A SIMPLE METHOD TO FORECAST THE ASCOSPORE DISCHARGE OF VENTURIA-INAEQUALIS

Potential ascospore release of Venturia inaequalis was monitored using a waterbath method during four release seasons (1994-1997). Overwinte red leaves were shaken with water and the ascospores discharged into t he water were quantified microscopically with a cytometer. A costly ce ntrifuge system to increase ascospore concentration could be omitted w hen a high volume cytometer was used. Ascospores were quantified weekl y before and during the annual period of primary infections and result s evaluated together with data obtained from volumetric spore traps an d scab-infected trees. In the orchard, the infections were detected wi th a bioassay using apple seedlings as indicators. Data from the water bath method correlated well with that obtained from the spore traps wi th advantage that ascospores could be detected earlier at the beginnin g of the season. Spore titers increased with the main aerial spore pea ks and decreased with the end of the ascospore discharge in the orchar d. The correlation of ascospore concentrations (waterbath method) with subsequent ascospore discharges (spore trap) caused by rain indicated the applicability of the method for prognosis.