Unit 10 Plant Diseases

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Lumber potato

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Peru introduced the potato to Ireland in 1590

characterized as a "wet, nasty, knobbly old potato”

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an airborne oomycete,

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Very similar to a fungus. spores carried by breezes from south america to ireland

Causal agent was _____originally transported on ships traveling from North America to southern England .

Spores settled on the leaves of healthy plants, multiplied & were carried by breezes to surrounding plants & fields

Rotten potato leaves and potatoes

single infected plant could infect 1000s more in a few days

An odor of decay permeated the air & a thick blue fog descended on the countryside

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Blue fog

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odor of decay permeated the air and this ____ to descend on the country side

People thought that it was causing the rotting of the potatoes and leaves

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How was the airborne oomycete pathogen carried?

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Winds from southern England carried it to the countryside around Dublin

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Phytophthora infestans

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Causes a disease known as late blight or potato blight

Can also infect tomatoes

2013 it was determined to be a previously unknown strain HERB-1

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Charles Trevelyan

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Created a Relief Commission to help those who didn’t have any food

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Robert Peel

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British Prime Minister that tried to distribute Indian corn imported from the U.S.

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Why did many people became ill due to eating the indian corn

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because they were not used to it & it lacked vitamin C (scurvy)

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Deaths from starvation were kept down due to:

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• Imports of Indian corn

• Survival of about 1/2 the potato crop

• Many sold off their livestock & pawned their possessions whenever necessary to buy food

• Some borrowed money at high interest from petty money-lenders

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Charles Trevelyan

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adhered to the laissez-faire principle

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laissez-faire principle

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To stop outside food help,

- Closed food depots selling Peel's Indian corn, rejected another boatload of Indian corn, free food handouts were rebuked

- Many began to die, mostly due to typhus or starvation

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obligate intracellular parasitic Rickettsia bacteria

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Causes Typhus.

the bacteria can grow from your body and if not treated spread to the brain and kill you

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Spread of Typhus

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by body lice

beggars and homeless paupers carried it from Irish town to town

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Typhus

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•Rash spreads over most of the body & then meningoencephalitis

Untreated cases are often fatal

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became known as coffin ships

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•People began trying to leave Ireland for Canada; 1 in 5 died on their travels over

1 million emigrated, 1.5 million died

even till today the population of Ireland is very small

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cold winter & diversified the crop

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was key for the complete end of Irish Potato Famine in 1852

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f they had planted many kinds of potatoes ....

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the famine would have been avoided. The “Lumper” potato was the only potato affected by the fungus, but all potatoes were “Lumpers” in Ireland

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Genetic Diversity in Planting is Key to stop the famine

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Many began to include corn & other vegetables in their diet & rely less on the potato

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Control of P. infestans us difficult to control but an integrated controll approach includes

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1.Breeding for resistant plants – limited success2.Reducing source inoculum – good quality seed3.Fungicides

4.Crop rotation

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Aristotle

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wrote about plant diseases in 350 BC

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Theophrastus (372-287 BC)

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theorized about cereal & other plant diseases

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•Shakespeare

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mentions wheat mildew in one of his plays

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•Duhumel de Monceau

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described a fungus disease & demonstrated that it could be passed between saffron plants

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seed treatment

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were used for wheat

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Heinrich Anton DeBary,

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known as the father of modern plant pathology, supplanted the theory of spontaneously generated diseases with the germ theory of disease

also published a book identifying fungi as the cause of a variety of plant diseases

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Disease Triangle Paradigm

the existence of a disease caused by a biotic agent absolutely requires the interaction of a susceptible _____, a virulent ______, & an _____ favorable for disease development

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Host

Pathogen

Environment

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Environment part of the paradigm include

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•Little thermal storage capacity

•Their immobility precludes escape from an inhospitable environment

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Host part of the paradigm includes

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•The sophisticated adaptive immune system found in mammals is absent in plants

•Places emphasis on the host's genetic constitution

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Pathogen part of the paradigm

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•Predominance in phytopathology of fungi, which are also highly dependent on environment

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Plant pathogenesis can be understood by studying the 4 phases of infection of the pathogen

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1.Attachment

2.Entry

3.Colonization & infection

4.Dispersal (i.e., movement in the plant host)

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Inoculation in attachment phase

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Initial contact between infectious agent & a potential host plant

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before they can enter the plant, pathogens .....

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these must germinate & grow on the surface of the plant before....

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Fungal agents adhere either by

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1. by the presence of moisture or

2. excretion of enzymes & mucilaginous substances

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Cell wall of underlying cells is reinforced specifically at discrete sites of interaction with potential pathogenic microbes

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This makes receptor-mediated endocytosis impossible for plant viruses

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Brute Force

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Direct penetration of the plant surface to enter host

Often through enzymatic degradation of the cuticle & cell wall

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(appressoria),

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specialized organs produced by the germ tube of many fungal pathogens

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Infection peg

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presses against epidermis

•Turgor pressure exerted punctures the host cuticle & cell wall

(insert picture)

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Infection life cycle of a general fungal pathogen

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Back (Definition)

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Biotrophic

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These fungal pathogens Establish an infection in living tissue. we see just viral symptoms

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Haustoria

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Biotrophic fungal pathogens obtain nutrients from living host tissues via specialized cells (_________) that form inside host cells

- this is used to penetrate the cell wall and suck out nutrients

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arthropod vectors, chewing insects, sap sucking

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Virtually all viruses & some other pathogens are transmitted by__________

Some are chewing _____ others are _____

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•2-step conversion of sucrose

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•Problem is plant sap (sucrose) is really sweet & the osmotic pressure exerted by ingested sap on the vector gut could lead to dehydration & death therefore, sucrose is converted to glucose and fructose to avoid dehydration

