Progress 05/04/98 to 05/03/04
Outputs
Perceived and real financial risk due to crop loss can be a serious barrier to the widespread adoption of integrated pest management (IPM) farming practices. Informed disease management decisions depend on both knowledge of local (in-field) inoculum levels and on accurate evaluations of "immigrant" inoculum coming from afar. The objective was to develop a biophysically-based model of pathogen dispersal to quantify amounts of inoculum arriving in a field from nearby and distant sources, to evaluate relative probabilities of alternative methods of dispersal, and to determine whether and when the obstacles of space and time can limit the spread of a pathogen enough to render insignificant its impact on crop yield before harvest. Two types of models were used to examine transport over a wide range of distances. A Lagrangian particle trajectory simulation model was used to evaluate spore release at the source and their transport for short to intermediate distances (less
than 1 km), and a Gaussian puff model was used to estimate transport over distances of tens to hundreds of km. Biophysical parameters, such as ascospore discharge distance, spore settling speed, washout by rain, and sensitivity to UV-B radiation was used to set criteria for choosing transport models appropriate for the scale of the problem (see, for example, Agric. For. Meteorol. 97, 275-292, 1999; J. Appl. Meteorol. 40:1196-1208; Phytopathology 91:1189-1196; Ecology 84, 1989-1997, 2003). The models also incorporate the temporal development of pathogen inoculum and the susceptibility of host tissue. Source strength for a well-defined source can be quantified using the LS model, however, the temporal development and availability of inoculum over a wide geographical area remains problematical. In gusty winds, as many as half of the spores released at or above mid-canopy height can escape from the canopy, resulting in a spore dispersal function with a long tail extending far from a
source. On the scale of a field, this tail can lead to a disease wave front that accelerates with distance from a focus. On a landscape or regional scale where hosts are discontinuous and patchy, however, wave front speed may be steady or may decelerate or even halt altogether. The seasonal expansion of a blue mold disease front over a range of distance scales (e.g., from northern Florida to Connecticut) was quantified in terms of the location of the outbreak versus the date of disease onset, and the rate for the northward advance of the disease front. This wave front speed was modeled using a combined infection-threshold finite-leap model, where the leap distance was derived using the spore dispersal model. The pathogens of apple scab, wheat stem rust, tobacco blue mold, and potato late blight were used to examine the possible existence of disease spread horizons in each case. The model suggests that there is a practical limit for the long-distance dispersal of a pathogen, which
depends strongly on fecundity and survival, and that the "green wave" of planting and the "golden wave" of harvest can impose definable constraints on the rate of propagation of the disease wave.
Impacts
Understanding the biophysical factors that control periodic reintroduction of diseases into geographical areas is important for guiding regional disease control strategies such as quarantine, sanitation, and pesticide use. By accounting both for aerial dispersal of pathogen inoculum and the seasonal development of crops, the model allows predictions of disease spread in time and space and thus allows alternative control strategies to be assessed and employed. Growers that have been apprised of these findings have renewed their efforts at on-farm sanitation of blue mold.
Publications
- No publications reported this period
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Progress 01/01/03 to 12/31/03
Outputs
Plant pathogens can be transported long distances through the atmosphere by wind and can potentially spread disease hundreds of kilometers from a source. A model of long-distance spore dispersal (Ecology 84, 1989-1997, 2003) was developed that allows this possibility to be examined. In the model, the pathogen is assumed to spread between two regions at some level of probability when a critical number of propagules of the pathogen arrive at the target region and both weather and host susceptibility allow local establishment of the pathogen. A logical outcome of the model is the possible existence of disease spread horizons--these are distances beyond which disease spread by aerial transport of the pathogen is highly improbable when compared to other possible means of transport, such as human-mediated transport. The model describes disease spread horizon distances in terms of biophysical characteristics unique to a particular host-pathogen system. For comparable size
sources of inoculum, the suggested horizon of spread for Venturia inaequalis (the cause of apple scab) is of the order of 5-10 km; for Phytophthora infestans (the cause of potato late blight), it is of the order of 40-50 km; for Peronospora tabacina (the cause of blue mold of Nicotiana spp.), it is of the order of 700-1,000 km; and for Puccinia graminis (the cause of wheat stem rust), it is of the order of 1,500-2,000 km. Source sizes and weather conditions that would allow these horizons to be realized occur relatively infrequently. The probability that disease will become established as a result of aerial spore dispersal between two regions depends on the product of the number of spores deposited on a target during a dispersal event and the probability that a single spore can start an epidemic. Usefulness of the model is currently limited by the incomplete knowledge of two factors: 1) inoculum production over large areas, and 2) the probability of pathogen establishment and
colonization on a distant host. Much work remains to be done to quantify these factors but, fortunately, both the total inoculum production and the probability of pathogen establishment enter the disease spread horizon distance calculation inside the argument of a logarithm, which substantially reduces the sensitivity of the calculation to either of them.
