Biology and Control of Root-Infecting Pathogens in Greenhouse Production

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Biology and Control of Root-Infecting Pathogens
in Greenhouse Production

• Michael E. Stanghellini
• Professor and Chair
• Department of Plant Pathology
• University of California
• Riverside

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Root-Infecting Pathogens

• Place constraints upon production and
  marketability
• Management is based upon preventive and curative
  control strategies
• Preventive strategies are preferred (i.e.,
  exclusion)
• Exclusion is based upon knowledge of the
  source(s) of pathogen introduction into and
  spread within a commercial facility

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Sources of Pathogen Introduction or Spread

• Airborne
• Planting stock or seed
• Potting medium or soil
• Irrigation water
• Insects

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MAJOR OBJECTIVES

• To determine the significance of insect vectors
  in the spread of root-infecting pathogens in
  greenhouses and to develop strategies for their
  control. (Conduced by Dr. Zeinab El-Hamalawi,
  postdoctoral researcher)
• To evaluate biological surfactants for the
  control of root-infecting zoosporic pathogens in
  recycled irrigation water. (Conducted by
  Carrieann Nielsen, Ph.D. graduate student)

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Insect Vectors vs Root-Infecting Pathogens

• SHORE FLIES
• FUNGUS GNATS
• MOTH FLIES

• VERTICILLIUM
• FUSARIUM
• THIELAVIOPSIS

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Why these particular insects?

• They are among the most abundant insects in the
  greenhouse
• They have been historically regarded merely as a
  nuisance
• They have recently been implicated as aerial
  vectors of some soil-borne, root-infecting fungi

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Aerial Transmission of Root-Infecting Pathogens

• Adult Shore Flies Adult
  Fungus Gnats
• Pathogens
• Fusarium ---
  Yes
• Pythium Yes
  No
• Thielaviopsis Yes
  ---
• Verticillium ---
  Yes

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Why these particular pathogens?

• They are all root-infecting fungi
• They all cause significant disease problems in
  commercial greenhouse facilities
• They all produce spores on the lower stem of
  infected plants (i.e., above-ground life stage)

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Significance of an above-ground life stage to a
root-infecting pathogen

• In general, soil-borne, root-infecting
  pathogens (compared to foliar pathogens) have a
  slow rate of spread within a plant population. An
  above-ground life stage of a soil-borne pathogen,
  however, could significantly increase the rate of
  spread via air dispersal or dispersal by insects.

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Common Characteristics

• All three insects can complete their entire life
  cycle (i.e., egg to egg) solely on a diet of each
  of the three fungal pathogens.
• Larvae of all three insects ingest and excrete
  high populations of viable (gt90 germination)
  spores of each of the three fungal pathogens.

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Research Emphasis Adult Shore Flies

• Stronger fliers and potentially more
• dangerous than fungus gnats as an aerial
  vector
• Externally and internally contaminated with
  fungal pathogens
• Internally-infested larvae give rise to
  internally-infested adults

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Interactions Assayed

• Attraction of adult insects to the pathogen
• Pathogen acquisition by adult insects (external
  or internal contamination)
• Pathogen retention after acquisition
• Pathogen distribution (temporal and spatial)
• Pathogen viability after distribution

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Attraction
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Preferential Attraction of Adult Shore Flies to
Various Substrates
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Pathogen AcquisitionIngestionExternal
Contamination
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Acquisition of Fusarium by Adult Shore Flies
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Pathogen Viability after Excretion by Adult Shore
Flies

• Verticillium
  Fusarium Thielaviopsis
• No. of frass deposits
• per adult shore fly 46
  40 39
• No. of spores/frass
• deposit 530,240
  80,320 235,520
• Germination of
• excreted spores () 96
  95 92

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Retention
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Retention of Verticillium After Ingestion by
Adult Shore Flies
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Distribution in Time and Space
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4.5 hr
Shore fly spatial distribution
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4.5 hr 9 hr
Shore fly spatial distribution
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4.5 hr 9 hr 18 hr
Shore fly spatial distribution
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Temporal Distribution By Adult Shore Flies
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Origin of Thielaviopsis colonies
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Verticillium conidia on the leg of an adult shore
fly
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Fusarium macroconidia on the wing of an adult
shore
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Thielaviopsis endoconidia on the wing of an adult
shore fly
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Adult shore flies on a plant leaf
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Aerial Transmission of Soil-Borne Pathogens by
Adult Shore Flies

