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:
- wind
- water
- infected soil.
A rose with black spot.This rose plant has black spot. It is a very common fungal disease.
Disease symptoms are:
- spots
- rot
- mildew and mould
- a furry appearance when they are mature and producing spores (this is one way of telling the difference between fungal and bacterial infections).
Common fungal diseases
| Type of diseases | Symptoms | Example | Plants infected |
|---|---|---|---|
| black spot | Black dead areas on leaves, stems, fruit. |  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 |
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 |
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 |
grapes, onions, cabbage seedlings |
| mould | Rotting tissue, which goes grey or brown. |  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 |
stone fruit |
| damping off | Seedlings rot and die. |  Fungi attack stem at soil level |
young seedling of many plants |
| wilts | Flow of water into plant is interrupted and plant collapses. |  Phytophthora wilt |
rhododendrons |
Fungal diseases can be prevented by:
- maintaining good hygiene (disposing of infected plant material, cleaning tools)
- making sure the plants in greenhouses have good ventilation and avoiding the wetting of plant leaves
- using chemicals called fungicides that cover plant surfaces
- using resistant plants.
Open HT1103A [PDF 643kB] to see more examples of fungal diseases.
Complete the table by dragging the disease name and its description into the correct space.
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:
- water
- insects
- infected seeds
- infected soil.
It can be difficult to tell the difference between bacterial and fungal diseases.
Disease symptoms include:
- spots
- lumps
- withered leaves
- collapsed tissue or rot.
Common bacterial diseases
| Type of diseases | Symptoms | Plants 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 can only reproduce by infecting and taking over living plant or animal cells, and then they use the host cells to make many copies of themselves. They reproduce very rapidly, meaning that a viral disease can spread quickly through a crop.
Viruses are spread by:
- insects such as aphids
- infected plants or cuttings
- infected soil
- tools and hands
- seeds.
Disease symptoms are:
- stunted, curled or deformed leaves
- mottled (mosaic) or patterns on leaves
- streaked petals.
Open HT1103A [PDF 643kB] at photos 19/20 to see more examples of viral diseases.
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.