Lecture 11: control of crop diseases Flashcards
diseases are caused by:
fungi, bacteria & viruses (pathogens) –> they cause greater economic loss than insect pests
crop diseases are transmitted in a number of ways e.g.:
- in the soil
- in the air
- by a vector (i.e. insect)
3 important soil bored diseases in agricultural systems:
- Verticillium root rot (fugal,affects root first then spreads)
- Eye Spot (affects cereal plants)
- Clubroot (swelling, plant tumour)
2 airborne diseases spread by the wind:
- rust on wheat
- Powdery Mildew on barley
3 airborne diseases spread by the wind and / or rain splash:
loose smut on oats
- septoria leaf spot on wheat
- potato leaf blight
many plant viruses are spread by __ vectors
APHID
- they are viruses:
- -cauliflower mosaic virus on cauliflower and canola
- -turnip mosaic virus on cabbage
types of damage caused by diseases:
- acute damage
- chronic infection
- direct damage to produce
- post harvest damage
acute damage:
- leads to sever injury within a few weeks e.g. late blight of potato (Phytophthora infestans)
- caused the Irish potato famine of 1845-55. 1M people dies and another 2M left Ireland for Great Britain, Canada, and the USA
chronic infection:
gradually debilitates the plant and decreases plant productivity
e.g. apple canker caused by the fungus Nectria galligena
Direct damage to produce:
destroys or lowers its value e.g. apple scab, (Venturia inaequalis) or potato scurf (Rhizoctonia solani)
post harvest damage:
pathogens may be on the produce prior to harvesting or arrive post harvest e.g. green (Penicillium italicum) and blue (Penicillium digitatum ) mold on citrus or grain molds (various fungi)
who are FERA
monitors diseases of plants in the UK
methods of disease control:
- Cultural practices (rotation, sowing date, cleaning equipment, killing weeds which harbour disease, burning debris etc)
- quarantine schemes
- chemical control
- biological control
- traditional plant breeding themes (resistant cultivars)
- genetic engineering of crops for resistant
for plant quarantine to be effective __ must be for filled:
CERTAIN CONDITIONS
- pathogen must be transported on diseased material or by a vectors that can be controlled
- regulations must be relevant and sensible
quarantine can be enforced in ___ ways
several
- inspectors at ports and airports
- growers can only import stock from nurseries that are inspected for disease
- can be a very effective control measure if applied at the correct time
example of quarantine taking place now
a total ban on import of a specific host plant e.g. import of Spanish chestnut into the UK
-to prevent the spread of Chestnut Blight caused by the fungus Endothia parasitica
Ash dieback caused by fungus Hymmenoscyphus fraxineus: What is it?
die back of ash is a serious disease caused by the fungus Hymenoscyphus fraxineus
Ash dieback caused by fungus Hymmenoscyphus fraxineus: where is it found? And when was it discovered
ash trees suffering with infection have been found widely across Europe since trees started to die in large numbers in Poland in 1992
Ash dieback caused by fungus Hymmenoscyphus fraxineus: has caused:
widespread damage to ash populations in Europe
-between 60 & 90% of Denmarks ash trees have died
Ash dieback caused by fungus Hymmenoscyphus fraxineus: Ash is the ___ species in many ancient woodland in the UK
DOMINANT
Ash dieback caused by fungus Hymmenoscyphus fraxineus: the disease causes (affect to tree)
leaf loss, crown dieback, and tree death
Ash dieback caused by fungus Hymmenoscyphus fraxineus: in Feb 2012:
the disease was found in a consignment of trees sent from a nursery in the Netherlands to on e in Buckinghamshire
Ash dieback caused by fungus Hymmenoscyphus fraxineus:in October 2012
FERA scientists confirmed cases in E. Anglia which didn’t appear to be connected with imported trees
-Govenrment banned the import of ASH SEEDLINGS into the UK
Ash dieback caused by fungus Hymmenoscyphus fraxineus: controlled or spreading?
SPREADING
stock certification:
seeds must be tested for the presence of pathogens
stock certification: to have seed stocks certified as pathogen free, seeds must be obtained from crops gown in:
- an area free of or isolated from the pathogen
- an area not suitable for a pathogen
- an area not suitable for the vector of a pathogen
chemical control of diseases:
- FUNGICIDES & BACTERIOCIDES
- large amounts of fungicides are used annual in combination with other strategies
- chemicals can be inorganic or organic moleulces
fungicides are classified according to
type and mode of action
- non systemic
- systemic
non systemic fungicides:
- contact fungicides
- protective, muti site inhibitors, broad spectrum & low risk for resistance
systemic fungicides:
- taken up into plant (apoplectic/symplastic pathway)
- protective, curative or eradicant, they are site specific inhibitors, selective and high risk fro resistance
fungicide resistance:
- this is the most significant problem with the use of chemicals to control diseases
- the pathogen may become less permeable to the chemical
- the chemical may become bound to the cell wall
- there may be changes in the metabolism of the compound so that toxic intermediates are not formed
- the fungus may be able to bypass a block in a biochemical pathway
the development of resistance to Erwinia amylovora (fire blight)
- Erwinia causes fire blight of trees
- bacterial disease are often controlled by spraying with antibiotics (bactericides)
- in many cases the application of antibiotics leads to the development of resistance
strengths of fungicides & bactericides:
- some can control multiple diseases
- some are curative (systemic) as well as protective
- many cause little or no injury to the crop plant
- they can be applied over a large area rapidly
- they control diseases quickly
weaknesses of fungicides & bacteriocides :
- they may be toxic to humans and other animals
- improper application may result in injury to the crop
- repeated use of a pesticide may lead to the evolution of resistance in fungi & bacteria
the use of resistant cultivars: STRENGTHS
• Use of disease resistant cultivars is a key to
environmentally friendly and economically sustainable disease control in modern crop production.
