Plant Pathogens and Biocontrol Agents

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Plant Pathogens and Biocontrol Agents
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Plant Pests

• Pathogens
• Predators
• Weeds

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Symptoms of Microbial Diseases in Plants

• Necrosis - death of plant cells may appear as
  spots in localized areas
• Canker - localized necrosis resulting in lesion,
  usually on stem
• Wilt - droopiness due to loss of turgor
• Blight - Loss of foliage
• Chlorosis - loss of photosynthetic capability due
  to bleaching of chlorophyll
• Hypoplasia - stunted growth
• Hyperplasia - excessive growth
• Gall - tumor

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Anton de Bary

• German botanist
• Investigate Irish potato blight in 1861
• Proved experimentally that Phytophthora infestans
  was actually the cause of the disease

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Birth of Plant Pathology

• de Bary gave birth to the science of plant
  pathology
• Soon other plant pathogenic fungi were described
• Pathogenic bacteria and viruses were identified
  later in the 19th century

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Dispersal of Pathogens

• Necessary for repeated cycles of infection and
  multiplication and, therefore, the spread of an
  epidemic
• Understanding of dispersal phase is necessary for
  predicting the onset and severity of disease
• Within the realm of dispersal are the processes
  of release, transport, and deposition
• Pathogens are dispersed by wind, rain, soil
  water, insects, nematodes and humans

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Scope of Dispersal

• Spread of plant disease can proceed over short
  distances through focal spread as well as over
  long distances
• Terminology of disease spread
• Microscale
• Mesoscale
• Synoptic scale (macroscale

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An infection focus

• Area of a crop with a contagious disease
• Foci often circular
• If strongly affected by wind, may be comet-shaped
  or v-shaped
• generally have a constant radial expansion
• a few cm per day for a localized infection
• hundreds of km per year for a pandemic

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Dispersal Mechanisms

• Airborne Dispersal
• Passive discharge
• Active discharge
• Rain splash

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Long Distance Transport

• Explains the introduction of a pathogen to a new
  area
• Also explains the yearly reintroduction to areas
  where overwintering cannot occur
• Most of the well studied examples of long
  distance transport involve fungal spores
• Due to environmental hazards only a small percent
  of spores are able to survive long range
  transport

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Boundary layer

• Before spores reach the free air above the crop,
  they must pass through the boundary layer
  surrounding the crop
• Possibly as much as 90 of the spores are
  deposited within the crop itself
• The percent that escape from the canopy depends
  on the balance between deposition and turbulence
  with greater escape during more turbulent winds
• Position in canopy also important

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Pathogens Viruses

• Transmission of viruses
• Insect vectors - especially aphids, whiteflies,
  leafhoppers, mealybugs, ants
• Nematodes
• Seeds from infected parent plants
• Airborne transmission
• Infected plant parts
• Aphids
• Pollen

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Pathogens Bacteria

• Generally Gram-negative bacilli species of
  Erwinia, Pseudomonas, Xanthomonas, Agrobacterium,
  and Corynebacterium
• Dispersal from plant generally passive by water,
  wind-blown water, animals, agricultural workers
• In warm, humid climates, where dew and rain are
  common, dispersal of bacteria by rain-splash is
  the major means of disease spread
• Airborne spread on rafts of plant material

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Pathogens Fungi

• Over 70 of all major crop diseases are caused by
  fungi
• Thousands of fungal species recognized as plant
  pathogens
• Fungal diseases cost more than 3.5 billion to US
  farmers alone
• In general spores of most fungal pathogens are
  adapted for airborne transport

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Late blight of potato (and tomato)

• Caused by oomycete Phytophthora infestans
• Occurs wherever potatoes are grown
• All potato cultivars are susceptible
• Without fungicidal protection, a blighted field
  can be destroyed within a couple of days

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Cool, wet weather promotes disease spread

• Method of germination is dependent on
  meteorological conditions
• Cool, wet weather promotes zoospores
• Warmer, drier conditions promote germination of
  the sporangium itself

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Potato late blight forecasting

• 40 year history in many potato growing regions
• When meteorological conditions indicate that
  Phytophthora spread is likely to occur warnings
  are issued to apply fungicides
• Forecasting models have been successful in
  reducing the number of fungicide applications.

