Low vapour pressure deficits: historical frequency and effect on tomato yield in southwestern Ontario

An historical analysis of vapour pressure deficit was done to determine the frequency of very low vapour pressure deficits during June, July and August of the growing seasons, in Harrow, Ontario. The impetus for the analysis was a lack of yield response of processing tomatoes to fertigation in 1995 compared to other years or sites. In the historical analysis, 2 years, 1964 and 1995, were identified as having a high frequency of very low daily vapour pressure deficits during the growing season. The site of the experiment where low VPD was recorded was about 5 km north from the shore of Lake Erie. A site 27 km away from Harrow, and about 10 km north of Lake Erie, near Leamington, Ontario, had a normal, increased yield response to the fertigation treatment. Key words: Relative humidity, vapour pressure deficit, yield, tomato