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Life history of Sclerotinia sclerotiorum in relation to its use as a mycoherbicide to control Cirsium arvense in pastures. Drawing courtesy of Dr Ian Harvey, Plantwise, Lincoln, New Zealand.
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In New Zealand, research is being performed on the use of the ubiquitous plurivorous ascomycete Sclerotinia sclerotiorum, as a biocontrol agent for Cirsium arvense in pasture. As a consequence of the wide host range of this fungus, the proposed biocontrol may pose a risk to non-target arable plants. Crop disease risk is primarily due to the formati...
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... hyperparasites (McQuilken et al., 1995) or by flooding the thistle biocontrol site (Moore, 1949). In addition, it may be possible for a farmer to reduce the fraction of ascospores escaping from pastures undergoing biocontrol by ensuring high pasture plant cover densities during the time of year when sporulation is occurring. The latter method is a logical consequence of the models concerning the escape and dispersal of ascospores from treated pastures that are discussed in the following pages, and will be explored in more detail in a subsequent communication. Risk acceptance is a subjective matter concerned with peoples’ percep- tions of risk, guided by the objectivity of a ‘risk determination’. It may be addressed by establishing a risk reference, such as the ratio of inoculum added due to biocontrol: natural inoculum and soliciting the opinions of risk referees such as growers of crops susceptible to S. sclerotiorum and the pesticide regulatory authority. According to the risk determination framework (Figure 2), the survival of sclerotia in the soil must be taken into account. The sclerotia produced by S. sclerotiorum in the stems of a biologically controlled population of C. arvense become soilborne during the summer, autumn and winter of the year of mycoherbicide application as the killed stems disintegrate. These perennating bodies, the primary source of phytosanitary risk, may directly infect susceptible crop plants sown at the biocontrol site by myceliogenic germination. In addition they may form fruiting bodies (apothecia) that release ascospores that disperse on air currents to infect susceptible crops sown at, or downwind from, the biocontrol site (Figure 1). We therefore must understand and predict the decay of soilborne sclerotia, a process that is apparently regulated largely by hyperparasites in the soil micro-flora. More than 30 species of bacteria and fungi have been implicated as antagonists or mycoparasites of Sclerotinia sp. sclerotia, and in some cases, e.g., Coniothyrium minitans and Trichoderma viride , there is proof of parasitism under natural conditions (Adams and Ayers, 1979). Many decay phenomena in nature follow an exponential law according to the ...
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... it will be necessary to validate the multi-layer model of spore escape by comparing its output with data on ascospore levels in the field. Such validation will however be unnecessary for the Gaussian plume model since it is commonly used in air quality studies and is known to provide reasonable estimates of the average concentration of airborne particles during a defined time interval. Evaluation of the whole aerobiological process chain, from fructification through to disease in susceptible crops (see Figure 1 in De Jong et al., 1990a), is impossible and arguably unnecessary. Nevertheless, using the methods described, predictions can be made of the downwind atmospheric concentrations of ascospores due to an intended biocontrol, and these can be compared with measured background concentrations. Eventually, high-level integration could be shown in a map of New Zealand with risky and moderately risky locations based on the results of experiments on ascospore dispersal and sclerotium survival. This map will be an important part of an application to the NZ Pesticides Board for registration of a Sclerotinia sclerotiorum mycoherbicide to support management recommendations and suggested safety constraints on the use of the intended product for thistle control in pastures. At present, a map of a representative area (60 km by 100 km) in New Zealand’s province of Canterbury has been constructed (Figure 9), displaying agricultural activities mapped at a 2 km by 2 km scale. To facili- tate a risk evaluation, current locations of large thistle infestations are also drawn on the map. Around these thistle infestations (the potential biocontrol sites), a safety zone might be declared with due regard for spore dispersal of S. sclerotiorum . To be conservative in our risk analysis, we should choose a worst-case scenario of spore dispersal in setting such a safety zone. The methodology presented in this paper enables us to calculate a safety zone. The very crude example given here, using preliminary figures as input to the models, indicates that it may be approximately 1 km. At a distance of 1 km downwind from a hypothetical biocontrol site, the Gaussian plume model showed that the aerial spore concentration had approximated 10 spores m − 3 (Figure 8). Such a level is hardly detectable by standard aerobiological equipment. However this in itself, is no indication of the relative disease risk to a crop 1.0 km downwind from a treated 100 m × 100 m pasture. We have accounted neither for the background concentration of S. sclerotiorum ascospores, nor set an acceptable value for the ratio of added to natural ascospores (see Figure 2). Furthermore, 10 spores m − 3 is probably an overestimate of the concentration of ascospores to be expected 1 km from a source with R spor = 10 6 spores s − 1 since our calculated escape fraction (0.815) using the single-layer model is an overestimate. This is because this simple ...
