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 Ven turia inaequalis are surface wetness, temperature, and humidity. Becau se infection risk criteria tend to assume that there is uniformity of microclimate within a tree, and between sites within a district, the p arameters are usually monitored with one or a few sensors at each site . To examine this uniformity, studies were made of the occurrence of w etness on four electrical grid sensors within an apple (Malus domestic a Borkh.) tree canopy at each of seven sites, and of-dry bulb (DB) tem perature and relative humidity (RH) at the seven sites within an apple growing district. Wetness observations were made on days when a Reute r-Stokes ''Apple Scab Predictor'' registered wetness at one site. Temp erature and RH observations were made at 0300, 0900, 1500, and 2100 h on the same days. The probability of one wetness sensor detecting wetn ess was c. 0.6 and the spatial location of the sensors within the cano py did not in most instances affect wetness detection. On only c. 5% o f occasions were all sites wet at the same time. On two thirds of occa sions, within a tree, the start of the wet period varied by > 1 h acro ss the four sensors, and on one third of occasions by > 2 h. The lengt h 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 w as extended by the time the RH remained > 90%, as is used in some V. i naequalis 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 degrees C in December to 6 degrees 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 sen sor on a single weather station to provide accurate data on infection conditions across an apple growing district. Leaf wetness detection re liability could be substantially improved by the provision of at least three wetness sensors at each site. Provision of a disease warning se rvice across a district may require weather stations to be located at multiple sites, depending on the topography of the district, to provid e accurate information.