This document covers a plant health week focusing on plant disorders, garden chemicals, and weed identification. The learning objectives are to describe physiological disorders and their causes, explain the modes of action of pesticides, fungicides, and herbicides, identify active ingredients in examples of each, state weed life cycles and name examples, and identify weeds botanically. Key topics include nutrient and water disorders and remedies, modes of action being contact or systemic, herbicide modes of selective and residual, and examples of chemicals and their modes of action. Common weeds are also defined botanically.

1. RHS Level 2 Certificate
Plant Health Week 21– Plant disorders; Review
of Garden Chemicals and modes of action;
Botanical names and Weed identification.

2. Learning Objectives
By the end of the session, learners will be able to:
• Name and describe three plant physiological disorders with different causes.
• state the principle modes of action for pesticides and fungicides – contact and
systemic – and describe each
• state the principle modes of action for herbicides – contact, systemic,
selective – and describe each
• identify the active ingredients of examples of contact, selective and systemic
herbicides
• identify the active ingredients and mode of action of an example of a
pesticide and a fungicide
• State the life cycle types of weeds and name two of each of perennial, annual
and ephemeral weeds
• Botanically name and identify four common weeds from images

3. Name the
physiological
disorder. How
could you
remedy this?

4. Name the
physiological
disorder. How
could you
remedy this?

5. Name the
physiological
disorder. How
could you
prevent this?

6. Physiological Disorders
• Plant problems caused not by pests or disease but by some problem
in the environment.
• Nutrient deficiencies – e.g. interveinal chlorosis in Rhododendron sp
caused by iron deficiency in high pH soil.
• Lack of light – etiolated or leaning growth caused by plants growing
with insufficient light.
• Water availability: drought or water logging.
• Temperature: too low or too high.
• Damage to plant cells or structures. Fasciation: distorted growth due
to damage at the growing tips of stems, flowers and fruit.
• Rose balling – flowers unable to open due to petals soaked by rain,
drying to form a shell.

7. Control of physiological disorders.
• Nutrient related – correct soil pH for plants grown.
Careful use of fertilizers.
• Water related – good, consistent and appropriate
irrigation. Appropriate drainage for species grown.
• Temperature related – harden off bedding plants; do
not plant out until after last frost. Avoid planting
shade lovers in full sun.
• Light related – good light levels for seedlings.
• Damage related – control aphids, prune appropriately
with sharp tools, good air circulation around roses.

8. Garden Chemicals – a quick reminder of types
• Pesticides – toxic to pests, that is insects,
molluscs or mites that damage or destroy
cultivated plants.
• Fungicides – toxic to or preventative of invasion
by fungal diseases that damage or destroy
cultivated plants.
• Herbicides – kill or prevent the growth of
weeds or unwanted vegetation.

9. Modes of Action
• Can be very complex indeed – but for our purposes
these are contact or systemic for all types of
chemical.
• Herbicides may also be selective or residual.
• Contact – requires that all affected parts of the plant
or pests, or every part of the weed to be killed must
be in contact with the chemical for it to be effective.
• Systemic – is transported in the xylem and phloem
to all parts of the plant. Often applied as a soil
drench for insecticides and fungicides (sometimes a
spray), or as a spray for herbicides.

10. Modes of Action - herbicides
• There are a couple of other MoAs that are specific to
herbicides:
• Selective – contact herbicide that interrupts the
production and effect of auxin (the PGR that controls
growth) in broad leaved (dicotyledon) weeds. The
weeds die but grasses are unaffected so these
herbicides are used on lawns.
• Residual – these remain active in the soil after
application and either kill newly germinated seeds and
seedlings or create a barrier preventing their
emergence from the soil.

11. Garden Chemicals - examples
Type of
chemical
Mode of action Active ingredient
1. pesticide contact Lambda - cyhalothrin
2. herbicide systemic glyphosate
3. fungicide systemic triticonazole
4. herbicide selective Mecoprop-P, Dicamba
5. herbicide contact diquat dibromide

12. Why botanical
names?
• Allows plants to be precisely identified – not
lumped together by a single characteristic.
• Same common names are often used for
unrelated plants.
• No language barrier – Latin is the botanical
naming convention all over the world.
• Gives information about the plant and about
its inheritance, where it grows etc.
This Photo by Unknown Author is licensed under CC BY-SA-NC

13. Ground elder Aegopodium
podagraria

14. Bramble or
blackberry
Rubus fruticosus

15. Urtica dioica
Stinging nettle

16. Elymus repens
Couch grass

17. Taraxacum officinale
Dandelion

18. Ranunculus repens
Creeping buttercup

19. Bellis perennis
Daisy

20. Stellaria media
Chickweed

21. Senecio vulgaris
Groundsel

22. Galium aparine
Cleavers/Goosegrass

23. Cardamine hirsuta
Hairy bittercress

24. Fat Hen
Chenopodium
album

25. Learning
Outcomes
• Name and describe three plant physiological disorders with
different causes.
• state the principle modes of action for pesticides and
fungicides – contact and systemic – and describe each
• state the principle modes of action for herbicides – contact,
systemic, selective and residual – and describe each
• identify the active ingredients of examples of contact,
residual and systemic herbicides
• identify the active ingredients of an example of a systemic
pesticide and a systemic fungicide
• State the life cycle types of weeds and name two of each of
perennial, annual and ephemeral weeds
• Botanically name and identify four common weeds from
images