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... substantially dependent upon the leafiness of the pasture, a consequence of the mucilage accompanying the discharged ascospores of this fungus, resulting in their adhesion to pasture foliage. Escape is also particularly sensi- tive to the exchange resistance, r , being reduced substantially with increases in r as may occur during misty weather conditions. In a risk determination, aerial dispersal of the ascospores should be quantified in order to assess infection dangers over both short and long distances from an imaginary biocontrol site. To more accurately quantify the dispersal of ascospores within a pasture, and their dispersal over short distances outside the biocontrol site, we need a more complex model than the one-layer model described above (see Van der Werf et al., 1989). While the one-layer model allows an estimate of spore scape, it does not allow calculations of spore concentration in the downwind direction, nor does it make allowances for the finite length ( L ) and width ( W ) of a spore source. A multi-layer model in which several layers within the grass canopy are distinguished such as (1) a spore production layer near the ground and (2) vegetation layers in which spores can be deposited to vegetation elements (leaves), will provide a better simulation of reality for a spore source when sporulation occurs close to the ground as in the case of S. sclerotiorum (Figure 1). This is because ...
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... this paper we have addressed phytosanitary risk management of biological weed control utilising native plurivorous plant pathogenic fungi. As a model for the development of an appropriate methodology, we have used the ubiquitous ascomycete Sclerotinia sclerotiorum ; a fungus intended for controlling Cirsium arvense in pastures in New Zealand. The methodology will be equally appropriate for a risk analysis of this fungus in the case of its use against other pasture weeds such as Cirsium vulgare, Carduus nutans , and Senecio jacobaea (Waipara et al., 1993) and Ranunculus acris (Green et al., 1996). Our proposition is that plurivorous fungi such as S. sclerotiorum may not necessarily create an unacceptable phytosanitory risk when utilized as mycoherbicides. This risk can be quantified in a manner that provides regulators with the necessary information to make an informed judgement about the exploitation and management of this form of weed biocontrol. The authors thank the New Zealand Foundation for Research, Science and Technology for funding this project, Dave Saville (AgResearch, Lincoln, N.Z.) for statistical analyses and helpful advice on the manuscript and Ian Harvey (Plantwise, Lincoln, N.Z.) for advice on mycological matters. We also thank Geoff Hurrell (AgResearch, Lincoln) for technical assistance in the field, Aaron Knight (AgResearch, Lincoln) for the artwork in Figures 7 and 9, and Ian Harvey for the artwork in Figure 1. We thank Walter Rossing, Dept. TPE, Agricultural University, Wageningen, The Netherlands, for help and advice on the manuscript. Thanks are also due to the editors of BioControl; Prof. Hokkanen and Prof. H. ...
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... model allows wind speed and the turbulent diffusion profiles within the pasture canopy, as well as the plant area density, to change with height, as occurs in a real pasture. S. sclerotiorum ascospores are forcibly released into the air from apothecia near the ground (Figure 1). Following release, ascospores are simultaneously transported in the downwind direction by the horizontal motion of the wind, diffused in the vertical direction by the turbulent fluctuations of the wind, and a portion are deposited on the vegetation in the pasture canopy. The airborne concentration of ascospores, C (ascospores M − 3 ), decreases with downwind distance, x , and vertical distance, z , from their point of release. Assuming that the source extends infinitely far in the cross-wind direction and that conditions are steady, an equation which describes C can be written as (Legg and Powell, 1970, Aylor, ...
