VARIATION AND ERROR IN ESTIMATES OF ASCOSPORE MATURITY AND DISCHARGE DERIVED FROM EXAMINATION OF CRUSHED PSEUDOTHECIA OF VENTURIA-INAEQUALIS

Crushed pseudothecia from overwintered, infected leaves are routinely used to assess the maturity and release of ascospores of Venturia inaequalis for the purpose of advising growers on timing fungicide sprays for the control of apple scab. A recent study has shown a disparity between the morphological maturity of ascospores and the physiological maturity of asci. Furthermore, the accuracy and reproducibility of assessments by different observers, using different methods of sample selection, preparation, and interpretation, have not been documented. We investigated variability in the ratings of a standard set of crushed pseudothecia by 17 observers and also quantified leaf-to-leaf, within-leaf, and temporal variation in ascospore maturity and discharge in populations of V. inaequalis. Ratings of the same pseudothecia by different observers were approximately twice as variable as repeated ratings by the same observer. Substantially more variation in ascospore maturity and discharge was encountered among populations from different leaves than within populations on a single leaf. The variance of assessments would, therefore, be minimized by sampling as many leaves as possible. A sampling plan was developed to specify sample sizes required to estimate the mean number of asci within a certain class based on the probable number of asci in that class (based on historical data) and the allowable error in the estimate. The sampling plan was validated with an independent set of observations of 1,800 pseudothecia. The statistical considerations of sampling and the delay between morphological maturity of ascospores and physiological maturity of asci are incorporated into guidelines for using squash mount assessments in disease management programs.