Donald E. Aylor

A framework for examining inter-regional aerial transport of fungal spores

Abstract Aerial transport of fungal spores has no doubt been responsible for occasionally spreading plant diseases over distances of 500 km or more. With currently available methods, however, there is little basis for estimating the likelihood of such occurrences. Models capable of estimating the relative probability of infection of a crop from known sizes of local or distant sources of pathogenic spores could help farmers with decisions about fungicide use, local sanitation and quarantine. A logical framework for estimating long-distance transport of viable spores is presented which encompasses: production of spores at the source region, escape of spores into the air above the diseased crops, transport and dilution of spores in the atmosphere, survival of airborne spores and deposition of spores onto a distant crop. Hypothetical examples of spore transport over 700 km are calculated to illustrate the relative magnitudes of the five parts of the spore transport process. Although the combined uncertainties in spore transport are estimated to be a factor of 10 4 –10 6 , this should be compared to the expected factor of 10 13 dilution of spore concentration experienced by a cloud of spores after having been transported through the atmosphere for 700 km. The largest uncertainties lie in our ability to estimate daily spore production in a geographic region, survival of spores while airborne and deposition of spores during rainfall. Survival of spores and deposition of spores during rainfall are difficult to predict because of temporal and spatial variations in weather. To account for this, it will be necessary in the future to combine the physical model of spore transport described here with weather records and air parcel trajectory calculations to develop a “climatology” of danger of disease transmission between geographic regions.

SPREAD OF PLANT DISEASE ON A CONTINENTAL SCALE: ROLE OF AERIAL DISPERSAL OF PATHOGENS

Successful transmission of plant diseases over long distances through the atmosphere depends on the reproductive rate of the pathogens, on the carrying capacity of the source locality, on atmospheric turbulence, stability, and wind speed, and on the survival of spores during exposure to inhospitable temperature and humidity and to UVB radiation from the sun. These interacting factors were incorporated into a model to estimate the rate and extent of seasonal incursions of disease from southern into northern areas of the United States. The model indicates a practical limit for long-distance dispersal (LDD) of a plant pathogen, which depends strongly on its fecundity and on its ability to survive in the atmosphere. Two classic plant diseases, stem rust of wheat and tobacco blue mold, are used to illustrate the model. Both diseases appear to spread northward on average at about the same rate as the seasonal advance of the “green wave” of available susceptible host tissue. The near concordance of the disease w...

Biophysical scaling and the passive dispersal of fungus spores: relationship to integrated pest management strategies

Abstract Successful integrated pest management strategies depend on an accurate evaluation of ‘immigrant’ inoculum coming into a managed area. Dispersal of plant pathogenic fungus spores is comprised of a series of inter-connected events. Starting with spore production, dispersal depends, in turn, on spore release or removal from a substrate, spore escape from the canopy space, transport and dilution of a spore cloud by turbulent wind, loss of inoculum viability during transport, spore removal from the atmosphere by precipitation, spore deposition on host tissue, and the infection efficiency of deposited spores on susceptible host tissue. All of these components change in time and space, challenging the aerobiologist to identify critical biophysical processes that control disease spread. Some of these processes will be illustrated for two plant diseases: apple scab caused by the pathogen Venturia inaequalis and tobacco blue mold caused by the pathogen Peronospora tabacina . Both these pathogens are dispersed by airborne spores. Ascospores of the apple scab pathogen become airborne primarily during rain events, while P . tabacina sporangia become airborne primarily during dry, convective conditions. Initial success of dispersal depends on wind gusts which allow escape of spores from a crop canopy or the boundary layer of air just above the ground. Microclimatic conditions at the time (e.g. day or night, rain or no rain) and the location of spore release (upper or lower canopy) can have a significant effect on subsequent dispersal of pathogens. The rate of advance of a disease frontal boundary during long-distance disease spread can be as much as six times faster than the local rate of disease spread. This apparent anomaly will be discussed using tobacco blue mold as an example. The rate of movement of the disease front in a spatially heterogeneous distribution of hosts, is determined largely by the relative rate of local disease development, the length scale of the dispersal function, distance between regions of host plants, and the area of those host regions. Factors such as ground transportation of diseased transplants and changes in the pathogen’s sensitivity to fungicides can, at times, override the biophysical constraints on long-distance spore dispersal.

An aerobiological framework for assessing cross-pollination in maize

Abstract Maize (Zea mays L.) is one of the world’s most important crops. Until recent times, improvement in maize resulted from the manipulation and exchange of genetic information within the genus Zea. With the advent of genetic engineering, genetic information from other species is routinely incorporated into the maize genome. Since maize is a wind-pollinated outcrossing species, questions arise concerning the flow of genetic information between genetically modified (GM) and non-GM maize. Because of the rapidly accelerating introduction of GM corn into agricultural production, improved aerobiological models to predict how far and to what extent maize pollen can be transported in the atmosphere are needed today. Models should provide quantitative understanding of both short- and long-range pollen dispersal, thereby offering a means to evaluate deposition of viable maize pollen in seed production fields as well as on neighboring farms. Central to pollen dispersal is the rich interplay between physics and biology and between space and time. We present an aerobiological framework for assessing corn pollen movement in the atmosphere that could also be applied to other crops and to uncultivated species. An important aim of this paper is to spark interest in the scientific community to generate a more complete framework of corn pollen dispersal. As such this manuscript presents a multidisciplinary albeit incomplete perspective to an emerging applied problem.