Impacts
Understanding the biophysical factors that control periodic reintroduction of diseases into geographical areas is important for guiding regional disease control strategies such as quarantine, sanitation, or pesticide use. By accounting for both aerial dispersal of pathogen inoculum and the seasonal development of crops, the model allows predictions of disease spread in time and space and thus allows alternative control strategies to be assessed and employed. Growers have been apprised of these findings and have renewed their efforts at on-farm sanitation of blue mold as a short-term benefit. The long-term impacts are: that plant diseases can be controlled earlier, thereby reducing damage to crops and that there will be ample supplies of fruits and vegetables available to comsumers.
Publications
- AYLOR, D.E. 2003. Spread of plant disease on a continental scale: Role of aerial dispersal of pathogens. Ecology 84:1989-1997.
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Progress 01/01/02 to 12/31/02
Outputs
Plant pathogens can be transported long distances through the atmosphere by wind and can potentially spread disease hundreds of kilometers from a source. A model of long-distance spore dispersal was developed to examine this possibility. In the model, the pathogen is assumed to spread by means of leaps between regions of host plants that are separated by many tens to hundreds of km. A leap of the pathogen between two regions is assumed to occur at some level of probability P1 when a critical number of propagules of the pathogen, Nc (P1), arrives at the target region and both weather and host susceptibility allow local establishment of the pathogen. The pathogen is assumed to leap the gap between two regions after a waiting time, T, during which the number of spores produced in the source region increases enough to counterbalance the dilution of the spore cloud and losses of spores or spore viability. The spore dispersal function used is a product of an inverse
power-law and a negative exponential of distance, the first to represent dilution due to turbulent dispersion (diffusion plus shear), and the second to represent depletion of viable spores. Disease increase in the source is assumed to follow a logistic law. The probability that disease will become established as a result of aerial spore dispersal between the two regions can be expressed as 1 - exp (-Nf) where N is the number of spores deposited on a target during a dispersal event, and where f is the probability that a single spore can start an epidemic. T is obtained by combining dispersal and disease buildup. For an obligate parasite, this probability is constrained by the location of the leading edge of the green wave of planting. T depends on the relative reproduction rate of the pathogen, the distance between regions of host plants, the initial level of infection, the carrying capacity of the source Q, a length scale for dispersal, and the probability level P1. Q, in turn,
depends on the acreage of host plants in the source region, partial resistance of host plants, unfavorable weather, or application of fungicides. T increases with increasing separation distance, but only up to a point. The model suggests that there may be a certain distance for which the probability of disease transmission is very low and may be considered to be practically zero compared to that for other potential modes of dispersal. The model also yields apparent average rates of disease spread between two regions that are in reasonable agreement with observed average spread rates for blue mold (approx. 13.9 km per day) and for wheat stem rust (35.2 km per day). Both diseases spread at rates that are similar to the rates of the northward advance of available susceptible host tissue in each crop system. The close agreement between the rates of advance of disease and of host susceptibility suggests that disease spread over long distances may be limited more often by pathogen
establishment than by dispersal. Finally, this constraint tends to reduce the stochastic variability of disease spread that might be observed in a uniformly distributed and uniformly susceptible host.
Impacts
Understanding the biophysical factors that control periodic reintroduction of diseases into geographical areas is important for guiding regional disease control strategies such as quarantine, sanitation, or pesticide use. By accounting both for aerial dispersal of pathogen inoculum and the seasonal development of crops, the model allows predictions of disease spread in time and space and thus allows alternative control strategies to be assessed and employed.
