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Horticulture 1000
HT1103 - Plant Management 3 - Diseases
Pests Pest and Disease Management
3

Diseases

Diseases are caused by fungi, bacteria or viruses.

Disease prevents plants functioning properly and interferes with growth and development. It can be difficult to work out exactly what has caused a particular disease, as many plant diseases have similar signs.

Fungi

Fungi are plants that cannot make their own food and rely on other plants or animals (dead or living).

Many fungi are beneficial, but some can damage or destroy the leaves, stems or roots of plants. They may attack and grow into and through the plant tissues as a mass of fine threads, feeding on plant cells as they spread.

Fungi reproduce by spores, and can spread rapidly. They prefer damp warm conditions for growth and reproduction.

Fungi are spread by:

A rose with black spotA rose with black spot.

This rose plant has black spot. It is a very common fungal disease.

 

Disease symptoms are:

Common fungal diseases

Type of diseasesSymptomsExamplePlants infected
black spot Black dead areas on leaves, stems, fruit. Apple leaf with black spot
Apple leaf with black spot
apples, pears, roses
rust Yellow or brown flat spots on upper surface, raised spots on lower surface. Chrysanthemum leaf with rust
Chrysanthemum leaf with rust
silverbeet, poplar trees, chrysanthemums, stone fruit
powdery mildew White spots that join on a leaf surface to form a mat. Pumpkin leaf with powdery mildew
Pumpkin leaf with powdery mildew
apples, grapes, pumpkins, cucumbers, roses
downy mildew Pale areas on upper leaf surface and soft white growth on lower surface. Grape leaf with downy mildew
Grape leaf with downy mildew
grapes, onions, cabbage seedlings
mould Rotting tissue, which goes grey or brown. Brown rot on peach
Brown rot on peach
greenhouse crops, grapes, strawberries, vegetable crops, stone fruit
wood rot Inner tissue of stems and branches are weakened. Leaves may turn silver. Silver leaf of peach tree with wood rot
Silver leaf of peach tree with wood rot
stone fruit
damping off Seedlings rot and die. Fungi attack stem at soil level
Fungi attack stem at soil level
young seedling of many plants
wilts Flow of water into plant is interrupted and plant collapses. Phytophthora wilt
Phytophthora wilt
rhododendrons

Fungal diseases can be prevented by:

Open HT1103A [PDF 643kB] to see more examples of fungal diseases.

Activity 3A Complete the table by dragging the disease name and its description into the correct space.


Activity 3B Test your knowledge of fungi below.


Bacteria

Bacteria are tiny single-celled organisms. They cannot make their own food, and rely on living on dead organic matter for energy. Many bacteria are beneficial, but some can cause diseases for plants. Bacteria spores reproduce rapidly. An infection can travel from one plant to a whole crop quickly.

Bacteria are spread by:

It can be difficult to tell the difference between bacterial and fungal diseases.

Disease symptoms include:

Common bacterial diseases

Type of diseasesSymptomsPlants commonly infected
cankers gradual decay of infected tissue
bark dies
gum oozes out
apple, rose
galls a lump or ball forms peach
soft rot damaged part moist and slimy potato
fire blight withered leaves and dieback apples, pears, cotoneaster, viburnum

Fire blight is a bacterial disease that can affect pipfruit in New Zealand. For many years, Australia has refused to take New Zealand apples because of the risk of importing the disease fire blight.

Open HT1103A [PDF 643kB] at photos 17/18 to see more examples of bacterial diseases.

Viruses

Viruses are not true cells. They consist of genetic material with a protein coat.

Viruses are spread by:

Disease symptoms are:

Open HT1103A [PDF 643kB] at photos 19/20 to see more examples of viral diseases.

Complete Activity 3C in your workbook

Complete Activity 3D in your workbook

Key points   Key points

Diseases are caused by fungi, bacteria or viruses.

Fungi

  • Fungi reproduce by spores that are spread by wind, water, or infected soil.
  • Disease symptoms include spots, rot and a grey, furry appearance.
  • Fungal diseases include black spot on roses, rust, botrytis a fluffy grey mould on fruit, club root on brassica plants (cabbage family), mildews and moulds and damping off on seedlings.

Bacteria

  • Bacteria are single organisms that can’t make their own food so they live off dead or live organisms.
  • They reproduce very fast by spores, which can be spread by water, insects, infected seed and infected soil.
  • Disease symptoms include rot, oozing, spots that may be weeping or slimy.
  • Diseases include cankers, galls, soft rot and fire blight.

Viruses

  • Viruses reproduce inside living cells using the host cell to make a copy of themselves.
  • They are spread by infected seeds, insects, soil, tools and hands, and cuttings from infected plants.
  • Disease symptoms include poor growth, mottled (mosaic) leaves, streaked petals, curled or deformed leaves.
  • Diseases include such things as tomato mosaic virus (TMV).
 

What's next?

Go to: 4 Pest and disease management.

Pests Pest and Disease Management