Use of a Rainfall Frequency Threshold to Adjust a Degree-Day Model of Ascospore Maturity of Venturia inaequalis.

Estimates of ascospore maturity generated by a model developed previously in New Hampshire, United States, were compared with the cumulative release of ascospores in southern Norway as monitored by volumetric spore traps at one site for 12 years, and at two additional sites for 2 years. In locations and years with frequent rain events, model-estimated ascospore maturity closely approximated observed ascospore release. However, in years with protracted dry periods of 1 to 3 weeks with no or little rain, not only was spore release delayed, but release continued to lag behind predicted maturity even after several rain events subsequent to the dry interval. By halting degree-day (base = 0°C) accumulation if 7 consecutive days without rain occurred, accuracy of the model during "dry" years was greatly improved, without substantially affecting accuracy in "wet" seasons. With minimal additional effort on the part of the user, this simple modification increases the accuracy of model-derived estimates of ascospore maturity when lack of rain slows ascospore maturation.