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Lu Zhang, Robert Howard Beede, Gary Banuelos, Christopher M. Wallis and Louise Ferguson

Springtime flail mowing of row middles for weed control in California pistachio (Pistacia vera L.) orchards blows dust into the leafless canopy if it occurs during bloom. The effect of dust on pistachio pollination and fruit set is unknown. Rachises were bagged prebloom and hand pollinated with pollen and dust mixtures at 1:0, 1:1, 1:4, 1:16, and 0:1 volume/volume ratios on five successive days. The 2016 and 2017 trials demonstrated that the inflorescences treated with a high pollen:dust ratio (0:1, 1:4, and 1:16) had significantly lower split nut rates (commercially less profitable) compared with low dust ratio tests (1:0 and 1:1). Our results also showed that dust damaged both pollen viability and stigma quality, particularly if contaminated with herbicide residues (GlyStar® Plus and Treevix®). Decreased yield was a function of decreased fruit set; increased embryo abortion, parthenocarpy, or both; and a lower split nut percentage. The GA3 content in flowers of both the pollen and dust treatments was significantly higher than that in nonpollinated flowers, suggesting dust stimulated parthenocarpy, resulting in empty nutshells, “blanks” at harvest.