Embryo Abortion in Pistacia vera 'Pete I'

California’s 300,000-acre pistachio industry is over 90% the Pistacia vera cultivar. Cultivars with an earlier or later harvest timing would relieve the pressure on processing facilities. A recently identified cultivar, ‘Pete 1’ harvests earlier but is producing unacceptably high percentages of early season aborted nuts, which abscise early or produce partially filled and blank nuts. Both overload the hullers and float tanks. Initial field observations suggested insect pressure and/or fungal infestation. In an insect exclusion study flower buds were both covered by insect-impermeable mesh and left uncovered on the ‘Pete 1’ trees to mature throughout the season. The resulting clusters showed no significant difference in the final percentage of aborted nuts between the bagged and unbagged samples. Laboratory plating of early- and mid- nuts season nuts for fungal pathogens, detected none beyond background levels. Therefore the high percentage of blanking and partially filled nuts in the ‘Pete 1’ pistachio cultivar is not due to either insect or fungal pressures. Other possibilities are pollen or ovule incompatibility, or self-thinning through embryo abortion.