• 
  Diseased Plants ()
• 
  Basil Carrot Geranium
• Treatments
• Verticillium ---
  --- 0
• Verticillium insects ---
  --- 50
• Fusarium 0
  --- ---
• Fusarium insects 75
  --- ---
• Thielaviopsis ---
  0 ---
• Thielaviopsis insects ---
  86 ---

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Fungus Gnatsand Moth Flies
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Pathogen Acquisition by Adult Fungus Gnats and
Moth Flies

• External Contamination Only

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Fungus gnat
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Macroconidia of Fusarium on leg of an Adult
Fungus Gnat
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Conidia of Verticillium on leg of Fungus Gnat
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Thielaviopsis endoconidia on leg of adult fungus
gnat
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4.5 hr
Fungus gnat spatial distribution
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4.5 hr 9 hr
Fungus gnat spatial distribution
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4.5 hr 9 hr 18 hr
Fungus gnat spatial distribution
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Temporal Distribution By Adult Fungus Gnats
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Temporal Distribution By Adult Shore Flies and
Fungus Gnats
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Verticillium conidia on the wing hairs of moth fly
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Fusarium macroconidia on the body of adult moth
fly
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Thielaviopsis endoconidia on adult moth fly wing
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4.5 hr
Moth fly spatial distribution
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4.5 hr 9 hr
Moth fly spatial distribution
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4.5 hr 9 hr 18 hr
Moth fly spatial distribution
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Temporal Distribution By Adult Moth Flies
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Temporal Distribution by Adult Shore Flies,
Fungus Gnats and Moth Flies
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New Research Area

• Biological Control of Adult Insects
• via
• an Entomopathogenic Fungus

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MAJOR OBJECTIVES

• To determine the significance of insect vectors
  in the spread of root-infecting pathogens in
  greenhouses and to develop strategies for their
  control. (Conduced by Dr. Zeinab El-Hamalawi,
  postdoctoral researcher)
• To evaluate biological surfactants for the
  control of root-infecting zoosporic pathogens in
  recycled irrigation water. (Conducted by
  Carrieann Nielsen, Ph.D. graduate student)

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Surfactants vs Zoosporic Root-Infecting Pathogens

• Zoospores are the primary root-infecting spore
  stage of Pythium and Phytophthora
• Zoospores, naked protoplasts, are the weak-link
  in the life cycle because they have no cell wall
  for protection
• When exposed to synthetic surfactants, zoospores
  are rapidly killed
• Synthetic surfactants are currently used as a
  chemical control strategy in cultural systems
  employing recycled irrigation water

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Objective

• To evaluate the efficacy of a naturally-occurring
  biosurfactant-producing bacterium (as well as
  the biosurfactant) for the control of zoosporic
  pathogens in recycled irrigation water

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Problem

• The biosurfactant-producing bacterium was
  identified as a quasi-human pathogen and
  therefore we turned our attention to evaluation
  of the biosurfactant ( which we identified as a
  rhamnolipid )as a biochemical for use in control
  of zoosporic pathogens.
• We identified a commercial company that produces
  large quantities of that biosurfactant and they
  are currently registering the rhamnolipid as a
  biopesticide. Product will be called
• ZONIX

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Biosurfactant Data

• The biosurfactant , when added to recycled
  irrigation water, lysed zoospores and was shown
  to be efficacious in disease control.

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Problem

• The biosurfactant is subject to rapid
  biodegradation when incorporated into the
  nutrient solution in the reservoir.
• Thus, we are now injecting the biosurfactant
  directly into the nutrient solution only during
  an irrigation event.

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Direct Injection System
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Direct Injection of Zonix
Percent mortality of pepper plants after
hypocotyl inoculation of one plant in
drip-irrigated recirculating cultural units with
Phytophthora capsici
Irrigation for 20 minutes/day
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Current Research Emphasis

• Identification of the optimal dosage and timing
  of direct injection of the biosurfactant into the
  irrigation water (rather than the reservoir).
• Effect of the duration of irrigation and the
  time of irrigation (a.m. or p.m.) on disease
  progression.

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Effect of Irrigation Duration on Disease Progress
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Effect of Irrigation Duration on Disease Progress
Disease onset occurred 2 weeks earlier with
longer irrigation and spread throughout the
system about three times faster than when shorter
irrigation duration was implemented
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The Influence of A.M. or P.M. Irrigation on
Disease Progress
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The Influence of A.M. or P.M. Irrigation on
Disease Progress
day
Disease onset occurred about 3 weeks earlier and
spread throughout the system about seven times
faster when irrigated at night rather than during
the day
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Results

• increases in the duration of irrigation enhance
  the onset and severity of disease
• there is a diurnal periodicity that influences
  disease incidence and severity.