Evaluation of three models for predicting <i>Venturia inaequalis</i> ascospore release in Southern Brazil

Abstract

Ascospores of Venturia inaequalis constitute the principal source of primary inoculum for epidemics of apple black spot (scab) Mathematical models that describe ascospore release have been developed in USA New Zealand and Italy These models are based upon degreeday accumulation In this work the seasonal pattern of released ascospores of V inaequalis in Vacaria Brazil was evaluated During four apple growth seasons (2009 to 2012) the natural release of ascospores from five orchards was recorded The cumulative percentage of ascospore release was predicted using three different degreeday models The released ascospore data showed the best fit to the model developed in Italy with a start date of 22 August rather than the phenological stage of apple bud break It is concluded that in regions with warm winter temperatures models that use bud break to initiate ascospore maturation will tend to underestimate availability of ascospores at the beginning of the apple growth season