FROST RESISTANCE IN SEVEN COMMERCIAL WALNUT CULTIVARS

Walnut orchards currently extend worldwide and are even found in areas less climatically suitable to obtaining large crops. When choosing cultivars, it is essential for bursting to take place outside the winter frost period, but the potential danger of early frost in autumn is often disregarded. Early frost events could be more damaging as the species are enlarging their vegetative cycles as the result of climatic change. Although walnut trees can thrive in continental climates and can withstand temperatures of as low as -30°C during dormancy, the response of the main walnut cultivars to frost damage has hardly been studied. In this work, we assessed the response of the main commercial cultivars planted in many new orchards throughout the world, such as: ‘Chandler’, ‘Fernette’, ‘Fernor’, ‘Franquette’, ‘Howard’, ‘Serr’ and ‘Tulare’ to sub-zero temperatures. Budsticks of these cultivars were submitted to sub-zero temperatures: -8, -12, -16 and -20°C, in a controlled climatic chamber. Frost damage was evaluated by the loss of electrolytes from basal slices of budstick in a water solution. Based on the damage observed at each of the temperatures studied, the value of LT₅₀ (°C) was calculated for each cultivar following adjustment to a “non-linear model (logistic 4p)” proposed for walnut trees. There were differences on winter frost resistance among cultivars, ‘Chandler’ was the most resistant and differed from ‘Tulare’, ‘Serr’, ‘Howard’, ‘Franquette’ or even ‘Fernor’. Damage by frost autumn simulation resulted in a different cultivar ranking of susceptibility being ‘Serr’ and ‘Fernette’ significantly less damaged by sudden autumn frost than ‘Chandler’, ‘Franquette’ or ‘Tulare’.