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... pastures, it is necessary to determine the magnitude of any additional disease risk and increased economic losses (Purdy, 1979) in these crops. This white mould fungus is notorious for the rots it causes in many arable crops. 1 After killing C. arvense shoots, the fungus forms sclerotia that become soilborne and may later infect susceptible crop plants sown at the biocontrol site through myceliogenic germination (Abawi and Grogan, 1979). Furthermore, soilborne sclerotia form apothecia in the spring time (September–November), from which ascospores are forcibly ejected at soil level, potentially escaping above the pasture canopy to disperse downwind, adding to the natural atmospheric spore concentration of S. sclerotiorum . Key events in the life history of this fungus are illustrated in Figure 1. The objective of this study was to use the biocontrol of C. arvense by S. sclerotiorum as a model system to develop a methodology that will enable an objective assessment of the phytosanitary risks associated with the use of plurivorous fungi as mycoherbicides. Quantifying the phytosanitary risks to non-target plants from this form of weed biocontrol calls for a systems approach integrating the disciplines of mycology for quantifying fate of soilborne sclerotia, and aerobiology and epidemiology for assessing dispersal of airborne ascospores. In this article we (1) define our risk analysis approach, (2) propose an empirical model for sclerotium survival, (3) propose mechanistic models for quantifying escape and dispersal of airborne ascospores, and (4) use cartographic techniques resulting in a preliminary risk evaluation. Phytosanitary risk refers to infection of non-target plants by soilborne and aerial inoculum arising due to the intended biocontrol. In this paragraph we will structure our thinking by dividing risk assessment into two compo- nents, ‘risk determination’ and ‘risk evaluation’ as proposed by Rowe (1980). Risk determination consists of ‘risk identification’ and ‘risk estimation’ and requires quantitative analysis of experimental data and model simulations. In our case, the research problem is to understand and predict the temporal and spatial processes regulating the population dynamics of S. sclerotiorum , so that the increased inoculum densities that may result from biocontrol, can be quantitatively compared with natural inoculum densities (Figure 2). This ‘relative risk’ approach was used for the woody weed mycoherbicide BIOCHON based on Chondrostereum purpureum (De Jong et al., 1990a,b, 1991; De Jong, 1992). For S. sclerotiorum key aspects to consider are: sclerotium survival, apothecium formation, ascospore formation and liberation, flight of ascospores within the treated pasture, escape of ascospores ...
Citations
... Thus, should the fungus be deployed for forestry weed control, the fruit trees would be at no greater risk of disease from the fungus than from natural sources. [18][19][20] The analysis therefore showed that the bioherbicide would meet the requirement of not subjecting susceptible fruit trees to more inoculum than they receive from natural sources. 12 In the case of S. sclerotiorum, proposed for pasture weed control in New Zealand, spatial analyses were conducted of multiple plumes from hypothetical 1.0-ha biocontrol sources of the spores of the fungus in sheep and dairy cattle grazed pastures, and multiple plumes within a hypothetical 49-ha market garden source. ...
... The environmental risk analyses conducted for S. sclerotiorum 15,22 and C. purpureum [18][19][20] assume that the bioherbicide strains of these two broad host-range pathogens are equally pathogenic on all nontarget host species. This assumption has not been tested, but research on other species suggests that it may not always hold. ...
