Potential ascospore dose of the apple scab fungus (Venturia inaequalis) from Indian Himalayas.

A study was conducted during 2002-05 in apple orchards in Indian Himalayas, pseudothecia development started during November - December and progressed steadily when moisture and temperature conditions were favourable. The pseudothecia continued to develop ascigerous stage through February end until the end of April, and discharged ascospores during April - June. Ascospore maturation peaked between pink bud and fruit set stages and was completed by fruit set stage or shortly thereafter (6 - 10 meteorological weeks). A linear relationship was observed between ascospore maturation and degree-days for 2002-05. The number of cumulative degree-days for 50 and 95% ascospore discharge was approximately 418 and 792 (R 2 =0.943) respectively for orchards situated at 1 900-2 200 m asl and > 1 182 (95% ascospore discharge; R 2 = 0.967) for orchards situated at >2 200 m asl. Thus, seasonal variation of ascospore discharge during experimental years differed at different locations in the region. Potential ascospore dose values were 50 times higher in the poorly managed orchards than in the integrated managed orchards.