4.3.3.1 Detection and identification of plant diseases/4.3.3.2 Plant defence responses

The common signs of plant diseases are:

● Stunted growth: indicating nitrate deficiency
● Spots on leaves: indicating black spot fungus on roses
● Areas of decay: black spot fungus on roses, blights on potatoes
● Abnormal growths: crown galls caused by bacterial infection
● Malformed stems or leaves: due to aphid infestation
● Discolouration: indicating magnesium deficiency, or tobacco mosaic virus
● Pests on leaves: such as caterpillars

You can identify the disease the plant has by:

Using a gardening manual or website
● Identifying the pathogen by observing the infected plant in a laboratory
● Using monoclonal antibodies in testing its to identify the pathogen

plant diseases

tobacco mosaic virus (viral disease) black spot (fungal disease)
aphids as insects.

Nitrate deficiency

can stunt growth
● Nitrates in the soil convert sugars made in photosynthesis into proteins
● These proteins are needed for growth

Magnesium deficiency

can cause chlorosis - leaves lose green colouration

● Magnesium is needed to make chlorophyll ● This pigment is green and is vital in photosynthesis

● If less is being made then parts of the leaves appear green and yellow which is known as chlorosis

potassium deficiency

production of flowers and fruit
poor growth of flowers and fruit, yellow leaves and dead spots

Aphids

Aphids have long sharp mouthpieces that pierce the surface of the stem and into the phloem so that they can feed on the sugar filled sap

extract nutrients such as sugars from plants, stunting its growth

They can infest a plant in large numbers and remove large quantities of the phloem sap. This can weaken the plant Aphids can also act as a vector for the transfer of diseases

how to control aphids

Biological pest control (using ladybirds, a natural predator of the aphid) to feed on them or using chemical pesticides are the best ways to control infestation with aphids

Physical defences: To prevent the invasion of microorganisms

● Tough waxy cuticle stops entry into leaves

● Cellulose cell walls form a physical barrier into the cells- difficult for microorganisms to penetrate

● Plants have layers of dead cells around stems (such as bark) which stop pathogens entering. The dead cells fall off with the pathogens.

Chemical defences: To deter predators or kill bacteria

Poisons (e.g from foxgloves, tobacco plants, deadly nightshades, yew) deter
herbivores (organisms that eat plants)
● Antibacterial compounds kill bacteria, such as mint plant and witch hazel

Mechanical defences

● Thorns and hairs make it difficult and painful for animals to eat them (but do not
defend against insects)
● Some leaves can droop or curl when touched which allows them to move away and
move insects off their leaves
● Mimicry to trick animals
○ Some plants droop to look like unhealthy plants so that animals avoid them
○ Plants can have patterns that appear to look like butterfly eggs, so butterflies
do not lay their eggs here in order to avoid competition
○ Species from the ‘ice plant family’ have a stone and pebble like appearance
in order to avoid predation

Adaptations Against Herbivores

Herbivores eating a plant can seriously damage the plant structures and they will not flower and reproduce
Smaller insect infections can remove essential nutrients and energy from the plant and act as vectors for pathogens

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sharp thorns

hairs-irritates the mouths of herbivores

leaves droop, curl when touched

white dead nettle look similar to stinging nettles but got no sting

identifying plant diseases

use a garden manual or website

take infected plant to a lab to identify the pahogen

we can use testing kits containing monoclonal antibodies

poisons

thorns

hairy stems

drooping or curling when touched

mimcry