Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2010
Publication Date: 2/1/2011
Citation: Carisse, O., Meloche, C., Turechek, W. 2011. Spatial heterogeneity, incidence-incidence, and incidence-lesion density relationship of apple scab (Venturia inaequalis) in managed orchards. European Journal of Plant Pathology. 130: 349-365. Interpretive Summary: Apple scab is one of the most important diseases in apple production and is the key disease for scheduling fungicide sprays in many apple-growing areas in the world. Although apple scab has been intensively studied, little is known about its spatial distribution within an orchard. In this study, 10 years of disease incidence and lesion density data collected in commercial orchards in Quebec, Canada was used to characterize the spatial pattern of apple scab, and subsequently to develop simple relationships for predicting disease intensity. Results showed that apple scab was aggreagted in the the orchard and that the degree of aggregation varied with season. This information was used to devleop a model to predict the level of apple scab on individual leaves (a good predictor of the level of scab on fruit), from quick assessments of the disease status of a tree. The knowledge acquired from this study can be used in making practical disease management decisions, including sampling plans.
Technical Abstract: The spatial pattern of apple scab was characterized using 10 years of disease incidence and lesion density data collected in commercial orchards located in Quebec, Canada. Distributional analyses indicated that scab incidence was better characterized by the beta-binomial than the binomial distribution in 53 and 65% of the data sets at the leaf and shoot scales, respectively. Median values of the beta-binomial parameter ', a measure of small-scale aggregation, were near 0 (0.003 and 0.028) at both sampling scales, indicating that disease incidence was close to being randomly distributed. For lesion density, the negative binomial distribution fit the data better than the Poisson distribution in 86% of the data sets at the leaf scale and median value of the index of dispersion k was 0.068, indicating that aggregation of lesions. For all apple scab measurements, the power law models provided a good fit to the data. The estimated slope and intercept parameters were significantly greater than 1 and 0, respectively, indicating significant aggregation. Results of a covariance analysis showed that heterogeneity of scab incidence at both scales and lesion density was not dependent upon shoot type but that heterogeneity of scab incidence and lesion density at the leaf scale was influenced by the sampling period. A hierarchical analysis showed that scab incidence at tree scale increased as a saturation-type curve with respect to incidence at the leaf or shoot scale. Similar relationship was observed for incidences at the shoot and leaf scales. The effective sample size model based on Madden and Hughes (1999) and based on the power law parameters gave the best fit to the leaf and shoot data, respectively. The incidence-lesion density relationship at both scales was well described by a CLL-LN model, however, the models tended to under estimate lesion density. The knowledge acquired from this study can be used in making practical disease management decisions, including sampling plans.