SOME RECENT ADVANCES IN EPIDEMIOLOGY OF APPLE SCAB A STENSVAND l, DM GADOURY 2, RC SEEM 2, T AMUNDSEN l , and SP Falk 2 l Department of Plant Pathology, Norwegian Crop Research Institute, Plant Protection Centre, Fellesbygget, 1432 As, Norway; 2 Department of Plant Pathology, Comell University, New York State Agricultural Experiment Station, Geneva, NY 14456, USA Background and objectives Apple scab is caused by the ascomycete Venturia inaequalis (Cooke) Wint. Ascospores released during rain from pseudothecia in overwintering leaf litter on the ground serve as the major primary inoculum in spring and early summer. Ascospore release in V. inaequalis is suppressed in darkness and at low temperatures. Because of either failures with proposed models for management of apple scab, more or less well documented results that contradict these models, or lack of easy rules of thumb to follow, new information on how climatic factors and tree growth is related to spore release and infection is often ignored by growers and grower advisors. We have confirmed and extended major findings of earlier work by studying the effect of (1) light and temperature on ascospore release; (2) protracted periods of dry weather upon spore maturation and release; (3) low temperatures on infection of ascospores and conidia; (4) plant target size, target susceptibility, and airborne inoculum on the relative risk of infection (RRI). Matedals and methods To study ascospore release under laboratory conditions, we constructed a wind tunnel in which light intensity and quality, simulated rain, relative humidity (RH), and temperature during ascospore release was precisely controlled [1]. Burkard volumetric spore traps were used to study ascospore release in the field and to monitor the airborne dose of ascospores for calculation of RRI in a mist chamber. Weather stations provided hourly climatological records. Light quantity and quality was recorded by means of LI-COR multispectral radiometers. Target size was assessed by photographing developing apple flower cluster's at all stages from green tip to petal fall, and subjecting prints to image analysis. Infection experiments at various low temperatures and investigations of target susceptibility were carried out in climate chambers. Results and conclusions In the laboratory, we observed no higher rate of ascospore release when light intensity was increased beyond a level recorded ca 1 h after sunrise on rainy days in the field. A higher than normal proportion of ascospores was released in darkness at low RH under lab conditions and towards the end of the primary inoculum season (after petal fall of the apple flower bud). Unexpected high spore releases during night in laboratory or field found by other researchers can partly be explained by these two factors. Low temperatures (1 -80C) reduced the rate of release, with a release rate directly proportional to temperature [2]. For practical purposes, in the period from bud break to petal fall, we suggest that if the temperature is 1 O"C or above, 5-1 00C, or 2-5"C and rain starts during night, a significant increase in spore release should be considered to start 1, 3, or 4 h after sunrise, respectively. After petal fall night time suppression of spore release should not be considered. Below 2"C spore release can be ignored. Protracted dry periods with no or little rain delayed spore maturation and extended the season for ascospore release. Subtraction of dry days beyond the 4th dry day in the degree-day accumulation in a spore maturation model for V. inaequalis developed in New Hampshire, gave a better estimate of spore maturation and prediction of the end of the pdmary inoculum season than using the existing model. Infections below 60C occurred after shorter time than previously reported, and based upon our data we propose a revision of the infection table for V. inaequalis [21. RRI (target size x target susceptibility x airborne inoculum dose) increased over 1 00-fold between green tip and bloom. The value changed substantially at each growth stage and dropped rapidly between bloom and petal fall. Because all three components of RRI are highly correlated with tree growth stage, it is very likely that infection risk eventually can be predicted from tree phonology alone. References 1. Gadoury DM, Stensvand A, Seem RC, 1996. Phytopathology 86, 596-601. 2. Stensvand A, Gadoury DM, Amundsen T, Semb L, Seem RC, 1997. Phytopathology 87,1046- 1053.