Our principal objective is to improve management of apple scab through development of accurate models of inoculum development and release, and thereby reduce fungicide use and yield loss. Apples are an important fruit crop in Norway, and worldwide. All ma jor apple varieties are highly susceptible to scab, thus the disease can only be controlled in commercial production by fungicide sprays. The causal pathogen overwinters in fallen infected leaves, and during spring rain release spores that infect emergent leaves and fruit. The need for these sprays (up to 20 per season) is closely tied to maturity and release of the overwintering spores. This relationship, which could largely determine the timing and efficacy of fungicide applications, has not been fully e xploited in management of apple scab due expertise of two leading labs in this field, a wealth of historical data, and novel equipment specially designed for these studies to fulfil our principal objective. Substantial reductions in fungicide use, yield l oss, development of pathogen resistance, and environmental degradation are all anticipated benefits of the proposed research.