Comparisons of Scion/Rootstock Circumference Ratios Among Pistachio Cultivars and Rootstocks in California

Similar growth rates of scions and rootstocks are desirable for fruit and nut tree crops to provide a strong graft union. In pistachio, a scion/rootstock trunk ratio close to 1 provides a uniform surface for connection of the mechanical shaker to the tree trunk during nut harvest. Typically, several pistachio trees are shaken per minute and large differences in the circumference of scion and rootstock can result in bark damage and delays in the harvest process.  Future possible graft incompatibility issues are also a concern as trees age although there is no evidence to suggest that incompatibility from overgrowth is an issue. Scion cultivars are Pistacia vera, and are grafted almost exclusively to P. integerrima or P. atlantica x P. integerrima hybrid rootstocks in the U.S. because of the resistance of P. integerrima to soil-borne Verticillium wilt and its ability to produce a rapidly growing, early fruiting tree.  The female cultivar ‘Kerman’ and the male pollinizer, ‘Peters’ are the primary cultivars  grown in California, and growth rates of scion and rootstocks appear to be similar. Newer alternative and improved  P. vera cultivars have been reported to show varying amounts of scion overgrowth when grafted to existing commercial rootstocks with P. integerrima parentage. Newer scion cultivars are important to the industry for extending the harvest season, improved nut quality and pest avoidance. Differences in growth rate of scion and rootstock were measured for a number of  P. vera cultivars on P. integerrima or P. atlantica x P. integerrima hybrid rootstocks of existing trees of different ages in multiple sites in the southern San Joaquin Valley of California. Differences were found in the ratio of scion circumference to rootstock circumference between standard industry cultivars (‘Kerman’ and ‘Peters’) and other cultivars.  The relationship between the scion/rootstock ratio with time developed in this study would provide a baseline for measuring progress toward developing a rootstock with a growth rate more similar to those of newly developed or introduced cultivars.