Intro to Pathology Flashcards
We heavily rely on a handful of crop species give some percentages.
What are the three species we rely on?
What does food security rely on?
Today, 75 % of the world’s food is generated from only 12 plants and 5 animal species.
Only 4 % of the 250000 to 300000 known edible plant species are used by humans – i.e. 150 to 200 species. Only three - rice, maize and wheat - contribute nearly 60 % of calories and proteins obtained by humans from plants.
Food security is heavily reliant on a firm understanding of plant-pathogen interactions.
What does climate change change?
It causes changes in phytopathogen and pest range
crop pests and pathogens move polewards in a warming world.
What is a plant pathogen?
What parts of the plant do they affect?
Viruses are non-cellular, and merely packaged nucleic acids
Nematodes are large, multicellular animals
Fungi and oomyctes are eukaryotes
Bacteria are prokaryotes
Any part of a plant may be susceptible to pathogens
Case study: Phytophthora infestans, plant destroyer
Potatoes were brought to Europe from South America starting in the 16th century, but the pathogen Phytophthora infestans was not observed in Europe until the 19th century
An outbreak potato late blight, caused by Phytophthora infestans, caused the Great Famine of the 1840s
1863 - Anton de Bary showed that Phytophthora causes late blight. De Bary transferred spores from a sick plant to a healthy plant, which then developed disease symptoms.
Pathologists unite in support of the germ theory
At the same time, Louis Pasteur and Robert Koch showed animal diseases were caused by microbes, and the germ theory was born.
Koch’s postulates to establish a microbe as the causal agent of a disease (1880s):
The microbe is always associated and isolated from the patient with the disease
The microbe must be grown in pure culture (hard to do this for plant pathogens)
The microbe can be injected or inoculated into an animal (plant) and cause disease
The microbe can be re-isolated in pure culture
Bacterial plant pathogens were described in the 1870s
T.J. Burrill (1878) demonstrated that fire blight of pear and apple was caused by a bacterium, Erwinia amylovora
T.J. Burrill (1878) demonstrated that fire blight of pear and apple was caused by a bacterium, Erwinia amylovora. How was this done?
By passing the extract from an infected leaf through an extremely fine filter, pathologists showed that the infectious agent was smaller than a bacterium, and named it “virus”
parasitic nematodes
Being much larger, plant parasitic nematodes were identified earlier
By the end of the 19th century, pathologists knew why plants got sick
Chemical warfare
Better hygiene (e.g. burning crop residue)
Genetics of disease resistance
Defining Plant Disease
Any organism that alters the normal physiological processes in plants leading to:
*Loss of leaf surface area
*Loss of yield component
-Grains/fruits
-Flowers
*Partial or complete plant death
-Individual plant
-Whole crop
Grey mould (Botrytis cinerea) a fungal pathogen
Defining Plant Disease
Name some plant pathogens
*Fungi
Rusts
*fungi like organisms
Oomycytes
Plasmodiophoromycetes
Myxomycota
*Bacteria
Erwinia
*Viruses
Mosaic viruses
Potyvirus
*Other organisms
Nematodes
Parasitic plants
Brown rot of apple (Sclerotinia fructigena) a fungal pathogen of fruits.Very common
But do not include!
Insect pests
Aphids
Leaf hopers
Spider mites
Galls
Locust
Weeds
These can cause significant checks on plant growth
But both of these groups are involved in plant pathology
Insects transmit viruses
Weeds act as secondary
hosts
Pathogens are biotrophs, necrotrophs or hemibiotrophs.
Describe what each of these do
Necrotrophs kill cells and then consume the contents (botrytis cinerea)
Biotrophs live within host tissue without causing death (hyaloperonospora arabidopsidis)
Hemibiotrophs can switch from biotroph to necrotroph (pseudomonas syringae)
Not all plant-microbe interactions are bad
Soil bacteria and fungi help plants fix atmospheric nitrogen and other nutrients in exchange for carbon containing molecules
Many form root associations with plants
Strategies of pathogenicity
What does a successful pathogen do:
Find the host and attach to it
Gain entry through the plant’s impermeable defenses (plants can close stomata and strengthen cell walls by reorganising cytoskeleton use of actin to stop fungal pathogens)
Avoid the plant’s defense responses
Grow and reproduce
Spread to other plants
Gray mold (Botrytis cinerea)
How can pathogens reach their host
Wind, water, insects and chemotaxis help pathogens reach their hosts
