Fungi & Diseases

A diseased plant could be defined as any plant that is in an unhealthy condition. Infectious or parasitic diseases are caused by micro-organisms that live off the plant (host) and cause damage (injury) to the plant.  Infections are caused by many different agents - bacteria, fungi, nematodes and viruses.

Bacteria have the ability to multiply rapidly and enter through wounds or natural openings. There are no controls so discarding infected plants is recommended. Insects play an important role in spreading bacteria.  
Viruses are tricky to identify because symptoms can be very common.  Plants will exhibit reduced or distorted growth, dwarfing, stunting, leaf variegation, ring spots, mosaics, tumors etc.  There are no chemical controls once a plant is infected with a virus.  Viruses are spread through direct contact of diseased plants with healthy ones.  Tasks such as dividing, vegetative propagation, gardening tools, hands, gloves or even insect feeding can spread viruses.  It is important to purchase seeds and tubers that are certified virus free and use good cultural / sanitation practices.
Fungi are parasitic multi-cellular organisms that can live on live or dead tissue. They reproduce by spores and can enter the plant through cracks, fresh cuts, and blossoms.  Fungi spread over great distances by wind, water, soil, air, birds, insects, man, and tools.

General Symptoms of Diseases:

  • rotten areas on leaves and/or fruit
  • slime and/or water spots on plants
  • wilting of plants
  • premature defoliation
  • twisted, curled and abnormal growth
  • rotten, decaying plant parts
  • plant dies for no apparent reason
  • leaves have dead and brown areas
  • mottling, dots, blotches and irregular brown areas on leaves
  • yellowing of leaves (chlorosis) on all leaves, on new growth only, on leaf edges and between the leaf veins

The environment can play a large roll in disease development of plants.  A plant that is thriving is less likely to be bothered by disease.  Where as a plant that is struggling because of environmental stresses is more likely to become infected.  It is important to select the right plant for the right spot to prevent environmental stresses which can lead to problems with disease.

Measures to prevent diseases from spreading:
  • remove all infected plants and plant parts
  • clean up infected leaves from the soil
  • clean up plant debris in the spring and the fall
  • rotate crops, plant resistant varieties, remove weeds and control insect pests
  • wash / disinfect all tools, gloves and hands after handling an infected plant
  • don’t compost infected plants
  • amend soil with organic matter annually to help create better growing conditions for plants 


Blossom Drop

Blossoms fall off before they get pollinated.  This environmental disorder is caused by hot, dry weather and dry soil.  It’s common when weather changes quickly from hot / dry to cool / wet.  To control, mulch ground, water well and avoid over fertilizing. 

Black Knot

Attacks plants in the Prunus genus.  This is a fungal disease that causes swelling (knot) on branches.  The swellings turn olive green then black harden and encircle the branch.  The knots will continue to expand and eventually kill the branch or in some cases the main trunk.  The best control is to prune out knots 6 inches below infection into healthy wood.  Make sure you disinfect tools between cuts. 

Black spot

A fungus that commonly infects roses.  Round black spots develop on leaves, stems and stalks.  Spots grow slowly and develop a yellow halo around the spot.  Leaves will yellow and can prematurely drop.  Black spot favours warm wet weather.  Try to keep water off the surface of leaves to help prevent the onset of this disease.

Botrytis (grey mold)

A fungal disease that causes young stalks to wilt suddenly and fall over; buds turn black and dry up and leaf spots can develop.  The infected areas of the plant become covered with a grey-brown felty layer.  Avoid overhead watering and overcrowding, cut back and clean up plants in the fall, destroy diseased plant parts immediately and make sure to follow proper sanitary practices - disinfect tools, wash hands

Browning of Evergreens

some needle shed in the fall is natural but excessive defoliation may result from different things:

  • Frost injury - new growth droops
  • Herbicide injury - twisting, distortion of stems, discoloration of needles
  • Drought - brown from the top down and from the outside in
  • Cold or winter injury - sudden reddish discoloration of branches
  • Salt injury / dog damage - localized discoloration near bottom of tree
  • Iron deficiency - yellowing of needles
  • Environmental - usually appears on one side of the tree; needles have a purplish colour

can be caused by fungus, bacteria or environmental stresses.  Cankers are dead sections on bark or branches.  In order for a canker to form a wound must be present.  Once the tree has been wounded bacteria and fungus quickly colonize tissue and can lead to girdling of branches. Remove cankered branches back at least 6 inches below infection into healthy wood, keep trees healthy and follow proper pruning practices - remove dead, drying and disease branches.

