

Over watering and poor drainage are the most common causes of poor plant health in our area, as we have heavy clay soil, which does not allow good drainage. If there is no oxygen in the soil, in the form of air between soil particles, roots rot takes place and can no longer supply the plant with moisture, so plants wilt and die. Probably the single most important component of soil is oxygen! It is important to assess the area for soil moisture and drainage before planting. Some trees, such as birch and willow, need a great deal of soil moisture; some, such as spruce and mountain ash, cannot tolerate it and will die in constantly wet soil.

To ensure proper watering, water must penetrate to the depth of the root ball. It's unfortunately common to have underground sprinklers which go on every day for ten minutes, whether necessary or not, which usually does not give the plant's roots sufficient water, but is assumed to be consistent watering. Depth of water necessary depends on the depth of the root system - a lawn does not need watering as deeply as a mature birch tree. It will need it more often, but a good, deep watering only when needed is much better than frequent sprinkling.

How well plants recover from drought depends on the root system. A plant with a thick, fleshy root can often recover because the root can shrivel somewhat, then absorb water when it is available. A plant with a very fine, hair-like root often reacts very poorly to drought. The fine roots shrivel quickly, and cannot recover - they die. Once a plant's roots no longer absorb water, the top of the plant cannot survive.

Temperature Extremes

Our chinooks can cause a plant that is dormant and tolerating cold well to become warm enough to break dormancy in the winter.

Temperature extremes are common in Calgary. Usually consistent temperatures at the appropriate times in the season are not stressful if the plant is hardy to the area and in an appropriate place (shade loving plants will have even more trouble with high temperatures if in too much sun). Problems arise when sudden temperature changes occur that alter a plant's normal growing cycle. Our chinooks can cause a plant that is dormant and tolerating cold well to become warm enough to break dormancy in the winter. When it gets cold again, the plant cannot go back to being dormant again and is now too tender to survive temperatures that previously caused no harm.

Plants normally become dormant in the fall in relation to day length. A plant becomes dormant when the day is shorter than the night, if so it can handle a good frost. Unusually long, warm fall weather can trick a plant that is 'programmed' to become dormant in plenty of time to be ready for winter into delayed dormancy and frost damage.

Late spring frosts that would do no harm when a plant is dormant will kill emerging buds and leaves. Often this means no fruit on apples, Nanking cherries, Mountain Ash and Lilac that form flower buds the fall before. Plants that flower on new wood (e.g. most roses, pink-flowering spireas, potentilla) will not be affected as long as there is healthy new growth. Young leaves that are killed are usually replaced with a second set a few weeks later, but on an occasion when the second leaves of Mayday trees are also killed by frost, the third set will be very small. In this case it is possible that the tree will not survive the following winter. The tree used so much energy producing three sets of leaves that they could not survive a relatively mild winter.

Alternate freezing and thawing, often on the sunny side of trees with soft bark, such as Mountain Ash, Amur chokecherry and Mayday, cause frost cracks. The sun alternately warms the bark during the day, then it becomes cold at night, and the bark eventually cracks. This isn't necessarily harmful at the time, but bacteria can easily enter the tree at this point and cause cankers. Wounds should be left open - exposed, dry wood rarely becomes infected. Covering the area with pruning paint or sealing it with home remedies traps any bacteria and fungi inside, and gives them a perfectly protected place to grow.

Humidity is more important to some trees than others. Cedars grow best where there is high humidity in the winter, so they suffer in our dry winters. Pyramidal junipers grow in the mountains, so are much more tolerant of our climate.

Sun reflecting off snow can cause browning of evergreens on the sunny side. New growth is usually OK. It can be controlled by placing a windscreen of burlap stretched between stakes out 18-24" from the tree on the sunny side before winter sets in. Do not wrap - it creates a greenhouse effect when warm air is trapped inside on sunny days. The next very cold weather causes even more damage.

Infectious Disease:

Fungus spreads by spores, which are single cells that move on wind or water droplets, people, animals, insects, tools and some seeds.


