An evaluation of sensors to estimate grape leaf temperature during frost risk periods

Abstract

Frost damage to grapevines is a threat in many wine grape production regions of the world. Improved information regarding frost temperature conditions will help farmers understand current conditions during frost risk nights and select the most appropriate strategies to avoid cold damage to their crops. Standard methods of measuring air temperature as currently used in many vineyards generally do not accurately indicate the temperatures experienced by sensitive plant tissues which are subject to radiation heat losses. The objective of this work was to evaluate alternative temperature sensors to determine if they provided more accurate measurements of leaf temperature as compared to standard air temperature readings. At a two-year old Cabernet Sauvignon vineyard near Templeton, California, the temperatures of 12 leaves were measured continuously from mid-September through mid- November 2014 using fine-wire thermocouples. These measurements were compared to standard shielded air temperatures, and to the temperatures of two sensors designed to mimic leaf temperature: 1) a 7 cm diameter thin copper disc positioned horizontally and painted flat black, and 2) the interior of a 15 cm diameter hollow copper sphere, also painted flat black. All temperatures were measured at a height of 1.8 m. A net radiometer was used to determine nighttime radiation loss conditions. The average leaf temperature was 0.9 °C colder than the shielded air temperature during the night, but under very clear sky conditions the leaf temperature was over 2 °C colder than the shielded air temperature. The copper disc and copper sphere were on average 0.4 °C colder than the leaf temperature, with the temperature of the copper sphere being slightly closer to the leaf temperature. The result of this trial demonstrates the advantages that non-conventional temperature sensors can have for improving our estimates of actual leaf temperatures under cold weather conditions.