Settling speed of corn (Zea mays) pollen

Abstract The settling speed of corn pollen is fundamental for determining the distance that corn pollen can be transported in the atmosphere and for determining its probability of being deposited on plants and the ground. The settling speed, vS, and the corresponding volume-equivalent diameter, De, of corn pollen grains were measured at various times after pollen was released from the anther. The geometric size of pollen grains decreased with increasing time after release from the anther due to loss of water during drying. The density of corn pollen ranged from 1.25 g cm −3 when freshly collected to 1.45 g cm −3 when dry. Values of vS ranged from 21 cm s −1 for pollen grains with De of 76– 80 μm to 32 cm s −1 for pollen grains with De of 103– 106 μm . Over a wide range of pollen sizes (76– 106 μm ), which included pollen collected from two hybrid varieties and one inbred line, vS was described well (P

Quantifying the Rate of Release and Escape of Phytophthora infestans Sporangia from a Potato Canopy

ABSTRACT A means for determining the rate of release, Q (spores per square meter per second), of spores from a source of inoculum is paramount for quantifying their further dispersal and the potential spread of disease. Values of Q were obtained for Phytophthora infestans sporangia released from an area source of diseased plants in a potato canopy by comparing the concentrations of airborne sporangia measured at several heights above the source, with the concentrations predicted by a Lagrangian Stochastic simulation model. An independent estimate of Q was obtained by quantifying the number of sporangia per unit area of source at the beginning of each sampling day by harvesting diseased plant tissue and enumerating sporangia from these samples. This standing spore crop was the potential number of sporangia released per area of source during the day. The standing spore crop was apportioned into time segments corresponding to sporangia concentration measurement periods using the time trace of sporangia sampled above the source by a Burkard continuous suction spore sampler. This apportionment of the standing spore crop yielded potential release rates that were compared with modeled release rates. The two independent estimates of Q were highly correlated (P = 0.003), indicating that the model has utility for predicting release rates for P. infestans sporangia and the spread of disease between fields.

Survival of Phytophthora infestans sporangia exposed to solar radiation.

ABSTRACT The effect of solar irradiance (SI) on the viability of sporangia of isolates belonging to two clonal lineages, US-1 and US-8, of Phytophthora infestans was assessed. Exposure during a 3-h period on sunny days (SI > 600 W/m(2)) drastically reduced germination regardless of the time of day of the exposure. After 1 h of exposure on sunny days, the viability of sporangia decreased by approximately 95%, and the effective time necessary to inactivate 95% of the sporangia was 1.1 h. The effective dose to inactivate 95% of the sporangia on sunny days was 2.6 MJ/m(2). On overcast (SI < 300 W/m(2)) days, survival after 3 h was reduced only slightly. Thus, other variables being equal, sporangia will survive hours longer in the atmosphere on cloudy days than on sunny days.

Quantifying Aerial Concentrations of Maize Pollen in the Atmospheric Surface Layer Using Remote-Piloted Airplanes and Lagrangian Stochastic Modeling

Abstract The extensive adoption of genetically modified crops has led to a need to understand better the dispersal of pollen in the atmosphere because of the potential for unwanted movement of genetic traits via pollen flow in the environment. The aerial dispersal of maize pollen was studied by comparing the results of a Lagrangian stochastic (LS) model with pollen concentration measurements made over cornfields using a combination of tower-based rotorod samplers and airborne radio-controlled remote-piloted vehicles (RPVs) outfitted with remotely operated pollen samplers. The comparison between model and measurements was conducted in two steps. In the first step, the LS model was used in combination with the rotorod samplers to estimate the pollen release rate Q for each sampling period. In the second step, a modeled value for the concentration Cmodel, corresponding to each RPV measured value Cmeasure, was calculated by simulating the RPV flight path through the LS model pollen plume corresponding to the ...

Long-range transport of tobacco blue mold spores

Abstract The epidemics of tobacco blue mold occurring in Connecticut, USA in 1979 and 1980 probably resulted from spores blown by the wind from sources several hundred km away. This probability is supported here by calculations of trajectories of parcels of air and by calculations of the change in the aerial concentration of spores between the source and new hosts.

Modeling spore dispersal in a barley crop

Abstract A model of aerial spore transport by Legg and Powell (1979) is modified here to account for passive liberation of spores by wind and its effect on the subsequent transport and deposition of spores within the crop. Spores blown from surfaces by wind are removed mainly during gusts when the wind speed exceeds a threshhold value characteristic of the particular fungus. The integration of this threshold into a transport model is described here. This new model adequately describes the decrease in concentration of Erysiphe graminis spores observed downwind of a source of infection within a barley crop.