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Plants use combo of defense responses:

Defence mechanisms of plants against pathogens

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•Constitutive (pre-formed structures/compounds)

•Induced (immune system)

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Pre-formed structures & compounds

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•Cuticle, cell wall

•Antimicrobial chemicals (glucosides, saponins) •Antimicrobial proteins

•Enzyme inhibitors

•Detoxifying enzymes (breakdown pathogen-derived toxins)

•Receptors that perceive pathogen presence & activate inducible plant defences

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Immune system (induced defence response)

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•Not adaptive (i.e., no antibodies are produced) •2-tiered: interconnected innate system (PTI) with evolved specific immunity (ETI)

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2 tiers of immune system

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Pattern-triggered Immunity and Effector-triggered immunity

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Pattern-triggered Immunity

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(PRRs) Pattern Recognition Receptors are seen in this tier.

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Effector-triggered immunity

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The R genes (resistance genes) are seen in this tier

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The rice blast fungus infects rice plants through an appressorium

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insert picture

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What are PRRs

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are plant proteins that recognize evolutionarily conserved pathogen–associated molecular patterns (PAMPs) in broad groups of pathogens

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Whats the PTI reaponse

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PRRs then activate cellular defense response mechanisms that limit pathogen colonization

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What does an R gene do?

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recognizes a specific protein (effector) used by a specific pathogen during the infection process

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ETI response

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genes specific to one pathogen that activate defense response

The R gene will activate a defense tailored for the specific pathogen it recognizes

•Suite of R genes varies dramatically among species & even among cultivars/races within a species

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Breeding resistance

optimal germplasm

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R gene(s) tailored for a specific pathogens are often bred into __________ (e.g., quick growing, adapted to specific area, produce large fruit) •Agronomists test tons of crop cultivars until they find one that works against the pathogen

because the plant is optimized to a certain environment, it might not grow well however, the R gene that can combat the pathogen is isolated using different methods

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optimal germplasm

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Genetically modified organisms (GMOs)

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•Foods produced from organisms whose DNA has been altered in a way that does not occur naturally by mating &/or natural recombination

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Pathogen-derived resistance of Papaya ringspot virus

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This is achieved by inserting a virus gene from the pathogen into the transgenic crop

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Backlash against GMO papaya

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has resulted in destruction of many papaya plantations

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Virus resistance in papay ringspot virus?

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insert picture

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Resistance similar to a virus 1986

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In transgenic papaya plants, the coat protein was engineered into the papaya to create _____

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Sneak attack form of entry

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Pre-existing opening in the plant surface

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Cross protection.

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Natural phenomenon where a mild virus isolate/strain can protect plants against economic damage caused by a severe challenge strain/isolate of the same virus

Farmers spray the field with the mild strain to confer resistance

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Four virus families composed of plant & human & animal

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viruses: Tymoviridae, Bunyaviridae, Rhabdoviridae, Reoviridae

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Mild mottle virus found in feaces of humans

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Those with the virus were more likely to report fever, abdominal pain & itching than those without it

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Citrus greening

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• • • Causative agent: Liberibacter V Candidatus ectored by Asian citrus psyllids 33 countries; reached FLA in 2005; CA in 2017

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Blue stain fungus

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• Current Rocky outbreak along the Mountains has destroyed broad areas of lodgepole pine forest 1996 H as since since killed about 50 % forest in BC

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Stomatal pores, wounds, anthropogenic wounding

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Pathogenic bacteria & nematodes often enter through _________ when there is a film of moisture on the leaf surface •Plant viruses usually introduced via _________ by infected arthropod vectors or anthropogenic _______ from farming

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Colonization phase

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Pathogens can be extracellular like fungi or intracellular like viruses

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Intracellular

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Where by the pathogen infects the cell and spreads , the term is

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Extracellular

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The term referred to when the pathogen Colonize the area between the cuticle & the outer wall of the epidermal cells •May colonize deeper in the plant tissues (mesophyll & parenchyma infections

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Dispersal phase

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Phase has 2 categories

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The 2 categories in the Dispersal phase

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Biotroph and necrotroph

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Necrotroph

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These fungal pathogens kill cells before colonizing them, by secreting toxins that diffuse ahead of the advancing pathogen •Obtaining nutrients from the dead host tissue

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What is Blue stain fungus

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•Mountain pine beetles lay their eggs under the bark

•The beetles carry the fungus fungal spores within a specialized sac on it mouth parts

•The beetle starve the tree of nutrients & the fungi grow into the wood & disrupt water transport

(Beetles work with the fungus bcz of the defence mechanism of tree to deter bettle herbivory. beetle needs fungus to combat natural defenses of the tree)

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Attachment phase

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these pathogens must complete their life cycle before the enter plant and so require appendages

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Stromatal pore

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another sneak attack way of entry for viruses. through that involves water regulation by guardcells

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Insects

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vast majority of pathogen are spread by

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antiplant defense mechanism in the beetle

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the spit of the _____ (which contains virus). regurgitate due to the plants leaf expressing many defense mechanism when ______ chew on it

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Atkins diet

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ants symbiotic relationship with aphids; where ants grow a farm of aphids, because of the sugar based diet

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Fungal agents

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cause vast majority of plant disease because of the dependency on environment

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Page 5 pie chart

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the percentage of plant-infecting fungi was way more than the animal-infecting fungi.

(insert picture)

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Insect vector transmission more diverse

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Viruses localize to different sites in the plant-feeding insect vector depending on their modes of transmission. Non-circulative viruses bind to the insect stylet (see inset) or foregut.