Late Maturity and Excess Irrigation Trigger Kernel Darkening in ‘Howard’ English Walnut ( Juglans RegiaL.) at Harvest, but not in ‘Chandler’

English walnut ( Juglans regiaL.) kernel color is an important quality factor for producers, processors, buyers and consumers because it affects marketing, sales, brand reputation, and prices. In the field and during postharvest handling, oxidation of phenolics in the pellicle (seed coat) triggers kernel darkening, resulting in amber-colored kernels. Our two-year study confirmed that ‘Howard’ is highly susceptible to dark kernel color development due to excess irrigation and harvesting at later physiological maturity, unlike ‘Chandler’. In ‘Howard’, the combination that reduced most the percentage of light-colored kernels was later physiological maturity at harvest and excess irrigation: this may account for ~40% of losses due to amber kernel color at harvest. The percentage of light-colored kernels is also reduced during cold storage, but the impact of cold storage is small (~ 10% loss) compared to that from incoming kernel color quality pre-determined by maturity stage at harvest and irrigation. Our data confirm the benefits of carefully monitoring proper irrigation, physiological maturity, and low-temperature storage (~0°C) for California walnut cultivars.