Tomato leaf boundary layer climate: implications for microbiological whitefly control in greenhouses

The efficiency of mycoinsecticides as a means of whitefly control was examined under greenhouse conditions, focusing in particular on the high humidity levels favouring infection of the whitefly larvae infesting the under leaf surface. The theory of the moisture transfers occurring within the leaf boundary layer under laminar conditions is outlined and a model for the air temperature and humidity distribution is developed. Measurements were made of the humidity of the air in the boundary layer on the underside of leaves of a tomato crop grown in a full scale greenhouse. A considerable increase in relative humidity was detected 5 mm from the underside of the leaves, particularly during day-time when crop transpiration reached its maximum. These measurements were compared with the results of the model based on boundary layer theory and it was established that the data recorded were accurately predicted by the model during day-time from both the qualitative and quantitative points of view, but they were underestimated at night, The implications of these findings were examined and discussed in the context of the microbiological control of whiteflies under the conditions of confinement pertaining in various greenhouses and similar cultural systems. It is suggested that the concept of the microclimate pertaining in the leaf boundary layer developed here could be applied to the biological control of insects and microbial pathogens, in both protected and open crop growth systems. (C) 2002 Elsevier Science B.V. All rights reserved.