Organism can be microscopic size to large structures such as mushrooms. Fungus is composed of threads which can penetrate the epidermis, or skin, of a plant or enter through natural openings or wounds. Fungal diseases can kill leaves, stems or roots specifically, or enter at one point and spread to other parts of a plant. Fungus spreads by spores, which are single cells that move on wind or water droplets, people, animals, insects, tools and some seeds. Some can live for many years in soil, and some are air borne until they land on a suitable host. They can also live for many years in a plant, causing little damage, or cause great destruction very quickly. Not all fungi are bad - yeast makes bread rise, vinegar is formed when fungi cause fruit to ferment, antibiotics are derived from fungi. Some fungi, called saprophytes, are responsible for the decomposition of dead plant material. An example is mushrooms, which live on dead tree roots underground, and grow on lawns when poplar trees are cut down.

Three conditions necessary for fungal growth:

  • a suitable host
  • a pathogen (disease-causing agent)
  • a suitable environment.

An example would be a slice of damp bread (suitable host) on a warm kitchen counter (suitable environment). It becomes blue and furry much more quickly that a sandwich in a cold refrigerator (drier and colder).

Diseases of Deciduous Trees & Shrubs:

Black Knot:

Black knot is a fungus which causes large black growths on the branches of Schubert cherry, Mayday, pincherry trees and saskatoons. It continues to grow until the branch beyond the gall eventually dies.

Pruning the branch back below the gall is necessary because the gall prevents water from moving beyond it and it will die. In rainy weather black knot will become much more prevelant but causes only minor damage. There is no chemical control.

Dutch Elm Disease

Dutch elm disease is caused by a fungus, spread by the elm bark beetle, which prevents the movement of sap within an elm tree. Leaves wilt and turn yellow on one branch at a time, eventually falling off prematurely. It is a major killer of elm trees in some areas. If you suspect Dutch Elm Disease, contact the DED Hotline in your area.

Dutch elm disease There is no chemical control for Dutch Elm Disease. Watch for symtoms - wilting and yellowing leaves in one branch of an elm tree - and report any suspected trees to the DED Hotline in your area. It is illegal to bring firewood from any other province into Alberta, to prevent the insects that carry it from coming into the province. There is no Dutch Elm disease in Alberta at this time.

Shothole Disease:

shothole disease

Shothole disease is a fungus which causes brown spots in the leaves of Shubert cherry trees immediately after they emerge in the spring. As the weather warms up, the spots fall out and leave small round holes which are often thought to be caused by insects.

There is no chemical control of shothole disease. The holes are a little disfiguring but cause no harm to the tree.


Fireblight is a bacterial disease that infects flowering trees. It is most common in our area on pear, apple, crabapple, and mountain ash. Bacteria enters the flowers, which turn brown prematurely. Leaves on infected shoots turn brown and remain on the tree well into winter. The closer to the tip of the shoot, the darker the leaves are - the ends are almost black (as if scorched by fire) and curled over like a shepherd's crook. Cankers form further down the branch that exude ooze which spreads the bacteria through wind and moisture. It is also spread by insects such as aphids and leafhoppers, so controlling them is also helpful.

Pruning well below the infected parts can eliminate the disease in early stages. Clean tools with one part chlorine bleach to four parts water after each cut. Copper spray can reduce the chance of infection if fireblight is in your area. Badly infected trees will have to be removed. Many trees are mis-diagnosed - get a diagnosis from a person knowledgeable about tree diseases.

Cytospora Canker:

Cytospora Canker

Cytospora canker can affect flowering trees particularly apple, mountain ash, pear and hawthorn. It is very common inside cotoneaster hedges, where there is very little light or air circulation. It causes longitudinal splits in the bark, with the bark peeling back. Leaves emerge in the spring but die when hot weather arrives because water cannot move along the branch beyond the damaged areas.

Cytospora canker cannot be controlled with a chemical. Pruning a branch back below the affected area will allow new growth. Pruning out dead wood in the centre of a cotoneaster hedge will eliminate an ideal place for it to grow. Clean tools with one part chlorine bleach to four parts water after each cut.

Bacterial Canker on Willow:

Bacterial Canker on Willow

Bacterial canker on willow attacks trees that have been stressed by heat, drought, insects, winter damage, rapid temperature changes or previous infection. It causes well-defined, elongated dark brown areas in trunks and branches. New cankers appear in smooth bark as sunken areas that may girdle the stem.

Bacterial canker in willow trees cannot be controlled with a chemical. Prune the branch back well below the affected part. Clean tools with one part chlorine bleach to four parts water after each cut.