• Decrease losses caused by disease
• Reduce the need for pesticides.
• Pesticides and other control strategies can be used together with resistance cultivars in integrated control programmes.
resistance cultivars:
genetic diversity of crop in field, susceptible and resistant in field together, don’t get rapid spread from susceptible to susceptible, barrier of resistance in between
the use of resistant cultivars: WEAKNESS
resistant cultivars are put at risk by the evolutionary potential of pathogens to overcome disease resistance of crop cultivars
two basic types of genetic resistance:
- monogenic or major gene resistance controlled by since genes (R genes)
- polygenic resistance is controlled by multiple genes
monogenic or major gene resistance:
- controlled by single genes (R Genes)
- these act in a gene of gene manner with virulence/avirulence genes in pathogen races
- this type of resistance is often overcome quite quickly by the evolution of virulence in the pathogen
polygenic resistance:
- controlled by multiple genes
- it is often more durable & long lasting
Magnaporthe grisea - rice blast disease: rice is a staple crop for __% of the worlds population
50
Magnaporthe grisea - rice blast disease: rice is ______ to rice blast disease caused by M. grisea
very susceptible
Magnaporthe grisea - rice blast disease: how does the disease spread? and how does it appear
through multiple cycles of asexual spore production during the cropping season causing necrotic spots on leaves and panicles thus devastating rice yields.
Magnaporthe grisea - rice blast disease: interacts with the host on a
‘gene for gene basis’
Magnaporthe grisea - rice blast disease: the fungus consists of
many races which attack host genotypes with different resistance genes, thus host resistance genes often remain effective for only a few years before pathogens evolve a new race which can overcome the resistance.
Magnaporthe grisea - rice blast disease: GENETIC DIVERSITY & DISEASE CONTROL IN RICE (explanation & hypothesis)
ZHU ET AL 2000
- the use of more than one cultivar in fields of a particular crop (genetic heterogeneity ) is likely to provide better disease control than monocultures
- a study was carried out by farmer & researchers in YUNNAN PROVINCE CHINA to test this hypothesis
- Mixtures of rice crops susceptible and resistant to rice blast disease (Magnaporthe grisea) were planted in all rice fields in five townships in 1998 and 1999.
- Control plots consisted of monocultures of the cultivars used to allow the effect of crop diversity on rice blast disease to be calculated.
Magnaporthe grisea - rice blast disease: GENETIC DIVERSITY & DISEASE CONTROL IN RICE: results and experiment
ZHU ET AL 2000
• Two glutinous ‘sticky’ rice cultivars – Huangkenuo and Zinuo.
• These have high market value but are very susceptible to rice blast
• Two non glutinous hybrid rice cultivars Shanyou 63 and Shanyou 22.
• These are are less susceptible to rice blast and are attacked by a different spectrum of races of M. grisea.
-When grown in monoculture disease severity was much greater on the susceptible cultivars ( H and Z) in all areas and years than when grown with a more resistant hybrid cultivar (S63 or S22).
Biological control of diseases
some fungi and bacteria possess characteristics which make them useful for the control of other fungal & bacterial pathogens of crops
Biological control of diseases: Modes of action:
- competitive ability
- Antibiosis
- Mycoparasitism (fungi only)
biopesticides:
diseases controlled by diseases
biocontrol of fungal diseases by Trichoderma harzianum
- The strain, T-22, protects root systems against Fusariam, Pythium & Rhizoctonia diseases on a number of crops
- T-22 forms protective barrier against other fungi (not pathogenic to crop/plant) attacking roots (mycoparasite)
- T-22 is able to grow in a range of soil types at temps above 50 degrees fahrenheit
- T-22 has been commercially developed as a biofungicide F-Stop.
crown gall is caused buy
Agrobacterium tumifaciens
crown goal disease is an economically important disease in
nurseries
how is crown gall disease treated:
by treating seedlings with a related strain of Agrobacterium radiobacter K84
- the strain produces a bacterocin which specifically kills Agrobacterium tumifaciens
Biocontrol of Heterobasidion annosum by Phlebiopsis gigantea:
what is H. annosum
the most damaging root pathogen of coniferous trees in the Northern hemisphere
Biocontrol of Heterobasidion annosum by Phlebiopsis gigantea: H. annosum spores infect
cut tree samples
Biocontrol of Heterobasidion annosum by Phlebiopsis gigantea: H. annosum spreads progressively by:
- contact of healthy roots with infected roots, caused disease gaps.
- progresses from the roots into the base of a tree, causing an economically important butt rot
- once established in a site, the fungus is almost impossible to eradicate
Biocontrol of Heterobasidion annosum by Phlebiopsis gigantea: how is it controlled?
by inoculating stumps with spores of P. gigantea
- The hyphae of this fungus antagonise the hyphae of H. annosum on contact - a phenomenon termed hyphal interference
- In addition P. gigantea degrades cellulose and lignin allowing colonisation by Trichoderma species
- Trichoderma species produce antibiotics which help to kill H. annosum
Biocontrol of Heterobasidion annosum by Phlebiopsis gigantea: P. gigantea is commercially available in
Britain, Sweden, Norway, Switzerland & Finland, as either a spore suspension or a dry product (named “Rotstop”)
antibiosis:
an antagonistic association between two organisms (especially microorganisms), in which one is adversely affected.
mycoparasitism
Parasitism of one fungus by another
