1/22/2015. Introduction. Pesticide resistance. Pathogen population size. Dispersal of inoculum. Dispersal of inoculum

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1 PLP 6404 Epidemiology of Plant Diseases Spring 2015 Lecture 4: Influence of Pathogen on Disease Development airborne pathogens Prof. Dr. Ariena van Bruggen Emerging Pathogens Institute and Plant Pathology Department, IFAS University of Florida at Gainesville Introduction Pathogen characteristics that influence epidemic development: Pathogen population size (number of propagules) Level of virulence (to a particular host) Reproduction (sexual, asexual, rate) Ecology (autecology, synecology) Pesticide resistance (fungicides, antibiotics) Dispersal mechanisms Pathogen population size The greater the number of propagules near susceptible hosts, the earlier the epidemic starts (becomes visible) Disease incidence High inoculum Pesticide resistance When a fungicide-resistant strain of the pathogen emerges, the effect on the epidemic is a shift of the epidemic to the right compared to not using a fungicide. The epidemic rate will be the same (assuming that the resistant strain is as fit as the sensitive strain) No fungicide Low inoculum Disease severity Fungicide, pathogen resistant Fungicide, pathogen sensitive Time Time Louis Pasteur found biological particles in air; designed a simple air-sampling system in 1861! Since then the air has been studied extensively for microbes; Charles Lindberg exposed hand-held Petri dishes in flight Aerobiology is an interdisciplinary study of the biological components of the atmosphere. The emphasis is on the release, transport, atmospheric interactions, and deposition of microbes. Dispersal (Zadoks & Schein) is that part of the infection cycle that is composed of: liberation (takeoff) transport (flight) deposition (landing, catch) The spread of disease is usually from numerous, successive acts of dispersal. Without dispersal, there is NO disease 1

2 Fungal spores, pollen, seeds, etc. are passively dispersed by wind, water splash, running water, insects, man, etc Viruses are dispersed by insects, nematodes, sometimes soil particles or water Bacteria are mainly dispersed by splash dispersal and then carried further away in droplets dispersed by wind Splash dispersal: many small droplets emerge around the point where a larger droplet impacted With dispersal, there is a scatter around the point of liberation. The process of dispersal is influenced by many factors; the destination of a single spore or bacterium cannot be determined The AVERAGE distribution of spores or bacteria over an area and over a period of time can be analyzed Inoculum decreases as one gets farther away from the source (we will discuss the shape of the dispersal kernel later). Liberation Wind is the primary factor for spore removal Threshold of wind speed must be exceeded before the first spore is liberated, depending on spore mass, shape, height above surface (length of conidiophore), and strength of attachment Some fungi are readily dispersed by wind, some difficult or not: e.g., Erysiphe spp.(force 100X spore wt.) vs Helminthosporium spp. (force 2000X spore wt.) Other liberation mechanisms: RH, IR (infra-red radiation), vibration, electrostatic forces, and the hygroscopic movement of conidiophores Liberation There is a boundary layer of static air around leaf surfaces (and other objects) which is of particular interest to epidemiologists (more in lecture 10). wind sp. b.l. leaf A strong wind of 25m sec-1 above the crop canopy may average only 5m sec-1 at the leaf surface. Wind is NOT steady, but it occurs in gusts, or puffs, and the maximum speed, even for a split second, is enough for spore liberation (exceeding the threshold of detachment force). So, TURBULENCE enhances spore removal. Liberation Once spores are deposited on a leaf after dispersal, they can be removed again by wind, i.e., reentrained. The same forces are needed for reentrainment as for original removal, but now the spores are in the phylloplane rather than extending into or beyond the boundary layer. If the leaf surface is sticky, then reentrainment is more difficult. There is also electrostatic attraction; plants have a negative charge (-), spores have a positive charge (+). Liberation - transport The number of spores in the air can be expressed as concentration (C t m -3 ), usually monitored by spore traps The number of spores in the air depends on the number of spores at the source (plot, field, cull pile) expressed as Q t. C t is also a function of transport, f (T). C t m -3 = Q t * f (T). We cannot estimate the source strength, Q, if we have measured C, because of incomplete knowledge of f (T). Quantification of transport is difficult because of various wind vectors over time and diffusional characteristics of the atmosphere. 2