Common Stem Canker of Roses:

Common Stem Canker of Roses

Common stem canker of roses is caused by a fungus which enters the stems through wounds in the bark, often from thorns whipping into other stems in storms. This creates black dead areas in the bark. Water cannot move upwards through the canker so the branch dies beyond the canker.

Common stem canker of roses can be controlled by pruning branches off well below the affected part. There is no chemical control.

Bacterial Wetwood:

Bacterial wetwood (slime flux) is a common disease of poplars and elms. The infection usually occurs in crotches of trees or where water accumulates, such as bad pruning or mechanical damage. Diseased wood inside the tree is dark brown, and foul smelling liquid may ooze from cracks or wounds. Foliage wilts in diseased areas.

Improving drainage (e.g. angling wounds downwards and using drains) and cutting out damaged wood can preserve the tree. Clean tools with one part chlorine bleach to four parts water after each cut. There is no chemical control.

Bacterial Blight of Lilac:

Bacterial Blight of Lilac:

Bacterial blight of lilac causes black, shrivelled tips, especially on white-flowering ones. Flowers blacken and immature leaves die quickly. It rarely affects older growth.

Bacterial blight on lilac cannot be controlled with a chemical. Prune back well below the affected part. Clean tools with one part chlorine bleach to four parts water after each cut.

Aspen Blight:

Aspen Blight

Aspen and poplar twig blight is caused by a fungus. Young shoots blacken and wilt, the trees are disfigured and new growth is severely affected, but it does not signficantly affect the health of the tree. Infection on older trees is rare.

Aspen and poplar twig blight cannot be controlled with a chemical. Prune the branches back well below the affected part. New growth will quickly replace affected areas.

Clematis Wilt:

Clematis Wilt

Clematis wilt is caused by a fungus. One branch of a clematis will suddenly turn dark brown and wilt, while others are unaffected. The roots are not damaged.

Clematis wilt can be controlled if the affected branch is cut back to ground level, and a systemic fungicide is applied to the soil at that time and again in two weeks. New growth will occur very quickly.

Black Leaf of Saskatoon:

Common Stem Canker of Roses

Black leaf of Saskatoon is caused by a fungus which covers the undersides of infected leaves with an olive-brown fungal mat. Fruit production is severely reduced. It over-winters on dead, infected leaves.

Infected leaves should be removed and disposed of in the fall. Pruning out infected branches will reduce the spread of the disease, but removing the shrub may be the best answer. There is no chemical control.


Common Stem Canker of Roses

Silverleaf is a fungus which enters the tree or shrub through wounds and infect the sapwood. It spreads rapidly throughout the tree, causing a leaden or silvery sheen on the leaves. Later symptoms are browning of the leaf rib and margins. It is usually confined to several branches a year but each year more are affected, until the tree dies.

There is no chemical control - pruning out infected branches well below the affected parts and protecting trees from physical wounds may help control it. Removing the shrub is possibly be the best answer.

Leaf Spot on Poplar:

Common Stem Canker of Roses

Septoria leaf spot on poplar is a fungal infection that causes numerous black spots on poplar leaves. New, tender leaves are the first infected, and small spots enlarge under wet conditions. Trees may also develop trunk cankers from the same fungus. Sunken black areas may girdle the tree and cause death. This is more likely in young trees with soft bark. Native poplars are more susceptible than newer cultivars.

Septoria leaf spot is a disease of native poplars and doesn't usually affect domestic trees. There is no chemical control. Cankers can be cleaned out to healthy wood - leave them open and exposed to air to heal naturally.


Rust on hawthorn and saskatoon is caused by a fungus which needs two alternate hosts. The alternate host in this case is a juniper, where the disease overwinters. In the summer, yellow spots appear on the underside of leaves, and spots on the top surface have orange centres. Later, fruiting bodies that look like thorns appear on the underside of the leaves. Fruit can also have thready growths on them. It is much more prevalent in rainy weather.

Rust on hawthorn and saskatoon cannot be controlled with a chemical. The only control is to remove all the junipers within a two mile radius in every direction!

Iron Chlorosis

Iron chlorosis is a yellowing of leaves between the veins, with the vein remaining green. It is caused by an inability of the plant to absorb iron from alkali soil. Our water, and therefore our soil, is very alkali in Calgary.

