plant disease and immunity

what are the symptoms of plant disease?

Fungi:

• grown as thread like structures (hyphae)

• spread via spores

• cause things like mould, rot and leaf spot

• fungi like microorganism - oomycetes - spread via water

bacteria:

• single celled organisms

• cause leaf spots, rot, wilting, ooze

viruses:

• spread by insects (vectors)

• cause discolouration, stunted growth, deformed leaves

nematodes:

• attack roots

• tiny worms in the soil

• cause poor growth, root damage, swelling

what are the signs of plant disease?

symptoms = visible or detectable abnormalities arising from the disease

signs are the visible pathogen itself

• rusts spores

• smut spores

• powdery mildew

• mycelium and sclerotia

what are fungal disease symptoms?

Eukaryotic
• Heterotrophic
• Hyphae
• Mycelium
• Fruiting bodies
• Spores

symptoms:

• soft rot

• dry rot

• leaf spots/scald

• leaf curl

signs:

• powdery mildew

• rust

• smut

• mycelium

• fruiting bodies

what are bacterial signs and symptoms?

• leaf spots

• wilts

• cankers

• diebacks

• rots

• galls

• scorch

• ooze - most distinctive sign

viruses

• transmitted by vectors

• rna or dna wrapped in protein coat or sometimes lipid membrane

symptoms:

• necrosis

• mosaics

• chlorosis

• stunting

• crinkling

nematodes

• unsegmented small worms

• microscopic

• some feed on bacteria some on plants

• ecto- and endo parasites

signs and symptoms;

• stunting

• discolouration, decline

• dieback

• necrosis

• chlorosis

• local lesions

• galls

• cysts

• very similar to oomycete symptoms

how can plant disease be diagnosed

1. kochs postulates

• suspected causal organisms should be consistently associated with symptoms

• organism is isolated from diseased host and grown in a pure culture

• healthy host is infected with isolated organism and should show same symptoms

• reisolate organism and should show same symptoms

2. molecular techniques

• dna extraction followed by pcr - expensive equipment

• take time to develop initially

• knowledge of potential pathogen required

• fast and efficient

what are the pathogen lifestyles

1. commensal - lives on plant but doesnt affect it

2. mutualistic - both plant and microbe benefit

3. pathogenic - cause disease

types of pathogen

• biotroph - feeds on living plant cells, keeps plant alive

• necrotophs - kill plants cells and then feed on them

• hemibiotrophs - starts of a biotroph then become a necrotoph

• saprotroph - feed on dead material

how pathogen cause disease

• toxins - damage plant cells

• ezymes’s - help enter plant

• effectors -trick plant defences

• hormones - alter plants growth

how do we manage plant disease

cultural methods: (prevention)

• clean tools

• remove infected plants

• crop rotation

• use healthy seeds

chemical control:

• use fungicides

• must be used carefully

• problems - expensive, harmful, pathogens can evolve resistance

biological control:

• use natural organisms to fight pathogens

resistant plants

• breed plants that are resistant

biotechnology:

• GM crops

• add resistance genes

• use tools like CRISPR

physical defences

• strengthen the cell wall locally

• close stomata

• formation of tyloses

chemical defences

• defensins

• antimicrobial metabolites

• hypersensitive responses

• systemic acquired resistance

• induced systemic resistance

what are the two components of immune system

1. basal resistance - recognises microbes, triggers generalised defence response

2. specific resistance - gene-for-gene hypothesis, triggers localised then generalised

Basal resistance

• generalised defence response

• pathogen recognition receptors (PRR) recognise MAMP’s/PAMP’s and DAMP’s

• also known as pattern triggered immunity

• triggers production of defensins and phytoalexins

• defensins are toxins, anti microbial properties

• phytoalexins are alkaloids, cyanide etc

specific resistance

• plants have a resistance gene

• pathogens have an avirulence gene (Avr)

• the plants R protein interacts with the pathogens Avr protein to signal the pathogens presence

• R/Avr interaction is an example of co evolution

what is the gene for gene hypothesis

whats the hypersensitive response

• plant cells produce large quantities of reactive oxygen species which induce rapid cells death around site of the attack

what is systemic acquired resistance (SAR)

• Triggered by pathogen attack

• uses salicyclic acid - produces defence proteins

• long lasting protection

• biotrophs and hemibiotrophs

what is induced systemic resistance

• triggered by helpful microbes in the soil

• uses jasmonic acid and ethylene

• prepares plant for attack

• necrotrophs and insects