Plant pathogens with a broad host range are commercially more attractive as microbial bioherbicides than strictly host‐specific pathogens as a result of the wider market potential of a product capable of controlling multiple species. However, the perceived spatiotemporal disease risk to nontarget plants is a barrier to their adoption for weed control. We consider two approaches to managing this risk. First, we consider safety zones and withholding periods for bioherbicide treatment sites. These must ensure inoculum spreading from, or surviving at the site, exposes nontarget plants to no more inoculum than from natural sources. They can be determined using simple dispersal models. We show that a ratio of added:natural inoculum of 1.0 is biologically reasonable as an ‘acceptable risk’ and a sound basis for safety zones and withholding periods. These would be analogous to the ‘conditions of use’ for synthetic chemical herbicides aimed at minimizing collateral damage to susceptible plants from spray drift and persistent soil residues. Second, weed‐specific isolates of broad host‐range pathogens may avoid the need for safety zones and withholding periods. Such isolates have been found in many broad host‐range pathogen species. Their utilization as bioherbicides may more easily meet the requirements of regulators. Mixtures of different weed‐specific isolates of a pathogen could provide bioherbicides with commercially attractive spectrums of weed control activity. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
... However, its effectiveness to control plants in the family Aizoaceae has not yet been tested. Use of this fungus as a mycoherbicide on several invasive plants has been widely studied (Cother 2000;De Jong et al. 1999;Green et al. 1998;Saharan and Mehta 2008c), usually with the mycelia as inoculum (Cother 2000;Green et al. 1998). The ascospores can be dispersed as far as 250 m, giving an idea of the safety zone for application (Bourdôt et al. 2001;De Jong et al. 2002). ...
Carpobrotus edulis is a highly invasive plant in coastal temperate areas worldwide. In a preliminary attempt at biological control, we evaluated the potential use of the fungus Sclerotinia sclerotiorum and the insect Pulvinariella mesembryanthemi as biocontrol agents. We carried out a greenhouse-experiment to evaluate the effects of both agents, separately and together, on short-term (physiological) and long-term (survival, growth, biomass) estimators of plant performance. We compared the susceptibility to both agents in plants originating from native and non-native areas. The fungus had immediate and negative, but short-lasting effects on chlorophyll content and photosynthetic-radiation use efficiency. No significant effects on plant survival, growth and biomass were observed after 1 year. Infestation with the insect increased photosynthetic performance and decreased the root/aerial biomass ratio of the plants, suggesting a counteractive response to insect feeding. After 5 months, the reflectance parameters were negatively affected. Only half of the infested plants survived for a year, and the growth and biomass were lower in the surviving plants than in untreated plants. In half of the surviving plants, the most heavily-infested parts died. The insect-infested plants were usually infected by mould. The number of insects per plant was lower in native populations and when both biocontrol agents were used together. Nevertheless, no long-term synergistic/additive effects of the insect and fungus were observed, and the susceptibility of native and introduced plants was not different. Therefore, use of the insect seems to be the best strategy for controlling C. edulis, as it decreases plant growth and increases mortality and stress susceptibility.
... Viable spores must escape the canopy in order to spread an epidemic widely and rapidly; otherwise they deposit on the ground and canopy elements within a few metres of the source (Aylor 1986). The product of release rate and escape fraction is commonly used to quantify emission of spores from canopies in plume models that predict the spatial distribution of spores deposited downwind and the spread of spores between fields, farms and crop-growing regions (Aylor 1999; de Jong, Aylor & Bourdôt 1999). The fraction of spores that escape the canopy increases with wind speed, turbulence level and release height (Aylor 1999; Aylor et al. 2001). ...
Modelling the dispersion of small particles such as fungal spores, pollens and small seeds inside and above plant canopies is important for many applications. Transport of these particles is driven by strongly inhomogeneous and non-Gaussian turbulent flows inside the canopy roughness sublayer, the region that extends from the ground to approximately three canopy heights. A large-eddy simulation (LES) approach is refined to study particle dispersion within and above the canopy region. Effects of plant reconfiguration are parameterized through a velocity-dependent drag coefficient, which is shown to be critical for accurate reproduction of velocity statistics and mean spore concentrations. The model yields predictions of turbulence statistics that are in good agreement with measurements. This is particularly true of the stress fractions carried by strong events, as revealed by standard quadrant analysis of the resolved velocity fluctuations, which is a known weakness of earlier LES studies of canopy flow using a constant drag coefficient. Experimental data on spore dispersal inside and above a maize canopy are reproduced successfully as well. Characteristics of the particle plume are analysed using LES results, and a pre-existing theoretical framework is adapted to model particle dispersal above the canopy. The results suggest that the plume above the canopy can be approximated using a simple analytical solution if the fraction of spores that escape the canopy region is known. Source height and gravitational settling have strong effects on the plume inside the canopy region and consequently determine the escape fraction. These effects are parameterized in the theoretical model by using the escape fraction to rescale the source strength.