Sulphur added to the soil can make the soil less alkali - read the label carefully before use. When iron is added it can be absorbed by the plants. Compost (your own or purchased) can help somewhat. Do not add lime to soil in our area.

Heat Scorch

Common Stem Canker of Roses

Heat scorch causes dark, irregular blotches or dark edges on leaves of susceptible plants. It is not a disease or insect but a reaction of the plant to extreme heat. Often, the plant's roots cannot absorb as much water as the leaves are losing from evaporation, so leaf tissue dies. It is common in hot weather in newly planted trees, particularly mountain ash, which have inefficient roots and cannot tolerate soggy soil.

Heat scorch cannot be controlled, as we cannot modify the weather. Giving the plant extra water does not help - it depends on whether the roots can absorb it fast enough. Too much water in the soil will do more harm than good. It all depends on the tree's ability to survive.

Diseases of Evergreens

Juniper saskatoon and juniper hawthorn rust

Juniper/saskatoon and juniper/hawthorn rusts are the most common rust in our area. Rust is a fungus which generally needs two host plants growing in the vicinity. Most need a deciduous host and an evergreen host. This fungus overwinters on a juniper, which develops brown, jelly-like telial galls on the branches in the spring. These harden in the summer, and needles beyond the galls die.

Pruning the branches of junipers back below the galls will help somewhat in reducing further rust infection. Prevention of galls on hawthorn is only possible by eliminating all deciduous hosts (juniper or saskatoon) for a radius of two miles! There is no chemical control.

Western Gall Rust

Common Stem Canker of Roses

Western gall rust affects pine trees, causing a swelling in the branch that remains localized and usually does not spread to the main stem. It does not need a second host and can persist for many years without causing serious harm. Trees can become stunted or malformed and infected branches are susceptible to breakage in a storm.

Removing the galls prevents the windborne infection from spreading to nearby trees the next year. There is no chemical control.

Needle Rust

Needle rust on spruce causes powdery orange spores on the current year's needles, which drop prematurely. A wildflower, Labrador tea, is an alternate host, so affected trees are usually close to natural areas.

Spruce needle rust is mostly cosmetic damage and although unsightly should not harm your tree. Affected areas may be pruned off and reducing watering may eliminate problem. There is no chemical control.

Cytospora Canker

Cytospora canker (also called leucostoma canker) is a common fungus on spruce trees, usually those stressed by severe drought, mechanical injury, poor nutrition, or hail injury. It begins on lower branches and moves upward, first causing dying needles as sap flow is inhibited. It causes cankers in the bark of branches that ooze amber colored sap. As it dries, it becomes a white crust.

Cytospora canker cannot be controlled with any chemical. It can deform a spruce tree and eventually destroy it.


Nectria canker is a common secondary infection where other cankers are present. It consists of small orange spots that live on dead wood. It is not a problem in itself, but simply indicates dead wood.

Nectria canker does not harm its host. It lives on dead wood. It is a good indicator for pruning out dead wood, particularly on the inside of cotoneasters.

Armillaria Root Rot

Armillaria root rot causes reduction of growth and reddish discoloration of needles in pine trees. White, thready fungal growth at the base of the tree, and yellowish-brown mushrooms in the soil around the tree, are indicative. Small trees may be killed quickly, but larger ones can live for many years. The disease tends to affect already weakened trees.

Armillaria root rot cannot be controlled with a chemical. It is wise to remove infected trees and not plant pines in the affected area.

Diseases of Houseplants

Cultural Diseases


Edema causes cork-like swellings on leaves of susceptible plants such as schefflera, ivy geraniums, pothos and philodendron. It is caused by high humidity and overwatering - cells absorb more water than can transpire into the humid air.

  • Cultural control - increase light levels, so plants photosynthesize at an increased rate, increase air circulation, allow plant to become as dry as that plant can be before watering, then water well (not every Sunday!), do not allow water to stay in the saucer, check drainage hole (can be plugged by pebbles in bottom of pot).


Guttation is also a watering problem - droplets of sticky moisture appear on uninjured leaves as a result of too much soil moisture and too much humidity in the air. Moisture is not lost as quickly through evaporation in very humid air as is being absorbed by the roots, so it oozes out of plant pores. It is often seen as a sugary-looking crystal at the base of a leaf of a weeping fig and assumed to be an insect.