Cedar / Apple Rust

A fungal disease that requires two hosts to complete its life cycle.  Junipers / cedars are the primary host and the symptoms are reddish brown galls on the branches that enlarge to form a gelatinous, yellow-orange mass in wet weather.  The secondary hosts of this disease are Hawthorns, crabapples, Saskatoon’s and other rosaceous plants. Bright orange-yellow leaf spots develop on the upper surface of the leaf and a horned structure develops on the lower surface of the leaf. Two hosts must be planted in close proximity to each other in order for this disease to complete its life cycle. 

Dutch elm disease

a fungus that over winters in infected and recently killed elm trees, fire wood and old stumps. It is carried and spread by a bark beetle.  This disease causes yellowing and wilting of leaves usually in the upper crown and isolated to one branch  this is called flagging. In subsequent seasons you will see the same symptoms occur one branch at a time progressively to larger branches and eventually the entire tree.  Not present in Calgary yet!


a bacterial disease that affects plants in the Rosaceae family.  


  • Infected blossom suddenly wilt and turn light to dark brown
  • Leaves on infected branches become brown, shriveled and appear to be scorched by fire. Affected leaves remain on the branches well after normal leaf drop  
  • Blackened new growth is often curled at the tips causing a shepherds crook
  • Cankers begin to develop and are discoloured, slightly sunken and tend to crack at the edges.  Cankers may eventually encircle the branch causing death.
  • In spring during warm, moist weather cankers may exude an amber coloured slime.
  • Young infected fruit becomes oily in appearance and exudes clear milky or amber coloured ooze.  Fruit shrivels, becomes dark brown and remains attached to the tree.
  • removal of diseased parts promptly is imperative.

bacteria that causes swollen knots to develop along stems, on the roots or on the crown.  Galls are white turning brown and rough / warty as they age.  Infected plants may or may not show a reduction in vigour.  Bacteria enter the plant through wounds in the bark, pruning cuts, cultivation, and insect damage. Bacteria can survive for long periods in fertile soil. 

Leaf Scorch / Yellowing

Leaves turn yellow, browning between veins and along edges.  Leaves appear wilted; pale green sometimes entire leaf may turn brown and dry out.  Pre-mature leaf drop can also occur. Usually caused by environmental conditions such as transplant shock, drought, nutrient deficiency, heavy wet soils, herbicide injury or salt toxicity.

Powdery Mildew

a fungal infection that causes a greyish-white powder (mold) that covers the leaves stems flowers, buds and can cause distortion of fruit. Mildew favours shady areas, humid conditions with widely fluctuating temperatures and in crowded plantings where air circulation is poor.


a fungus that appears as orange or reddish spots that later form raised black spots (pustules) on the underside of the leaves. Leaves turn yellow and drop prematurely. Avoid overhead watering and ensure plants are not overcrowded and have enough air circulation.


insufficient water is the most common cause.  There are however certain disease organisms that interrupt the water flow in plants and cause wilting.  Fungal wilts can be identified by discolored brown areas on stems and twigs.  Bacterial wilts can be identified by a sticky substance that is present in the stems. Good cultural practices are the best form of defense against wilts.  


Fungicides are products used to destroy or inhibit the growth of fungi.  Here are the most common fungicides:

Bordeaux mix

this mixture of copper and hydrated lime has low toxicity level. Effective against scab, anthracnose, brown rot, black rot, black knot, cherry leaf spot, leaf spot, fire blight, peach leaf curl and other fungal diseases.


organic / low toxicity is quite effective against black knot, brown rot, anthracnose, cherry lead spot, leaf spot, scab, powdery mildew, rust, black rot, cedar apple rust, and other fungal diseases.  Don’t use on apricots, raspberries, members of the melon family or D’Anjou pears.

Lime Sulphur

usually used with dormant oil in late winter to kill over wintering diseases when the plants are dormant.  Helps control black knot, apple scab, powdery mildew, brown rot and other fungal diseases.

Always read and follow label directions when applying any pest control product.