... However, the results obtained with the mixed-cellulose-esters membranes did not correspond to those reported by Hunter et al. (1982). In our study, the ascospores were first harvested in deionized water prior to being collected and dried on the membranes, which may have decreased their viability by affecting metabolism or nutrient status, as observed in other fungi (Nickerson et al. 1981), or by causing the removal of the mucilage around the ascospores (de Jong et al. 1999). The best storage medium was glycerol, likely because of its cryoprotectant properties (Fennema et al. 1973). ...
Assessing the reaction of canola plants (mainly Brassica napus) to the stem rot fungus, Sclerotinia sclerotiorum, in controlled environments, using ascospores, most closely resembles natural infection. This approach requires a ready supply of viable ascospores with germination ability, while their production under laboratory conditions is often inconsistent. The objective of the present study was to determine whether ascospores of S. sclerotiorum could be effectively conserved in deionized water or glycerol and used directly for inoculating plants. Ascospores were collected in sterile deionized water from apothecia produced under controlled conditions and were stored in deionized water, in 30%, 40%, 50%, or 60% aqueous solutions of glycerol, or on hydrophilic, mixed-cellulose-esters membranes at –20 or –80 °C for 1, 3, 6, 9, 12, and 24 months. After 12 months of storage at –20 or –80 °C, there were significantly higher proportions of viable ascospores in 30% and 40% glycerol than in the other storage media. Ascospores remained infectious for as long as 12 months of storage at both storage temperatures in most treatments. Overall, the study indicated that ascospores stored at –80 °C in 30%–40% glycerol for up to 12 months were a reliable source of inoculum for pathogenicity tests.
... and ''how long should a withholding period be?'' Using inoculum dispersal and decay models, these distances and times respectively can be defined in terms of the ratio of the density of inoculum added by the bioherbicide (to the susceptible crop's environment) to the density of naturally occurring inoculum (Bourdôt 1998;de Jong et al. 1999). This idea is based on the plant pathology principle that ''inoculum density'' (number of infective propagules), together with ''infection potential'' (environmental conditions), plays a fundamental role in a disease epidemic, whether inoculum density is set at the beginning of a growing season (as for monocyclic pathogens) or grows with the epidemic (polycyclic pathogens) (Lucas 2002). ...
... First, the dispersal of the pathogen's inoculum from, or its decay at, the application site must be quantified. This can be achieved by conducting appropriate experiments and analyzing the resulting data using modeling approaches such as those used by de Jong et al. (1999) and Bourdôt et al. (2006). Second, the relationship between the amount of disease in susceptible crops and the density of the pathogen's inoculum needs to be quantified. ...
Broad- host-range pathogens are appealing as candidates for commercial development as bioherbicides because of the wider market potential of a product that is effective against a range of weeds. But when these pathogens are able to spread in space (or time), a risk analysis is necessary. Here we test the hypothesis that a safety zone around a bioherbicide application site is adequate so long as it is wide enough to ensure that dispersing inoculum has diluted sufficiently that the density of inoculum occurring naturally in a susceptible crop is no more than doubled by the influx of bioherbicide spores. To this end the plant disease–pathogen inoculum density relationship using data from nine published experiments was modeled using the logistic equation. This revealed that a doubling of the natural spore density of a plant pathogen in the range of 103.4 to 106.7 spores/ml may generally be expected to result in unacceptable increases in disease in a susceptible crop. A doubling outside this range (< 103.4 or > 106.7) is less likely to do so. Therefore when the natural density of inoculum in a crop's environment occurs outside this range, an “acceptable” safety zone for the pathogen's use as a bioherbicide can in most cases be defined by the 1 ∶ 1 ratio of added ∶ natural inoculum. However, if a more “risk averse” safety zone is desired, it can be defined using a 1 ∶ 10 ratio of added ∶ natural inoculum.