3 Transport of inoculum An aerosol of spores can be characterized by several variables: Source strength: the quantity of spores of the pathogen that can be airborne and remain viable; Decay rate: the reduction of the concentration in the aerosol as a function of age. Spore or particle size: particles less than 1 m remain airborne for long periods, very large spores and particles settle out quickly. Relaxation time: 1) the time needed to bring a spore up to moving speed or 2) the time needed for a spore to come to a stop. Transport of inoculum Friction in the air will slow down a horizontally moving object. Relaxation time τ = time needed to reduce speed v of a horizontally moving spore to v/e where e=euler s constant ~ 2.7 Relaxation time is an example of a time constant [T], the inverse of a rate of change [T -1 ], in this case settling rate Relaxation time is determined by the speed and frictional resistance, which is larger when the surface to mass ratio is larger Spore landing: Relaxation time There is an interesting comparison of the surface : mass ratio of man vs. the surface : mass ratio of a spore: Man= surface/mass = 1.5m 2 /70 kg = 0.021m 2 /kg Spore (10 m radius) = 4.2 m 2 /14 g= 300m 2 /kg The surface : mass ratio for a spore is about 10,000-fold greater than for a man; thus wind (as counterforce friction or as moving force) has a great effect on a spore, little effect on a man walking. How do the spores get out of the air? The most important factor for the removal of spores from the air is gravity. As an object falls through the air, it increases in speed to a point where the resistance on the falling spore is balanced by the pull of gravity. This speed is terminal velocity. Stokes law If the weight of a falling object does not change, then the terminal velocity is constant until the object reaches a solid surface. Consider a skydiver, who can control his terminal velocity by altering his resistance in the air by extension of his arms and legs, and horizontal position. A fungal spore may become desiccated in falling, and then this loss of weight would slow its descent (spore is less dense). Stokes described the relation between the size and density of a sphere and its terminal velocity. Stokes law was: V = 2/9 * [(p-o)/ ]*gr 2 ; where: V = terminal velocity (cm/sec) p = density of sphere (for spores, assume p = 1) o = density of medium (negligible) g = acceleration due to gravity (981 cgs; cm-g-sec) r = radius of sphere (in ) = viscosity of medium (air = 1.8*10-4 ) 3

4 For spores, p (density) is difficult to determine as it depends on hydration. Any deviation from a smooth, spherical form also affects velocity. A spore with spines or appendages or a rough surface would fall more slowly than a smooth spore of the same weight and radius of nearly spherical spores of the agaric mushroom were tested in moist air. The spores fell 50% faster than predicted by Stokes law. Why?? Droplets of water were found on spores when they were discharged, increasing the effective size and weight of the spores. When these spores were dried, they fell one-third as fast as turgid spores Example of a skewed distribution The terminal velocities of many fungal spores are in the range of cm/sec. For many spherical spores, pollen grains, etc., researchers have found Stokes law to be exact. A population of cereal rust spores has a skewed distribution for density and terminal velocity. This is because rust spores frequently occur in clumps of 2, 3, or more spores. The clumps of spores would fall faster than individual spores. Skewed rain drop size distribution Horizontal: diameter Vertical: number of drops per unit diameter and per volume of air Relatively few large drops Niu et al., 2010 Stokes law applies only to smooth, spherical particles. Most fungal spores are NOT isodiametric, they are usually elongate. The length of spores is frequently 10 to 20 times the width. Many elongated spores fall horizontally (the greatest surface area of the spore is presented to resistance). Spores of Alternaria spp. fall with their long axes vertical. The beak of the spore feathers the fall like a shuttle cock. Many fungal spores fall about 1 cm/ sec. Question: Do elongated spores tumble in falling? Besides sedimentation according to Stokes law we distinguish: Turbulent deposition Rain deposition Impaction and Interception 4

5 Quantification of inoculum General monitoring considerations: Target population of propagules to be monitored Dispersal and survival of propagules Epidemiological importance of propagule Propagule viability and infective ability (efficacy) Density of inoculum and collection efficiency (number per volume of air, water, or soil) Frequency and timing of sampling Location Identification Quantification of inoculum Quantification techniques similar to those used for sampling dust and pollen (1-40 µm) Methods based on: settling (gravity) impaction (inertia) By suction into the sampler By a moving sampler Quantification of inoculum: settling Quantification of inoculum: forced air impaction Intake errors: Undersampling (wind speed > suction speed) Horizontal collection surfaces Glass slides, petridishes Anderson sampler (six stages); can also be impaction trap if air is sucked through Isokinetic sampling (wind speed = suction speed) Oversampling (wind speed < suction speed) Quantification of inoculum: forced air impaction Quantification of inoculum: forced air impaction Burkhard spore trap adhesive-coated tape on drum, with 7-day clock overall high efficiency samples 10 L/min relatively expensive High-volume cyclone sampler Can sample larger volumes of air 16.8 L/min collects particles directly into Eppendorf tube multi-vial sampler or single-vial sampler 5

6 Quantification of inoculum: moving samplers Rotorod sampler high-speed whirling arm trap, inertial impaction Very efficient for spores 20 µm diam. Not significantly affected by wind speeds (<6.2 km/h) Mechanically simple, lightweight Collection efficiency depends on spore density and size Can only be operated continuously for short periods Distribution of spores trapped during a day Cercospora spores trapped during a day RH drops in the morning spore release If there is rain, more spores may be released or washed out Bait plants Susceptible plants are more likely to collect pathogen spores than any artificial samplers they sample large volumes of air (depending on exposure time) they are the real target propagules are viable and infective but only semi-quantitative timing not known, unless you replace plants frequently Factors to consider: Biological (mode of propagule dispersal, spore size, spore concentration) Economic (cost, ease of use, time period, ease of replication) Conclusion Many pathogen characteristics determine spread of disease and epidemic development Which factors? Dispersal consists of three phases Which phases? How do propagule characteristics affect these phases? Which sampling technique is the best? Depends on: 6