  • Cultural control - as edema above

Excess Salts:

Excess salts can build up in soil from too much fertilizer over a long period of time, and not watering thoroughly. As the soil becomes dry, salt concentration in soil moisture is increased, and plant tissue that absorbs this moisture is scorched. Dark edges and blotches near edges on long, grassy leaves are symptoms, particularly on dracaenas and spider plants (which are susceptible specifically to fluoride in the water). Many more plants are harmed by over watering than under watering, but drought can cause leaf damage other than the expected wilt.

  • Cultural control - water enough at one time to thoroughly soak the rootball, but do not leave water in the saucer. Increase humidity in the air (not practical in Calgary in the winter - it just freezes on the windows). Do not over-fertilize - if a little is good, a lot is not a lot better!

Poor Flowering:

Poor flowering is caused by factors specific to each plant, and lack of fertilizer is not one of them! Each plant has its rule for flowering - some need a long day and short night, some a short day and long night, some a warm day and cold night, some need a great deal of sun to flower, some just have to become mature enough. There is no fertilizer that will make a plant bloom, although a healthy, well-nourished plant will produce more spectacular flowers than a sickly one. The only way to make a plant bloom is to learn what it needs and supply it - not always easy!

Infectious Diseases

Powdery Mildew

Powdery mildew is a fungus which causes a white, dusty coating on flowers and leaves of susceptible plants such as begonias and African violets. Leaf edges become brown and crisp, and flowers turn brown and die.

  • Cultural control - increase air circulation, water the soil, not the leaves, make sure plant becomes as dry as that plant can be before watering, remove damaged leaves, maintain consistent temperatures (too cold can increase susceptibility). If flowers are affected, remove every flower and bud at the same time. If it is an inexpensive or short-term plant, consider throwing it out - the cure will cost more than the plant is worth.

  • Chemical control - systemic fungicides registered for indoor use, such as Benomyl or Funginex, used as directed


Viruses can cause streaks and strange patterns of discolored and abnormal growth of foliage and flowers. All viruses are not harmful - streaked tulips, variegated foliage of pothos, hoya, etc. are caused by a specific virus. Tobacco mosaic virus is a specific virus which causes yellow mottling on orchids, and cacti. Many viruses are spread by insects, so insect control is especially important if viruses are a concern.


  • Cultural control - if the plant is unthrifty and degenerating, destroy the plant.

Chemical control - There is no cure.

Black Rot Fungus

Black rot fungus causes soft, rotted areas, commonly on new growth, of orchid foliage. It spreads down into the rhizome, then upward into older leaves. Advancing edges are yellow as tissue dies.

  • Cultural control - cut off infected areas, clean tools. Do not overwater - soft, succulent growth is more susceptible to infection.

  • Chemical control - systemic fungicide such as Benomyl, used as directed.

Diseases of Flowers

Black Spot

Black spot is a fungus that causes circular black patches surrounded by yellow halos, up to 1/2" across, on rose leaves. Leaves eventually turn yellow and die. Some roses, such as Persian Yellow and Austrian Copper, are more susceptible; most of the Explorer series are resistant.

  • Cultural control - fungus over winters in dead leaves on the soil, so removing debris in the fall is important. Water in the morning, and water soil, not leaves if possible.
  • Chemical control - systemic fungicides such as Funginex (liquid) and Benomyl (wettable powder), and the contact fungicide Folpet (wettable powder) will control it if used as soon as disease is seen. It is important to alternate fungicides, as fungus builds up a tolerance to specific chemicals.




Botrytis fungus causes young shoots of peonies to wilt and fall over, and a black rot develops at the base of the plant. Gray mold forms on stems just above soil level, and flower buds wither, turn dark and die. Open flowers can also be attacked - they turn brown and the leaves become covered in gray mold, with irregular brown patches.

  • Cultural control - remove all leaves from plant to just below ground level in the fall, increase air circulation and available sunlight by pruning and/or thinning plants.
  • Chemical control - apply a systemic fungicide such as Benomyl as soon as shoot growth appears in the spring to plants that were affected the year before.

Clematis Wilt

Clematis wilt is a fungus which causes wilting and die-back of shoots in the large-flowering varieties of clematis. It affects plants just above ground level, and young leaves are affected first - eventually the whole stem turns brown and wilts. Stems can't recover, but it rarely affects the roots.

