Aspects of microclimate variation within apple tree canopies and between sites in relation to potential Venturia inaequalis infection

The microclimate variables of major consideration for infection by Venturia inaequalis are surface wetness, temperature, and humidity. Because infection risk criteria tend to assume that there is uniformity of microclimate within a tree, and between sites within a district, the parameters are usually monitored with one or a few sensors at each site. To examine this uniformity, studies were made of the occurrence of wetness on four electrical grid sensors within an apple ( Malus domestica Borkh.) tree canopy at each of seven sites, and of dry bulb (DB) temperature and relative humidity (RH) at the seven sites within an apple growing district. Wetness observations were made on days when a Reuter‐Stokes “Apple Scab Predictor” registered wetness at one site. Temperature and RH observations were made at 0300, 0900, 1500, and 2100 h on the same days. The probability of one wetness sensor detecting wetness was c. 0.6 and the spatial location of the sensors within the canopy did not in most instances affect wetness detection. On only c. 5% of occasions were all sites wet at the same time. On two thirds of occasions, within a tree, the start of the wet period varied by >1 h across the four sensors, and on one third of occasions by >2 h. The length of the wet period varied by > 1 h on 97% of occasions and was >5 h on almost two thirds of occasions. When the length of the wet period was extended by the time the RH remained >90%, as is used in some V. inaequalis infection risk criteria, almost 50% of occasions varied by >5 h between the four sensors. The range in DB temperature across the seven sites was c. 2°C in December to 6°C in February. The range of RH across the district was c. 15% in December to 23% in February. Our results question the reliability of a single wetness sensor on a single weather station to provide accurate data on infection conditions across an apple growing district. Leaf wetness detection reliability could be substantially improved by the provision of at least three wetness sensors at each site. Provision of a disease warning service across a district may require weather stations to be located at multiple sites, depending on the topography of the district, to provide accurate information.