Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2002
Publication Date: 7/20/2002
Citation: CHAREST, J., DEWDNEY, M., PAULITZ, T.C., PHILION, V., CARISSE, O. SPATIAL DISTRIBUTION OF VENTURIA INAEQUALIS AIRBORNE ASCOSPORES IN ORCHARDS.. PHYTOPATHOLOGY. 92: 769-779. 2002. Interpretive Summary: Apple scab, caused by Venturia inaequalis, is the most important disease on apples. This study looks at the spatial distribution of ascospore inoculum in an apple orchard, by trapping spores, and measuring the potential ascospore dose based on ratings on the fallen leaves. Using a number of analytical techniques, the ascospores showed a non-random, aggregated pattern of distribution.
Technical Abstract: Apple scab (Venturia inaequalis) causes important economic losses in many apple production areas of the world. The disease is controlled by numerous fungicide applications regardless of the presence of ascospores in the orchard. Airborne ascospore concentration (AAC) can be measured in real time to time fungicide applications. However, the level of heterogeneity of the ACC in commercial orchards was unknown. Consequently, the spatial distribution of V. inaequalis ascospores was studied in a commercial orchard of 0.43 ha. The potential ascospore dose (PAD) and AAC were measured in 40 quadrats each of 108m(2). In each quadrat, the ACC was monitored during the major rain events in spring 1999 and 2000 using spore samplers. The variance-to-mean ratio for the PAD and for most of the ACC sampling dates was >1, indicating an aggregated pattern of distribution. None of the frequency distributions of the most important ascospore ejection events followed the Poisson probability distribution, indicating that the pattern of distribution was not random. For all events, ACC has an aggregated pattern of distribution as suggested by the negative binomal distribution. Geostatistical analyses confirmed the aggregated pattern of distribution. The cultivars had an effect on the PAD and AAC distribution pattern, but both PAD and AAC were not uniformly distributed within a block of the same cultivar. Therefore, the number, location, and height of samplers required to estimate AAC in orchards need to be investigated before using information on AAC for decision making.