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Long distance dispersal ofPhytophthora infestans

• Intercontinental migration has been associated
  with the transport of infected plants or tubers
  by humans
• This occurred in the 1840s and again before the
  1980 outbreak of the A2 mating type
• Long distance dispersal over tens of kilometers
  is attributed to wind-blown sporangia
• Maps showing the rapid progress of blight
  epidemics in the 1840s suggest that a
  Second-Order Epidemic of late blight could
  possibly occur during a single growing season.

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Tobacco Blue mold

• Caused by oomycete Peronospora tabacina
• Unpredictable disease of both wild and cultivated
  tobacco causing devastation some years and not
  appearing at all during other years

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Blue mold

• First described in Australia during 19th century
• In North America the disease was confined to
  seedbeds until 1979
• 1979 the first serious epidemic occurred

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North American Epidemics

• The infection rate was especially severe in both
  1979 and 1980
• Second-Order Epidemic advancing at rates of 10-32
  km/day northward in the eastern United States to
  southern Canada
• Crop losses in the U.S. and Canada during these
  two years were estimated at approximately 350
  million

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Source areas

• Both host plants and pathogen exist year-round in
  tropical areas such as the Mediterranean and
  Caribbean basins.
• In temperate regions, tobacco is grown as an
  annual
• P. tabacina is not able to overwinter in
  temperate zones
• As a result, the long distance transport of
  inocula from tropical regions

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Disease Cycle

• Infection can occur within four hours after a
  sporangium lands on the leaf
• Symptom-free incubation period 5-7 days
• Appearance of yellow lesions and the development
  of new sporangia

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Conditions for Dispersal

• Cool, wet, overcast weather, favors the rapid
  advance of the fungus
• Clear, hot, dry weather stops disease spread

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Dispersal

• Each spring in U.S. weather conditions are
  favorable for the northward transport of
  Peronospora sporangia from southern sources
• Case studies of epidemics occurring from 1979 to
  1986 suggest at least two likely pathways of
  disease spread
• Northward from Florida and Georgia, Cuba
• North and Eastward from south Texas and Mexico
• Forecasting systems can potentially provide time
  for tobacco farmers to apply fungicides

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Forecasting Blue Mold

• Predictive models make use of extant disease
  outbreaks, weather fronts, and weather forecasts
• HY-SPLIT trajectory model successfully used since
  the spring and summer of 1995 to predict
  outbreaks of blue mold
• The model used to plot trajectories of
  inoculum-laden parcels of air

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The Daily Blue Mold Forecast

• Forecast produced by HY-SPLIT describes future
  weather conditions at the source and along the
  anticipated pathway
• Emphasis given to atmospheric conditions that
  favor sporulation at the source, survival during
  transport, and deposition
• Overall outlook describes the likelihood of blue
  mold spread over the next 48 hours
• Available on-line at www.ces.ncsu.edu/depts/pp/bl
  uemold/

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Rust Fungi

• About 6000 species
• Attack a wide range of host
• Cause some of the most destructive plant diseases
• Basidiomycetes but no fruiting body

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Coffee Rust

• Destroyed coffee plantation in Ceylon in 1870s
  and 1880s
• Today threatens coffee wherever it is grown
• 1966 outbreak in Angola produced spores that were
  carried across the Atlantic Ocean and washed out
  by rain in Brazil 5 to 7 days later

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Wheat rust fungi

• Stem rust (black rust) caused by Puccinia
  graminis f.sp. tritici
• Leaf rust (brown rust) caused by Puccinia
  triticina (syn. P. recondita f.sp. tritici)
• Stripe rust (yellow rust) caused by Puccinia
  striiformis.