  • Cultural control - none
  • Chemical control - an infected stem can be cut back to soil level, or just below, treated with a systemic fungicide such as Benomyl two or three times at 10 day intervals as soon as the problem is noticed, and a new stem will grow.

Bacterial Leaf Spot

Bacterial leaf spot and stem rot affects geraniums, causing two entirely different symptoms. Small, water-soaked spots develop on the underside of leaves, the spots spread, and turn dark brown and dry. Bacteria enters the stem, causing stem rot where leaves join the stem, and leaves wilt and fall off. Another symptom is wilting of leaf margins, which eventually spreads to form large, dead areas enclosed by leaf veins. Stems become black, dry and shriveled, and yellow bacterial ooze is visible when they are cut. Infected cuttings fail to root, but develop a rot which moves up the stem. Bacterial stem rot moves more slowly than damping off, which can kill a plant very quickly.

  • Cultural control - spread on tools, soil, physical contact, insects, so sanitation is important. Plant disease-free stock, root in individual pots so that infected plants can be easily eliminated.
  • Chemical control

Lawn Diseases



Mushrooms in lawns grow on decomposing plant material. The most common cause is roots of trees previously removed. The roots take many years to decompose and mushrooms will grow along the root when the soil is wet for long periods of time.

It is important to water your lawn thoroughly and then let it become dry well below the surface before watering again. A good rule of is to water for an hour every 7-10 days. Mushrooms will be much more of a problem in wet, rainy weather. There is no chemical control.

Fairy Ring

Fairy ring is a fungal infection of lawns that causes circles of darker green grass that enlarge over time. Mushrooms often appear in the ring, and a mat of white threads develops below the surface of the soil that water cannot penetrate.

There is no chemical control. Aerating the area will allow water to penetrate the fungal mat and water to reach the roots of the lawn. Use a pitchfork to aerate the area, starting at the outside edge and working inwards, to prevent spreading the infection from soil on tools. Sterilize tools with one part bleach to ten parts water after using. Water well and allow to become dry. Fertilize with a slow release fertilizer regularly. Fairy rings will disappear as they reach a sidewalk, driveway, flower bed, etc.

Snow Mold

Snow mold is a fungal infection that is most prevalent after winters with heavy snowfall. The fungus lives under snow that stays the longest - on the north side of buildings, where snow has been packed because of traffic or has been thrown when shovelling. It causes grey, fuzzy mold on the grass blades, which often turn yellow or brown.

Packed snow that remains longer than usual should broken up to allow air to reach the grass. Shovel snow to a place where it will disappear sooner, if possible. Use a broom to break up the patches of mold. The grass roots are rarely affected and new grass will usually grow in again.

Slime Mold

Snow mold is a fungal infection that is most prevalent after winters with heavy snowfall. The fungus lives under snow that stays the longest - on the north side of buildings, where snow has been packed because of traffic or has been thrown when shovelling. It causes grey, fuzzy mold on the grass blades, which often turn yellow or brown.

Slime mold is a fungus that causes patches of sticky, jelly-like masses on lawns. It is more prevalent in wet weather, when the soil-borne fungus can reproduce.

Slime mold is unsightly but causes no harm. When it dries it can be swept off with a broom. No chemical control is necessary.

Diseases of Vegetables & Small Fruit


Botrytis Gray Mold

Botrytis gray mold is a fungus that can start first in flowers, which turn brown and dry. It moves to stalks and leaves, then fruit, causing gray 'fuzz' and rotting fruit. Botrytis is common on strawberries, raspberries, Saskatoon's, and chokecherries. It can also affect vegetables such as broccoli, cauliflower, cabbage, and lettuce. It causes brown, water-soaked areas, which become covered with a gray mold. Spores become air-borne and also spread when handled.

  • Cultural Control - Rotate vegetables, planting no vegetables of the crucifer family two years in a row. Make sure soil drains well, don't over water, and space plants well apart. Water plants in the morning so foliage is dry before nightfall. Our nights are cool, causing moisture to condense on cool foliage, which encourages infection. Greenhouses need good ventilation to reduce humidity and increase air circulation.
  • Chemical Control - none registered for domestic use.

