ELAD, Y., Department of Plant Pathology, ARO, Volcani Center, PO Box 6, Bet Dagan, 50250, Israel
ZIMAND, G., Department of Plant Pathology, ARO, Volcani Center, PO Box 6, Bet Dagan, 50250, Israel
ZAQS, Y., Agricultural Advisory Service, Department of Crop Protection, Ministry of Agriculture, HaQirya, Tel Aviv, 61070, Israel
ZURIEL, S., Agricultural Advisory Service, Department of Crop Protection, Ministry of Agriculture, HaQirya, Tel Aviv, 61070, Israel
CHET, I., Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot, 76100, Israel

A preparation of Trichoderma harzianum was sprayed on cucumber plants in greenhouses in order to control fruit and stem grey mould. Up to 90% control was achieved by the biocontrol agent (0·5–1·0 g/l) which in most experiments under commercial conditions was as effective as the dicarboximide fungicides iprodione or vinclozolin (0·5 g/l each) alone or alternated with diethofencarb + carbendazim (0·25 g/l each). However, in one experiment disease incidence in Trichoderma‐treated plots did not differ significantly from the control. A mixture of T. harzianum with a dicarboximide fungicide resulted in up to 96% control of grey mould. In this case control was always significant (P=0·05) but improvement of control compared with each treatment alone was not significant (P=0·05). The alternation of sprays with the biocontrol preparation and with a dicarboximide fungicide was tested in three out of the five experiments and was found to be effective, thus enabling a reduction in the use of chemical sprays. Populations of T. harzianum were on a level of 3 × 105‐8 × 105 c.f.u. per leaf and ten times lower on one fruit. They remained high after the second and third sprays. Conditions favouring the ability of T. harzianum to control grey mould were temperatures above 20°C and relative humidity between 80 and 97%. Copyright © 1993, Wiley Blackwell. All rights reserved