Simulation of vertical spread of plant diseases in a crop canopy by stem extension and splash dispersal

Upward displacement of light leaf spot lesions by winter oilseed rape stem extension and dispersal of Pyrenopeziza brassicae conidia by rain-splash were both incorporated into a model to describe vertical spread of light leaf spot on winter oilseed rape in the spring. Development of leaves, flowers and pods over the period from the start of stem extension to pod ripening was simulated. In the model, as new plant parts developed from the apex during stem extension, infections on plant initials were spread to the upper canopy by internode growth. In addition, conidia produced by the pathogen were dispersed in the canopy by rain-splash and produced infections at new sites. Vertical disease spread was simulated for a number of different light leaf spot distributions at the start of stem extension and with different crop structures and rain durations. Results showed that stem extension was an important factor in influencing vertical light leaf spot spread in the model oilseed rape crop. Rain events contributed to the splash dispersal of conidia to the plant apex and the resulting infections were directed vertically by internode growth. Periods with frequent rain events in a dense crop canopy (LAI constant with height) were most favorable for light leaf spot progress. The upward spread of light leaf spot on winter oilseed rape in field experiments showed the same trends as those predicted by the model. (C) 2002 Elsevier Science B.V. All rights reserved.