Powdery Mildew

Powdery mildew causes a white, floury coating of fungal spores on strawberries, raspberries, currants, gooseberries, peas and beans. It affects leaves, young shoots, flowers and fruit, and is most problem in humid, cool weather, and poor light, when it multiplies in moisture on leaves. It is spread by wind in warm dry weather. Leaves are distorted and curl upwards, and severely affected leaves may die. Powdery mildew over winters in leaf debris on soil

  • Cultural Control - Prune to encourage air circulation and increase light levels; remove leaf litter, plant disease-resistant cultivars. Baking soda sprayed repeatedly can give some measure of control, but can also increase pH levels, causing iron chlorosis, which is already a problem on raspberries and strawberries in Calgary.
  • Chemical Control – Use garden sulfur, copper spray, Folpet (common in insecticide/fungicide sprays for garden use), or Benomyl.

Wilt Fungus

Verticillium wilt fungus affects potatoes, tomatoes, raspberries and strawberries. It enters the plants through roots at flowering time. Lower leaves wilt first and as the fungus moves up the plant; leaves turn yellow, then brown. Only some stems are affected. Verticillium wilt comes from infected seed potatoes, or from infected soil, including soil in which purchased transplants are grown. It can last from two to seven years in soil.

  • Cultural Control - Rogue out affected plants, rotate crops.

Fusarium Wilt Fungus

Fusarium wilt fungus enters potato plant roots from cool, wet soil and causes the lower part of the stem to rot. When warm weather arrives, the plant wilts because water cannot reach top of plant through infected stem, which may be completely girdled. Leaves roll upwards, often turn purple and form tight rosettes. Spores over winter in debris and in infected potatoes saved to be used for seed.

  • Cultural Control - Plant resistant varieties; (marked V or F after name on tomato seed packages) Rotate crops, not using the same plants in the same soil for 3-4 years. Rogue out infected plants immediately. Improve soil drainage.
  • Chemical Control - none

Damping Off

Damping off is caused by several specific fungi which live in the soil and destroy seedlings just before they emerge from the soil, or causes them to collapse as the leaves begin to grow Stems often have a dark, thin, wire-like area near soil level which is narrowed, and prevents moisture from reaching tops of plants.

Cultural Control - Use sterile soil less mixes for starting seeds, clean containers, thin to prevent overcrowding, do not keep too wet or too cool, rogue out infected plants immediately.

Chemical Control - 'No Damp' is registered for this use.

Tobacco Mosaic Virus


Tobacco mosaic virus is one of several virus infections causing mottling of leaves and distorted growth of tomatoes; tools and insects mostly spread it.

  • Cultural Control - spread by sucking insects, mostly aphids, so insect control is important.. Rogue out all infected plants.

Red Leaf of Rhubarb


Red leaf of rhubarb refers to two conditions - one bacterial, one viral.

The bacteria causes rotting of the crown until a cavity forms, and dull red leaves. The plant quickly dies.

The virus causes stunted growth and red leaves. The plant dies gradually over several seasons.

There is no cure for either disease - remove plants and plant new ones in a different location.

Blossom End Rot


Blossom end rot of tomato and pepper is a cultural problem. Water-soaked areas on the end of the fruit turn dark brown and become dry and leathery. Irregular watering prevents calcium uptake, even though there is plenty in our water and soil, so care taken to maintain sufficient moisture in the soil is essential. It does no good to add extra calcium. Some varieties of tomato are susceptible (Roma, Beefsteak), some are resistant (Early Girl, Champion, Husky Cherry Red).

Potato Greening

Potato greening is caused by exposure of the tubers to light - glycoalkaloids occur naturally in the tubers in low concentrations, but increase when exposed to light, and make them taste bitter. Peeling off the green layer makes them edible.

Bacterial Speck

Bacterial speck of tomato causes small, dark brown spots on the fruit, which do not extend deeper into the fruit. It is spread by rain and watering.

  • Cultural control - grow in sterile potting mix, water soil, not foliage, if possible, and early in the day. Rogue out infected plants, clean tools! Do not plant tomatoes in the same soil as infected plants have grown for at least two years.
  • Chemical control - none


Black leaf of Saskatoon is caused by a fungus related to the one causing black knot on Shubert cherries. The underside of the leaves are covered with a black fungal mat. Infected shoots produce witches' brooms, which enlarge each year. Dead leaves, containing the fungus, remain on the branches over the winter, which infects new growth in the spring. Berry production in affected.

