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Volume 10, Issue 2


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Individual-Based Models of the Spread of Disease, Weeds, and Insects
Art Diggle
Moin Salam
Marta Monjardino

Spread of Disease

In blackspot disease of field peas primary infection occurs by spread of fungal ascospores from infected residue of past pea crops. Maturation of the ascospores occurs at a rate that depends on weather conditions before and during the time that the pea crop is growing. Release of ascospores is triggered by rainfall, and the direction and median distance of spread are determined by the wind conditions following release. The number of ascospores produced during any triggering event depends on the stage of maturation of the ascospores and the age of the residue.

Our model of the spread of blackspot disease is driven by hourly wind and rainfall data, with a spread event occurring in any hour where the amount of rainfall exceeds a threshold. Spread occurs from each cell on the map where peas have been grown. The number of ascospores spread from each cell is the product of the absolute number produced and the probability that each of these ascospores will succeed in producing an infection if it lands on a pea crop. Cauchy distribution along a straight radial path is assumed with the median spread distance being a function of wind speed and the angle of spread being the wind direction.

Figure 1 was produced by the model using ListDensityPlot. It represents an approximately 40km square area of farmland near Esperance, Western Australia. The colours indicate the number of ascospores falling on each map cell. White denotes no ascospores, with the number of ascospores increasing in the order . The red and blue lines indicate field and property boundaries. They were produced by a GIS and superimposed on the figure as an Epilog.

Figure 1. Simulated pattern of ascospore showers near Esperance, Western Australia.

Results from the model can also be exported to a GIS as was done for Figure 2, which represents several Western Australian Shires.

Figure 2. Simulated pattern of ascospore in the Northam advisory district of Western Australia.

Models of this form are also suitable for simulating spread of other small wind-borne particles such as pollen.

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