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Puccinia graminis f.sp. tritici

• Globally the most serious rust pathogen
• Complex life cycle with 5 spore stages
• Major method of disease spread is by the
  uredospores which are easily carried by wind for
  hundreds or thousands of kilometers

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Puccinia graminis Life Cycle
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Wheat stems showing uredia (lesions) with
uredospores
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Uredium with uredospores
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Telial Stage
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Source Strength and Viability

• Mature uredium produces 10,000 uredospores/day
  over several weeks
• A 5 disease severity would produce 50 uredia
  so 500,000 uredospores/day
• Field of wheat with moderate infection 4 x 1012
  uredospores/day/hectare
• Nagarajan and Singh reported that spores had to
  be deposited within 120 hrs after takeoff to be
  infectious

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Survival of Spores

• Eversmeyer and Kramer studied survival of
  uredospores in field
• At subfreezing temperatures during winter no
  spores viable after 96 hrs
• During spring 10 to 20 viable after 120 hrs 1
  survived for 456 hrs
• In growth chamber spores viable for up to 864 hrs
  at temp between 10 and 30 C
• Although atmosphere conditions harsh, small
  percent will survive

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US Source Areas

• In south Texas and Mexico uredospore stage can
  survive all winter on winter wheat
• Gives rise to spring infection
• Spores carried north by southerly winds to
  northern states where spring wheat grown
• In late summer and fall uredospores can be
  carried back to southern areas
• Puccinia Pathway studied since 1920s Stakman
  first described the aerial dispersal

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Puccinia Pathway
Cereal Disease Laboratory, USDA-ARS, Univ Minn,
St. Paul
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• In some years the pathogen is spread gradually by
  anticyclones making a succession of short jumps
  with stops along the way where the inoculum
  multiples
• (Stakman and Harrar, 1957 Isard and Gage,
  2001)

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• In other years uredospores are transported by
  extratropical cyclones over a distance of 1000 km
  or more in 1 or 2 days
• In early June 1925 a huge spore cloud move 1,000
  km northward
• Spores were caught in traps throughout the area
  (previously rust-free)
• Field observations indicated that infection
  throughout the region was almost simultaneous
• (Stakman and Harrar, 1957 Isard and Gage,
  2001)

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LDT of wheat rust in Europe

• Two pathways studied
• An east European path originating in Turkey and
  Romania
• A western path from Morocco and Spain
• Both paths converge in Scandinavian countries

Nagarajan and Singh. 1990. Annual Review of
Phytopathology 28 139-153
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Brown and Hovmoller. 2002. Aerial dispersal of
pathogens on the global and continental scales
and its impact on plant disease. Science, 297
537-541
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Other Areas

• LDT of uredospores between eastern and western
  wheat growing areas of Australia
• Overseas LDT also studied - two strain in
  Australia identical to strains found in Africa in
  terms of pathogenicity and isozyme patterns
• LDT of uredospores in India
• LDT of uredospores of P. striiformis in China

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Brown and Hovmoller. 2002. Aerial dispersal of
pathogens on the global and continental scales
and its impact on plant disease. Science, 297
537-541
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Brown and Hovmoller. 2002. Aerial dispersal of
pathogens on the global and continental scales
and its impact on plant disease. Science, 297
537-541
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Asian Soybean Rust

• Caused by Phakopsora pachyrhizi
• Most destructive foliar pathogen on soybean and
  reports of loss range from 10 to 80.
• Yield losses over 50 are common when
  meteorological conditions favor disease
  development

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Disease effects

• The fungus causes numerous uredial lesions on the
  leaves
• Reduces photosynthetic capacity of the host and
  subsequently reduces the yield of soybeans
• Simple life cycle with uredospores and
  teliospores on same host
• Phakopsora pachyrhizi can infect more than 95
  species of plants including other edible legumes
  and also weeds as kudzu

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Phakopsora pachyrhizi history

