Fungal bacterial and viral plant diseases

1. RHS Level 2
Certificate
Year 2 Week 20– Fungal, bacterial
and viral plant diseases and
physiological disorders.

2. Learning Objectives
4. Know the problems posed by diseases in horticulture and ways in which
these can be minimised.
 Describe the damage caused by plant diseases to include: apple and pear
canker (pick this up in Week 21), fireblight, bacterial canker on Prunus, potato
leaf curl virus (and vector peach potato aphid), tobacco mosaic virus
 Describe TWO different methods of reducing the effects of EACH of the
diseases stated in above; methods should be selected from more than one of
the control options (chemical, physical, cultural or biological) available.
 Describe in outline the life -cycles of bacterial canker, potato leaf curl virus
 Describe methods of spread of EACH of the diseases named above
 Describe the damage caused by TWO named plant viruses. .
 Describe TWO methods of avoiding the spread of plant viruses.
 Explain how knowledge of the life -cycle and biology of diseases stated above
contribute to the success of their control

3. Plant Disease
 Damage caused to plants by some disease
causing agent – bacteria, viruses or fungi.
Collectively disease causing agents are known
as pathogens.
 Plants do not have an immune system, unlike
animals. If an individual plant survives a
disease it will not be more resistant to another
attack in the future.
 Some cultivars however are genetically resistant
to disease – they are less likely to get it or less
affected than others in the same species.

4. Bacterial Disease
 Bacterial diseases colonise plant tissues,
giving off toxic compounds that kill plant cells
and feeding on the remains.
 No chemical controls available – growing
resistant varieties and cultural controls are
the only solutions.
 Not all bacteria are harmful – without nitrogen
fixing and nitrifying bacteria the Earth would
be a very different place with far fewer plants.

5. Bacterial Disease - Fireblight
 Bacteria enter through the
nectaries and any wounds.
 Warm, moist conditions in spring
are required for infection – so it is
a sporadic problem in the UK.
 Flowers and foliage die and
blacken but often do not fall –
scorched appearance. Cankers
form on stems and root stock.
Cankers and dead shoots are a
source of future infection
 Affects plants in the family
Rosacea – Apples, Pears,
Roses, Rowan, Hawthorn
 Control – prune back to sound
wood and burn infected material.

6. Prunus bacterial canker
 Lesions and die back of
bark, gummy ooze in
spring.
 Girdles stems and
causes die back.
 Gum contains bacteria,
spread by insects, rain,
birds
 Control – prune back to
healthy wood in
summer. Grow
resistant cultivars.

7. Viral Diseases
 Viruses are the ultimate parasites – using living
cells by taking over their processes to replicate
viral DNA.
 Present in every part of an infected plant, even
if not showing symptoms.
 Spread by vegetative propagation from infected
plants. Some spread in seeds.
 Also spread in sap by aphids and transferred
between plants on tools and hands.
 No chemical controls available.

8. Tobacco Mosaic Virus
(TMV)
 Leaf mottling, poor
growth, small fruit.
 Spread by contact – on
hands or tools.
 Also spread in seed.
 Controls – resistant
varieties; aphid control;
clean hands; no
smoking; clean tools;
certified virus free seed.

9. Potato leaf roll virus
 Serious disease of Solanaceae – potatoes, tomatoes
etc. Leaves roll up, growth stunted, few tubers, severe
interveinal chlorosis.
 Spread by Peach-Potato aphid which lays eggs on
peach trees which overwinter, or overwinters as adults
on brassicas and Chenopodium album and related
weeds. Aphids migrate to potato crops, become
infected by feeding on infected plants and then spread
virus.
 Controls – control aphid by removing alternative hosts
and/or chemical control. Control virus by removing and
destroying all tubers and plant debris at harvest and
any affected plants during the season. Buy virus free
seed potatoes.

10. Learning outcomes
4. Know the problems posed by diseases in horticulture and ways in which
these can be minimised.
 Describe the damage caused by plant diseases to include: apple and
pear canker (pick this up in Week 21), fireblight, bacterial canker on
Prunus, potato leaf curl virus (and vector peach potato aphid), tobacco
mosaic virus
 Describe TWO different methods of reducing the effects of EACH of
the diseases stated in above; methods should be selected from more
than one of the control options (chemical, physical, cultural or
biological) available.
 Describe in outline the life -cycles of bacterial canker, potato leaf curl
virus
 Describe methods of spread of EACH of the diseases named above
 Describe the damage caused by TWO named plant viruses. .
 Describe TWO methods of avoiding the spread of plant viruses.
 Explain how knowledge of the life -cycle and biology of diseases stated
above contribute to the success of their control