... It may be possible to use broad-spectrum pathogens without intolerable non-target effects (De Jong et al., 1999; Pilgeram and Sands, 1999). Instead of replacing herbicides, synergy between pathogens and chemical herbicides may be a more successful approach to weed management (Boyette et al., 2007a). ...
... This general problem has been of particular importance in the analysis of the passive dispersal of atmospheric pollutants, pollen and spores, and has been resolved by incorporating wind speed and direction into the diffusion model to generate Gaussian plume or tilted plume models of directed dispersal (Okubo and Levin, 1989;Turchin, 1998). This approach has been used in the development of a risk analysis for fungal spores used in the biological control of weeds (de Jong et al., 1999(de Jong et al., , 2002 and for dispersal of predatory mites (Jung and Croft, 2001). For most invertebrate natural enemies, however, dispersal is an active process and any directionality in dispersal may be driven as much by landscape features as by wind. ...
Mark-release-recapture (MRR) experiments are considered the best approach to use in monitoring the dispersal of natural enemies from the target environment, in an assessment of the risk of non-target impacts from augmentative releases. Starting from some general considerations of the difficulties of using MRR, we specifically address marking techniques, the design of recapture grids and the limitations imposed by different sampling strategies for the recapture of the natural enemies released. Subsequently, we describe both an exponential and a diffusion model for dispersal that can be used to analyse the time-integrated density-distance data generated from MRR experiments, pointing out the need to examine and correct the data for directionality, if possible, or to use a diffusion model with displacement when correction is not possible. The application of the exponential and diffusion models of dispersal to the estimation of dispersal distance and density, the two most important metrics to consider in a risk assessment of non-target impacts of augmented natural enemies, is also discussed. Finally, we present a case study of an inundative release of Trichogramma brassicae in a meadow in Switzerland to illustrate how the data from an MRR experiment can be fitted to a dispersal model to estimate dispersal distance and the density of dispersing individuals at different distances from the release point.
... The approach taken in this paper is to consider the ratio of added to natural ascospores in the air above market garden crop land (de Jong et al., 1990a(de Jong et al., , 1999, and to make a judgement about the width of a safety zone on this basis. The distance from a pasture undergoing biocontrol of C. arvense by S. sclerotiorum at which the density of ascospores has declined to the density of ascospores occurring naturally in the air above market garden crops, i.e. ascospore density is doubled, may be acceptable as a safety zone (as in de Jong et al. (1990aJong et al. ( , 1990b). ...
Biological control of Cirsium arvense(L.) Scop. in pasture by the plurivorous plantpathogenic fungus Sclerotiniasclerotiorum (Lib.) de Bary mayresult in the formation, escape and aerialdispersal of ascospores, creating an additionaldisease risk in down-wind market garden crops. To determine the width of a safety zone for apasture subjected to this form of weed control,we simulated the spatial pattern in the ratioof added (due to biocontrol) to naturallyoccurring airborne ascospores (due to marketgarden crops) around a 1ha virtual biocontrolpasture under either sheep or dairy cattlegazing over a 91-day emission period in 1996 inCanterbury, New Zealand. This was achievedusing a unique combination of two computermodels; SPORESIM-1D (for spore escape from avegetation source) and PC-STACKS (a modernGaussian plume model for dispersal beyond asource). Plumes of dispersing ascospores weremodelled for each hour of the emission periodfor both the virtual market garden andbiocontrol sites, and the aerial density of theascospores was averaged over the period. Assuming that a 1:1 ratio of added to naturallypresent spores is acceptable, no safety zonewas necessary for either of the modeledpastures. A ten-fold ratio (1:10 added tonatural) necessitated safety zones of 300 and150 m for the sheep and dairy pasturerespectively. Uncertainties associated withextrapolation of this conclusion to individualpasture management scenarios, and to otheryears and climatically different regions arediscussed.
