- Author: Brooke Jacobs
University of California (UC) Cooperative Extension Advisors and Specialists have received an overwhelming number of inquiries from the press and growers regarding the effects of the current drought on California tree crops. To address these questions I will be posting several blogs over the next few months which highlight relevant research conducted by UC Professors, Cooperative Extension Specialists, and Farm Advisors.
Dr. Bruce Lampinen (UC Cooperative Extension Almond and Walnut Specialist) has been conducting applied tree crop research for over 15 years in California. Beginning in 2005 he became interested in understanding the effects of foliar clay mineral (kaolin) sprays on walnut and almond nut quality and tree physiology. Kaolin sprays are typically advertised to manage pests, reduce sunburn, and minimize water stress by lowering leaf temperatures. Dr. Lampinen conducted a series of experiments in almond (Nonpareil variety) and walnut (Howard, Chandler, Vina and Tulare varieties). His research team compared trees sprayed with commercially available kaolin foliar sprays and unsprayed control trees. They carefully measured many aspects of tree growth and physiology including photosynthetic rate, light interception, water use, and stem water potential. Overall, Dr. Lampinen's research yielded four conclusions regarding the use of foliar sprays to reduce sunburn or drought stress:
1) Sunburn temperature threshold for walnuts is 1220F. Although air temperatures never exceeded 122oF (50oC), hull temperatures over 122oF were observed in nuts fully exposed to afternoon sun. Temperatures above this threshold were most common in the south west section of trees due to increased light exposure at the time of day when air temperature is greatest.
2) Foliar sprays consistently lowered leaf temperatures resulting in increased water use. Trees sprayed with kaolin had lower leaf temperatures mid-day. As a result, the natural response of individual leaves to close stomata during dry, hot summer afternoons and prevent water loss was over-ridden. Some trials indicated that overriding the natural response of leaves to dry and hot conditions resulted in greater tree water use, potentially inducing water stress.
3) At the orchard level the rate of sunburn damage did not differ between trees sprayed with kaolin and untreated control trees. The primary determinant of sunburn was the location of individual nuts within the tree. Nuts in the south west quadrant of a tree had the highest levels of sunburn.
4) Foliar sprays did not significantly increase yield in any of the experiments. Occasionally yield in large, healthy trees was reduced when sprayed with kaolin. Although the effects of foliar sprays varied among experiments from neutral to negative, they did not result in a significant increase in yield. Occasionally, in large healthy trees with canopy light interception greater than 80%, foliar sprays reduced yield significantly.
Links to relevant publications and research reports:
B. Lampinen, K. Kelley Anderson, S. Metcalf, and C. Negron. 2006. Effects of kaolin clay particle film on leaf temperature, nut temperature and sunburn susceptibility in walnut. Walnut Research Report.
‘A. Rosati, S. Metcalf, R. Buchner, A. Fulton and B. Lampinen. 2006. Physiological effects of kaolin applications on well-irrigated and water stressed walnut and almond trees. Annals of Botany 98 (1): 267-275
‘A. Rosati, S. Metcalf, R. Buchner. A. Fulton and B. Lampinen. 2007. Effects of kaolin application on light adsorption and distribution, radiation use efficiency and photosynthesis of almond and walnut canopies. Annals of Botany 99 (2): 255-263
C. DeBuse, J. Hasey, K. Kelley Anderson, S. Metcalf, L. Contador, B. Stewart, M. Glenn and B. Lampinen. 2010. Kaolin particle clay film effects on physiology, quality, and productivity in Tulare and Howard walnut. Walnut Research Reports.