  • Cultural control - remove infected branches, clean tools!
  • Chemical control - none

Vegetable Diseases


Powdery Mildew

Powdery mildew looks like a dusting of icing sugar on the upper surface of leaves of susceptible annuals, perennials and roses. It is common in wet, cool weather.

Powdery mildew can be controlled somewhat by keeping plants well spaced apart, watering in the morning, and avoiding getting water on the leaves. Systemic fungicides can also help, but are expensive to use routinely.

Black Spot

Black spot is a fungal infection that causes black spots on the leaves of roses. It is spread in wet weather. Leaves eventually turn yellow and fall off. Many new rose varieties are resistant to black spot but some older ones are very susceptible.

Black spot can be controlled with systemic fungicides. It is also important to plant roses in the sun, water early in the day and avoid getting the foliage wet if possible.

Hollyhock Rust

Hollyhock rust causes numerous black spots on the undersides of hollyhock leaves, causing large yellow spots with red centres on the upper sides.

Hollyhock rust can be controlled somewhat by removing infected leaves as soon as they appear and cutting the plant down to the ground as soon as flowering is finished. It is much more prevalent in wet weather.

Botrytis is a fungus that affects peonies particularly, causing a black rot at the base of the stems, which wilt and fall over. Flower buds shrivel and young leaves have black spots. Be sure peonies are planted just barely below the soil surface.

Botrytis All infected parts should be cut away and destroyed. Systemic fungicides applied to the soil as soon as damage is seen and again in two weeks will keep it under control.

Aster Yellows

Aster yellows is caused by a virus-like organism that affects many plants such as delphiniums, asters, and carrots. It causes stunting, leaf yellowing and a witches' broom appearance, and softening of the roots.

Aster yellows is spread by leafhoppers, so keeping insects under control can prevent it from spreading. There are no chemical controls for viruses.

Mosaic Virus

Mosaic virus causes yellow lines and swirls in leaves of roses, tomatoes, and peppers. It weakens the plants and makes them less winter tolerant, but doesn't kill them.

There is no controlfor mosaic virus. It doesn't spread to other plants so they do not necessarily have to be removed immediately but they will not be good flower or fruit producers.

Blossom End Rot

Blossom end rot is a blackening on the end of tomatoes, caused by the plants' inability to absorb calcium when the soil is too dry.

We have ample calcium in our soil, so keeping the soil moist (not soggy) helps prevent the problem. Some varieties are very susceptible, while others are quite resistant, so choose varieties carefully if it has been a problem.



Physiological leaf roll of tomatoes is a curling and crisping of tomato leaves caused by inconsistent temperatures and watering, not a disease or insect.

Our nights are cool, so planting tomatoes in sheltered places and watering in the morning can help not in the evening. Watering in the evening is sometimes essential, but does mean that moisture on the leaves during our very cool nights stresses the plants. If you have a rain barrel with water warmed by the sun in it, so much the better!

Bacterial Spot of Tomatoes


Bacterial spot of tomatoes causes irregular black spots on the skin that do not penetrate very deeply. Stems and fruit may also be affected. It spreads during heavy rain. It is seed borne.

Remove affected plants immediately and do not plant tomatoes or peppers in the soil for at least two years.

Catfacing of Tomatoes


Catfacing of tomatoes creates lobes and scaly areas instead of the regular tomato shape. It is caused by fluctuating temperatures, and does not harm the tomatoes, although they are unsightly.

Catfacing of tomatoes does no harm to the plants and cannot be controlled. We can't do much about inconsistent weather! Planting in protected spot lessen the chances of catfacing.

Red Leaf of Rhubarb


Red leaf of rhubarb is a bacterial disease which causes a crown rot in the plants. As the root dies, spindly side shoots grow but die quickly and leaves turn a dull red.

Remove affected plant and do not plant rhubarb in that area. It can be spread by foliage feeding insects, particularly aphids, so control of insects can help prevent its spread.

Scab on Potatoes


Scab on potatoes is caused by a bacteria which is carried by manure. The skin appears rough and 'scabby'. The potatoes are safe to eat, although unsightly.