... In general these processes are influenced particularly by wind and air turbulence (McCartney and Fitt, 1985) as well as density of the vegetation. de Jong et al. (1999) have outlined an alternative, mechanistic approach employing a Gaussian plume model for dispersal that uses as input the emission rate of spores from a pasture (or 'source strength'), the product of the release rate of spores from the apothecia, a process quantified empirically by Bourdô t et al. (2001) and the 'escape fraction'. The escape fraction (Gregory, 1961) cannot be measured directly (Aylor and Taylor, 1983; Aylor and Ferrandino, 1985) but may be evaluated given a mechanistic understanding of the interacting processes that control the movement of spores within a vegetation canopy. ...
... Thirdly, the vegetation canopy (pasture containing grasses and clovers) is divided vertically into many horizontal layers. This modification of the 'one-layer' model SPORESIM-1L, used in de Jong et al. (1999), allows the vertical profile of pasture leaf area density to be taken into account when calculating the deposition of spores onto foliage due to turbulent impaction. In SPORESIM-1D, impaction is adequately treated and the spore source can be vertically shifted, making it a very flexible tool that has relevance for a wide range of dispersal phenonema in addition to the escape of S. sclerotiorum spores from a pasture. ...
... In this simulation an escape height, h, of 1.5 m was chosen, resulting in slightly reduced escape fractions compared to the previous analysis where the escape height was 0.25 m. Escape heights greater than 1 m are required for generating valid pointsource spore emission rates as input for a Gaussian model (Erbrink, 1995) an intended elaboration of the current study to evaluate longdistance dispersal and minimum isolation distances (de Jong et al., 1999). The simulated data (symbols inFig. ...
A multi-layer physical model, sporesim-1d, based on the gradient transfer theory (K-theory) of turbulent dispersal (analogous with the molecular diffusion of gasses) is described for the transport of Sclerotinia sclerotiorum ascospores within and above a grass canopy following their release from apothecia at ground level. The ‘steady-state’ diffusion equation is solved numerically and the spore escape fraction is estimated. sporesim-1d's context is the risk analysis of S. sclerotiorum used as a mycoherbicide to control Cirsium arvense in pasture. In validation tests sporesim-1d was internally consistent and produced a vertical wind speed profile similar to that measured in a grassland. In further validation tests, measured vertical profiles of atmospheric concentrations of Lycopodium clavatum spores in a wheat crop, and Venturia inaequalis spores in an apple orchard and in a grassland, were closely approximated by the model, as was measured data on the concentration of S. sclerotiorum ascospores deposited downwind of a small area source in a grassland. Escape fractions for grassland predicted by sporesim-1d, were 50% lower than predicted by both a Lagrangian model (Plant Disease 82 (1998) 838) and a one-layer version of sporesim-1d, sporesim-1l, indicating that the vertical compartmentalisation in sporesim-1d, allowing wind speed and pasture leaf area index (LAI) to vary with height, results in a more realistic estimate of the escape fraction. Simulations using sporesim-1d revealed an increase in the escape fraction with increasing wind speed, and an order-of-magnitude fall with increases in LAI from values typical of a closely grazed sheep pasture (ca. 2) to those of more laxly grazed cattle pastures and intact grassland (ca. 7). This result implies that any additional risk of disease in a susceptible crop growing downwind of a pasture treated with a S. sclerotiorum mycoherbicide may be reduced by grazing management. Reduction in the risk of sclerotinia rot in kiwifruit (Actinidia deliciosa) vines, and in apple scab disease in apple trees, caused by V. inaequalis, appears possible by maintaining a dense grass under-storey. A simple empirical model for spore escape with one parameter and two variables (LAI and wind speed) derived from the mechanistic model provided a good description (r2=0.998) of simulated escape fraction. Combined with information on release rates of S. sclerotiorum spores at a biocontrol site, this model will enable a times-series analysis of spore emission, and coupled with a Gaussian plume model, prediction of minimum isolation distances between a biocontrol site and a susceptible crop.
... However, a strict host-range requirement may not be economically feasible because the majority of agroecosystems are comprised of multispecies weed communities (Frantzen et al. 2001). Plurivorous pathogens may be used safely under certain circumstances when they can be separated sufficiently from nontarget hosts in space and time (De Jong et al. 1999). Plant architecture and morphology have played a role in the success or failure of bioherbicide agents. ...
