Dusty orchard conditions during the pistachio bloom period could be a reason for poor fruit set in pistachios.

University of California, Davis researchers Lu Zhang and Louise Ferguson are conducting a study on the effects of dust, and dust mixed with herbicide and pesticide residues, on pistachio flowers during the bloom period. This includes dust generated inside the orchard, and dust blown into orchards.

With this wind-pollinated species, the male staminate and female pistillate inflorescences are panicles composed of hundreds of individual small flowers. Infloresence buds begin to swell in late March and into the first two weeks of April when the 100-300 flowers per panicle are pollinated and set.

The florets of male and female flowers have no petals. The stigmas and anthers are fully exposed to air and vulnerable to environmental conditions.

Zhang reports that a successful fruit set for pistachio is determined by stigma condition, pollen tube health, and ovule longevity. Preliminary research in 2016 found that dust inhibits pistachio pollination; causing the flower stigma to wilt. Using a scanning electron microscope to see the effects of dust on pistachio flowers, Zhang and Ferguson found dust alone caused stigmas to wilt.

A mixture of dust, pesticide, and herbicide residue on the flowers also caused the disappearance of the papilla cells of the stigmas. Their preliminary study, says Zhang, suggests that the residues of herbicides and pesticides found on the ground in orchards are carried with dust, and during bloom it can cause unsuccessful fertilization.

They did not do a large scale trial in the field to determine which day during bloom that dust most adversely affects fruit set.

The main target for this year’s study will be determining how dust/toxic dust influences pollination, fertilization and parthenocarpy (seedless fruit). Zhang notes that he and Ferguson are checking pollen tube growth and possible dust movement in vivo and pollination-related hormone levels in flowers.

Zhang says specialized instruments made by the Amphasy Company were used to determine the dust level of the pollen and dust mixtures used in the hand pollination of pistachio flowers. To measure the natural dust level in the field, he says dust was collected from sticky tape attached to trees. Dust levels were also checked by microscope.

The 2016 trial included bagging Kerman pistachio flowers which were hand pollinated on three successive days with dust, pollen, dust and pollen, and dust mixtures during bloom. Zhang says the findings suggest that nut set and production were extremely low in clusters receiving only dust, compared to clusters receiving only pollen, plus pollen and dust mixtures.

The production of the clusters receiving dust was only 32 percent in nut count and 38 percent in nut weight compared to clusters with applied pollen. The higher percentage of blanks in the un-pollinated control confirmed observations of fruit development without fertilization as the more common reason for blanking.

Extremely dry conditions during the bloom period (except for this year) can cause high amounts of dust generated by equipment, including flail mowers. Zhang says avoiding work on orchard floors, wetting middles of rows, or spreading mulch in the middles can help minimize in-orchard dust during bloom.

He believes all varieties of pistachio are affected similarly by dust, regardless of their bloom period. Even earlier blooming varieties, including Kaleghouchi, Golden Hills, and Lost Hills, are no more than one week earlier than the dominant cultivar Kerman.