The effect of cultivation on the size, shape, and persistence of disease patches in fields

Abstract

Epidemics of soil-borne plant disease are characterized by patchiness because of restricted dispersal of inoculum. The density of inoculum within disease patches depends on a sequence comprising local amplification during the parasitic phase followed by dispersal of inoculum by cultivation during the intercrop period. The mechanisms that control size, shape, and persistence have received very little rigorous attention in epidemiological theory. Here we derive a model for dispersal of inoculum in soil by cultivation that takes account into the discrete stochastic nature of the system in time and space. Two parameters, probability of movement and mean dispersal distance, characterize lateral dispersal of inoculum by cultivation. The dispersal parameters are used in combination with the characteristic area and dimensions of host plants to identify criteria that control the shape and size of disease patches. We derive a critical value for the probability of movement for the formation of cross-shaped patches and show that this is independent of the amount of inoculum. We examine the interaction between local amplification of inoculum by parasitic activity and subsequent dilution by dispersal and identify criteria whereby asymptomatic patches may persist as inoculum falls below a threshold necessary for symptoms to appear in the subsequent crop. The model is motivated by the spread of rhizomania, an economically important soil-borne disease of sugar beet. However, the results have broad applicability to a very wide range of diseases that survive as discrete units of inoculum. The application of the model to patch dynamics of weed seeds and local introductions of genetically modified seeds is also discussed.

Impact of scale on the effectiveness of disease control strategies for epidemics with cryptic infection in a dynamical landscape: an example for a crop disease

“…A small number of sites, each comprising three or four randomly selected fields, represent the initial introduction of inoculum from outside the UK. Movement of inoculum within a field by machinery is described by a redistribution kernel (Truscott & Gilligan 2001),$p(r)\text{d}r=true\{\begin{array}{ll}left(1-p)\delta (r)+pk\text{exp}(-kr),\hfill & r>0\hfill \\ left0,\hfill & \text{otherwise}\hfill \end{array}$where p ( r ) is the probability density of a unit of inoculum being moved a distance r in the direction of travel. Applying this to the distribution of inoculum, M i at the end of a season gives the distribution for the start of the next season.…”

Heterogeneity in susceptible–infected–removed (SIR) epidemics on lattices

“…This class includes many plant pathogens, such as soilborne pathogens transmitted between neighbouring plants [6,7], or, at a larger scale, pathogens spreading within a mosaic of neighbouring susceptible fields or even farms [8,9]. The class also comprises animal pathogens that spread in host populations living in a fixed habitat, as has been shown, e.g.…”

Biotic changes in relation to local decrease in soil conduciveness to disease caused by Rhizoctonia solani

“…The high disease occurrence inside the patches has generally been attributed to a high density of the primary inoculum in the soil (Truscott and Gilligan 2001). The density of R. solani AG 2-2 assessed by real-time PCR using specific primers showed that the number of copies of target DNA was highest in the centre of the patches and it decreased along the respective transects.…”

“…Cultivation may lead to the disappearance of patches in successive seasons if the density of inoculum within a patch falls below the critical threshold. It may also increase the period for which a field is infectious without the infection being apparent (Truscott and Gilligan 2001). Conversely, there is also a threshold level with respect to the host population above which the plant pathogenic fungus may be able to invade plant roots (Gubbins et al 2000).…”

“…Similar information was reported concerning R. solani AG 2-t in tulips (Schneider et al 2001). Longdistance transmission is observed between seasons and is generally attributed to water movement and mainly to mechanical dispersal of inoculum during harvest and cultivation procedures (Gill et al 2002;MacNish 1996;Truscott and Gilligan 2001).…”