The Online Gardener's Handbook 2010
Chapter 1: Causes of Plant Injury
Disease Causing Plant Injury

Table of Contents

  1. Introduction
  2. Fungal and Bacterial Damage
  3. Fungi
  4. Bacteria
  5. Viruses
  6. Nematodes
  7. Learn More


Parasitic diseases are caused by fungi, bacteria, viruses and nematodes. These organisms attack plants, living at the expense of their hosts.

Fungal and Bacterial Damage

Fungal and bacterial diseases can be broadly categorised by the type of damage they cause.

Foliage diseases attack leaves. Leaves may turn yellow, drop off or become deformed due to vascular wilt or secondary factors such as air pollution or drought. Common diseases include Botrytis, powdery mildew, downy mildew, leaf spots, black spot, rusts, anthracnose and needlecast.

Crown and root rots often attack seedlings. The lower stem of the plant appears water soaked. Eventually, the plant rots off at the soil line. Soil fungi such as Pythium and Phytopthora attack herbaceous and woody plants. Rhizoctonia and Fusarium cause brown to black lesions on lower stems and roots.

Canker diseases are fungal, and commonly attack plants weakened by drought, root disturbance or poor root environment. Cankers kill bark and underlying tissue in patches on branches and stems. The bark splits as it dries and is easily removed, exposing the wood below. Fruiting bodies from the fungus are often seen on the dead bark areas.

Vascular wilts occur when fungi or bacteria enter a plant's vascular system and reduce water movement to the leaves. Leaves often recover in the early stages during cool, moist periods. Eventually, however, leaves and branches wilt and die; the entire plant dies soon after. Serious wilt diseases include verticillium wilt and Dutch elm disease.

Many fungal and bacterial diseases have similar names on different crops, even though they are caused by different pathogen species. For example the fungus causing powdery mildew on grapes is be different from the fungus causing powdery mildew on cucumbers. This can be important in some cases, because the course of the disease and the management practices taken can vary with the pathogen causing the disease. For example, downy mildew on cucumber is often much more serious than downy mildew on broccoli.


Fungi are a group of living organisms that have some similarities to plants, but unlike plants they do not have chlorophyll and so must take their nourishment from living or dead organisms to survive and reproduce. On infected plants, they appear as delicate strands (filamentous mycelium) that grow on or in plant tissue and may not be visible to the naked eye. Most reproduce by spores produced by the millions and spread by air currents, water, soil and insects. Under suitable environmental conditions, and if a suitable host is near, the spores will germinate and infect the plant. They enter through wound sites and natural plant openings or by forcing their way through the outer layer (epidermis) of plant tissue. Many fungi also produce resting spores that can withstand long periods of unsuitable conditions.

Fungi that attack foliage, flowers and stems are usually spread by air currents or splashing water. Fungi that attack plant roots, crowns or the sap stream of plants (wilts) survive indefinitely in the soil, until suitable germination conditions exist. The damage that they cause includes a breakdown of tissues (soft rot), cell death (necrosis or leaf spots) and the blockage of water conducting tissues.


Bacteria are microscopic single-celled organisms that reproduce rapidly inside the plant under favourable conditions. Bacteria enter plants through wounds, pruning cuts and the pores of leaves. They can also be spread mechanically, though splashing water and rain is the most common delivery system. Once inside, they affect most parts of the plant.

Bacteria are often classified as wilts (as in geranium wilt), blights (as in lilac blight), or soft rots. Wilt-causing bacteria tend to clog up the vascular tissue, inhibiting the flow of water and nutrients. Blights attack the young shoots and the outer layers of tissue first. Soft rots develop on fleshy tissues in wounded areas of the plant that remain wet for long periods.

Symptoms of wilt disease appear primarily on the leaves, and include large V shaped areas of dead tissue, yellowing, and a wilted appearance. Grey or black streaking may be apparent on leaves and stems. Under warm, humid conditions, plants collapse very quickly.

Blights may first appear as black spotting on leaves and stems. With many woody ornamentals, flower buds, young leaves, and shoots may turn completely black and die off. Infection occurs in early spring during extended periods of wet weather.

Soft rots often develop on fruits or vegetables when water sits next to them for long periods. This usually occurs where a wound already exists on the plant. Bacterial soft rots usually have a foul odor and appear wet and gooey.


Viruses are only visible with electron microscopy. They reproduce only in the body of a living host. The most common viral diseases are classified by their symptoms: mosaics, yellows, stunts, mottles and streaks.

Viral diseases exhibit a variety of symptoms. The most common is the uneven yellowing of the leaf, which gives rise to a mosaic pattern of yellow and green. With some systemic viruses, the areas adjacent to the veins turn yellow. Still other viruses can cause ring spots, stunting, distortions of leaves and flowers, and premature death.

Many viruses are carried from infected to healthy plants by insects. Infection can also be spread by humans when handling plants, taking cuttings or pruning. Many viruses are carried over from season to season in tubers, bulbs, corms and occasionally in seed.


Nematodes are microscopic worms that attack plant roots to obtain food. Leaves may also be attacked, though this is rare. In some cases, the roots become stubby, with swollen knots or growths. (Do not confuse the nitrogen fixing nodules on leguminous plants with damage from nematode infestation.) Small lesions are often also visible. As the disease progresses, root rot due to invasion by other organisms usually occurs. The plant gradually loses vigour, showing symptoms of drought, stress, nutrient deficiencies, and stunted growth. Nematodes thrive best in sandy soils.

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For more information:
Toll Free: 1-877-424-1300
Author: OMAFRA Staff
Creation Date: 03 May 2010
Last Reviewed: 25 June 2010