Publications
- AYLOR, D.E. 2001. Aerobiology of fungi in relation to capture and release by plants. Pages 341-361 in Phyllosphere Microbiology, S.E. Lindow, E.I. Hecht-Poinar, and V. J. Elliot, editors. APS Press, St. Paul, MN.
- LAMONDIA, J.A. and AYLOR, D.E. 2001. Epidemiology and management of a periodically introduced pathogen. Biological Invasions 3:273-282.
- AYLOR, D.E. 2003. Spread of plant disease on a continental scale: Role of aerial dispersal of pathogens. Ecology (in press).
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Progress 01/01/01 to 12/31/01
Outputs
It has long been assumed that plant pathogens can be transported great distances through the atmosphere by wind and could thus spread plant diseases hundreds, if not thousands, of kilometers from a source. Evidence for such long-distance dispersal is circumstantial at best, and attempts to quantify it are rare. To properly evaluate disease containment and control measures such as sanitation, eradication, and quarantines, it is important to determine the probability of long-distance dispersal in the atmosphere relative to other mechanisms for pathogen transport, such as movement by people. Tobacco blue mold and wheat stem rust are two diseases that are spread on a continental scale in the U.S. during many years. Successful transmission of plant diseases over long distances through the atmosphere depends on many things, such as the reproductive rate of the pathogens, the size of the source, atmospheric turbulence, stability, and wind speed, and the survival of spores
during exposure to an inhospitable environment. A simple dispersal model was developed and used to obtain order of magnitude estimates for the rate of advance of a blue mold and a stem rust disease front over large distances. In the model, the pathogen spreads by means of great leaps between regions of host plants that are separated by many tens to hundreds of km. A leap of the pathogen occurs when a threshold level of propagules of the pathogen arrives at a target region at a time when both weather and host susceptibility allows local establishment of the pathogen. The details of the buildup and spread of the pathogen on a small scale are ignored, and the pathogens are considered to be so virulent that their arrival in a region during conditions conducive for infection is enough to ensure disease spread. The major constraints for determining the temporal pattern of disease spread over long distances are the dilution and mortality of spores in the atmosphere during transport and
whether or not a sufficient amount of susceptible host lies ahead of the moving front. As with disease progress, the green wave of susceptibility for annual crops advances northward in the U.S. each season. The practical limit for long-distance dispersal of a pathogen depends strongly on its fecundity and on its ability to survive in the atmosphere. The model yielded theoretical rates of disease spread that were in reasonable agreement with observed average spread rates for blue mold (13.9 km per day) and for stem rust (35.2 km per day). Interestingly, both diseases spread at rates that are similar to the rates of the northward advance of available susceptible host tissue in each crop system. The close agreement between the rates of advance of disease and of host susceptibility suggests that disease spread over long distances may be limited more often by pathogen establishment than by dispersal. It also focuses attention on interstate transportation of seedlings, in the tobacco
system, and on possible resurgence of the alternate host in the wheat system.
Impacts
Understanding the biophysical factors that control periodic reintroduction of diseases into geographical areas is important for guiding regional disease control strategies such as quarantine, sanitation, or pesticide use. Quantifying aerial dispersal of inoculum in concert with the development of the crop and the disease in time and space will allow these strategies to be assessed and employed.
Publications
- Pinkerton, J.N., Johnson, K.B., Aylor, D.E., and Stone, J.K. 2001. Spatial and temporal increase of Eastern filbert blight in European hazelnut orchards in the Pacific Northwest. Phytopathology 91:1214-1223.
- Aylor, D.E. 2001. Aerobiology of fungi in relation to capture and release by plants. In: Phyllosphere Microbiology. APS Press (in press).