• First described in Japan in 1902
• By the mid-1930s the pathogen reported from
  several other countries in Asia and in Australia
• India in the early 1950s.
• By late 1990s pathogen had spread to several
  countries in Africa
• 2001 reported from Paraguay and Brazil
• Over the next few years, the pathogen spread
  through much of the soybean growing areas of
  South America causing significant yield losses
• First detected in the continental United States
  in November 2004

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Plant Pests and Their Control by Fungi and
Bacteria
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Plant Pathogenic Nematodes

• Obligate parasites
• Feed on roots of plants - may cause malformations
• Many have a sharp stylet that pierces plant cells
• Some never live in soil, they survive in host and
  are spread by insect vectors
• Reduces crop yield and increases risk of
  infection through wounds

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Insects

• Our greatest competitors for food
• Damage or destroy crops before and after harvest
• Larval stage often most destructive
• Injury plants directly by using plant for food or
  shelter and indirectly by spreading pathogens

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Basic feeding patterns

• Chewing Insects
• Either larvae or adults
• Tear or bite portions of the plants
• May eat their way through the plant causing holes
  and tunnels
• Lvs left as skeletons by some
• Others eat whole plant
• Sucking Insects
• Pierce the plant and sucks up the sap
• Results in curling, stunting, deformed parts

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Weeds

• Unloved plant
• Injurious to agricultural crops
• Loss is a direct result of competition for light,
  water, nutrients
• Unchecked can dominate crop plants
• Indirectly damages by harboring insect pests
• Crop losses by weeds in US 14 billion

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Control Measures

• We are dependent on healthy plants to feed the
  worlds population
• Chemicals widely used to control plant pests and
  diseases
• Dangers of pesticide use apparent
• Economic cost of pesticides may actually outweigh
  the value of the crop at harvest time
• Use other techniques to reduce pesticide use

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Integrated Pest Management (IPM)

• Multifaceted approach to disease control
• Sanitation
• Crop rotation
• Genetic resistance
• Biological Control

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Biological Control

• Use of living organisms to reduce disease due to
  competition or antagonism
• i.e.. ladybugs to control aphids
• Ultimate aim is to reduce dependence on chemicals
• Today emphasis on microorganisms
• Bacillus thuringiensis for insect control
• Several Pseudomonas species for control of
  bacterial and fungal pathogens
• Numerous fungi for insects, nematodes, fungal
  pathogens

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Bacillus thuringiensis

• Common soil bacterium well known for its ability
  to produce crystalline proteins with insecticidal
  properties
• Since 1960s Bt available as a safe naturally
  occurring biopesticide
• sold as a dried inoculum containing endospores
  and crystals of insecticidal proteins
• used as sprays or dusts for a wide variety of
  insects - especially Lepidopteran

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Bt Toxins

• Genes for insecticidal proteins on plasmids
• Many subspecies of Bt which differ in number and
  type of plasmids
• Over 1000 strains of Bt have been isolated
• family of toxins
• over 200 insecticidal proteins identified and
  sequenced

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Bt Toxins

• Toxins activated by enzymes in insect gut
• Kill insects by binding to membranes in digestive
  system and creating pores in membranecontents
  leak into body cavity
• Harmless to humans, natural enemies of
  arthropods, and non-target organisms

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Bacillus thuringiensis

• B.t. subspecies kurstaki is widely used in
  caterpillar control in agriculture and forestry
• B.t. subspecies israelensis is active against
  mosquitoes and black flies
• B.t. subspecies tenebrionis is active again
  beetle larvae

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Bt Uses

• Spray Applications
• Bt toxins degrade within a few days
• Endospores can survive for several years after
  spray applications
• Genetic Engineering with Bt genes
• Transfer into crop plants
• Transfer other bacteria

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Pseudomonas species

• Pseudomonas fluorescens for control of fire
  blight (also may control apple blue mold)
• Fire blight bacterial disease of apples and
  pears caused by Erwinia amylovora
• Pseudomonas out competes Erwinia on stigma
  surface
• Reduces use of streptomycin which has been
  helpful since many Erwinia strains resistant