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Progress 01/01/00 to 12/31/00
Outputs
Many practical problems, including spread of plant diseases and movement of GMOs in the environment, can be addressed using models of spore and pollen transport. These models require a description of turbulent air motions valid in canopies and at the same time extending seamlessly to large-scale motions in the atmosphere. Providing the wind statistics are known, Lagrangian Stochastic (LS) simulation is well suited for describing spore trajectories in canopies, and Large Eddy Simulation (LES) is well suited for describing these required statistics. Progress was made (with R. H. Shaw, UC Davis) in developing a framework for combining LS and LES analyses to describe spore motions over a wide range of turbulence length scales. LES and LS models could be linked in two ways. First, LES could be used to calculate velocity fields and Lagrangian velocity statistics, such as the Lagrangian time scale T, which would be used to run LS dispersion models. Problems exist with this
approach, however, since Lagrangian processes are non-stationary and their velocity statistics are not conserved through the life of a fluid element. Second, and more directly, LES could be used to study the dispersion of a large number of spores released within the canopy. LES can resolve (at substantial computational expense) a large portion of the energy spectral density, leaving only a relatively small fraction of the energy in the subgrid-scales still to be modeled. Spore dispersal is driven by a combination of resolved-scale simulated flow and SGS processes. A subgrid component is constructed from the SGS energy, an equation for which is carried along by the simulation. This construction of an SGS velocity would be similar in form to the usual Langevin type of formulation and would be based on a velocity variance extracted from the SGS energy, an estimate of an appropriate time scale, and a random variate. It is assumed that the Lagrangian velocity of a particle can be
partitioned between a resolved-scale velocity and a random subgrid-scale component. Spore release and capture are subgrid-scale processes, which are treated probabilistically. The best way to link the LES and the sub-grid scale processes is by no means clear, however. It is not entirely clear how to handoff heavy particles between LES and LS models since inertia tends to make the particle motions become uncorrelated with the turbulence. Also, it will be difficult to directly link the effect of gusts predicted at the LES scale with spore entrainment events taking place at the microscopic scale near the leaf surface. Thus, we are still short of being able to quantify actual numbers of spores that become airborne from lesions in a canopy, and further work is required to determine if the substantial computational expense of LES is warranted. Despite these and other uncertainties, the obvious advantage of LES to directly model coherent flow structures should ultimately lead to more
realistic estimates of canopy exchange of spores, particularly during convective conditions which play a major role in determining the geographic distribution of plant diseases spread by airborne spores.
Impacts
Understanding the biophysical factors that control periodic reintroduction of diseases into geographical areas is important for guiding regional disease control strategies such as quarantine, sanitation, or pesticide use. Quantifying aerial dispersal of inoculum in concert with the development of the crop and the disease in time and space will allow these strategies to be assessed and employed.
Publications
- AYLOR, D.E., AND SHAW, R.H. 2000. Integrating spore dispersal and models of turbulence. pp. 174-175. In Proc. 24th Conf. on Agricultural and Forest Meteorology, Amer. Meteorol. Soc., Boston, MA.
- WAGGONER, P.E., AND AYLOR, D.E. 2000. Epidemiology, a science of patterns. Annu. Rev. Phytopathology 38:71-94.
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Progress 01/01/99 to 12/31/99
Outputs
Several conditions must be met for a pathogen to spread far enough to cause disease to become pandemic. Necessary are presence of host, pathogen, a conducive environment, and transport of inoculum between regions. A model framework is being developed to examine spread of disease by long-distance aerial transport of pathogens in concert with the seasonal development of host tissue. Temporal dynamics of blue mold epidemics caused by Peronospora tabacina on Nicotiana tabacum is being used as a model system. With minor modifications the model should be applicable to other diseases such as downy mildews and powdery mildews of vegetables. Spread of disease from a focus is modeled by the location in time and space of a given constant level of disease severity, S(x,t). Curves of constant S (isopaths) can appear to travel as a wave along a characteristic curve; depending on the distance scale and inoculum transport conditions, an isopath can appear to move either as a
traveling wave with constant speed or as a dispersive wave with a speed that increases with increasing distance from a focus. Identifiable traveling waves with constant speed occur only if the tail of spore dispersal distribution is of exponential order. Over distances much larger than a field or a farm, inoculum mortality, washout by rain, and transport of infected transplants by people can lead to an exponentially-bounded or constant length scale dispersal function. The average rate of spread, V, of five recent pandemics of blue mold in the eastern U.S. ranged from 11 to 19 km per day. By contrast, the disease spread in Connecticut and Massachusetts in 1998 at an average local rate of about 2.5 km per day, or only about 16% of V. Furthermore, V is only about 4% of the average transport wind speed in the planetary boundary layer. Thus, local disease spread has an "apparent length scale" that is much shorter than for long-distance spread. In both cases, the apparent transport length
scale appears to be small compared to an average advection length scale for transport in the atmospheric boundary layer. Interestingly, V is not greatly different from the average rate of northward movement of the mean monthly 12.7 C isotherm, and in at least one year (1980) appeared to be constrained by it. This indicates that the "green wave" of planting for annual crops that progresses from south to north during spring and early summer can impose a definable constraint on the rate of propagation of disease. How often and to what extent this constraint comes into play is being addressed. The model also indicates the possibility that the pathogen can survive winter in northern locales needs to be examined more closely. For those growing seasons when a trajectory burns out before exhausting the geographic range of the host is very interesting from the standpoint of IPM. This occurred frequently between 1981 and 1996, probably mostly due to applications of an effective fungicide.
Clearly, it would be possible to save pesticides if this situation could be predicted.
Impacts
Understanding the biophysical factors that control periodic reintroduction of diseases into geographical areas is important for guiding regional disease control strategies such as quarantine, sanitation, or pesticide use. Quantifying aerial dispersal of inoculum in concert with the development of the crop and the disease in time and space will allow these strategies to be assessed and employed.
Publications
- AYLOR, D.E., AND IRWIN, M.E. 1999. Aerial dispersal of pests and pathogens: Implications for integrated pest management strategies. Agric. Forest Meteorol. 97:233-234.
- AYLOR, D.E., AND IRWIN, M.E. 1999. Editors, Special Issue on Aerial Dispersal of Pests and Pathogens: Implications for Integrated Pest Management. Agric. Forest Meteorol. 97:233-349.
- DE JONG, M.D., AYLOR, D.E., AND BOURDOT, G.W. 1999. A methodology for risk analysis of plurivorous fungi in biological weed control: Sclerotinia sclerotiorum as a model. BioControl 43:397-419.
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Progress 01/01/98 to 12/31/98
Outputs
Some plant pathogens rarely, if ever, survive the winter season in northern latitudes of the mid-Atlantic and northeastern United States and are thought to be introduced anew into these growing regions from inoculum sources in the South. Blue mold of Nicotiana tabacum, caused by Peronospora tabacina Adam, is an example of a disease that apparently must be introduced each season into northern growing regions. Blue mold occurs in the southeastern U.S. in most years, but appears as far north as Connecticut on an infrequent basis, appearing in Connecticut in significant amounts in only 5 out of the last 40 years (1979, 1980, 1996, 1997, and 1998). Disease spread and temporal progress of the pandemics is being described in terms of a combined infection-threshold, finite-leap model. The model expresses the rate of advance of the disease front in terms of: disease infection rate, length of the pathogen's latent period, size of the crop areas, distance between crop areas,
wind speed and turbulent diffusivity, and time scales for removal of airborne spores by rain and loss of spore viability due to exposure to ultraviolet rays in sunlight. The dispersal function used contains a self-similar term due to turbulent atmospheric dispersion and an exponential term due to scrubbing by rain and spore mortality. The model appears to be able to mimic the observed rates of disease spread using realistic values of the model parameters. Complicating factors such as ground transportation of diseased transplants and changes in the fungus response to temperature and its sensitivity to fungicides on rate of disease spread need to be assessed. In cooperation with M.D. De Jong (Wageningen Agricultural University, The Netherlands) and G.W. Bourdot (New Zealand Pastoral Agriculture Research Institute, Lincoln, New Zealand) progress was made on analyzing the risk of using the plant pathogenic fungus Sclerotinia sclerotiorum as a biological control of California thistle
(Cirsium arvense). A preliminary analysis indicates there are some areas in New Zealand where disease risk in arable fields may not be increased beyond that due to background levels of inoculum by introduction of the S. sclerotiorum mycoherbicide. However, there are many other areas around population sites where its use would definitely not be practical.
Impacts
(N/A)
Publications
- AYLOR, D.E. 1998. Biometeorological parameters governing long-distance spread of plant disease by atmospheric transport of inoculum. pp. 311-312. In Proc. 13th Conf. Biometeorology and Aerobiology, Amer. Meteorol. Soc., Boston, MA.
- DE JONG, M.D., AYLOR, D.E., AND BOURDOT, G.W. 1999. A methodology for risk analysis of plurivorous fungi in biological weed control: Sclerotinia sclerotiorum as a model. BioControl (in press).
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