S13 - Session O3 - Accounting for crop load to identify rootstock differences in the incidence of nutrient-related physiological disorders in apple

Authors: Lee Kalcsits *, Bernardita Sallato, Juan Munguia Delacruz

Nutrient-related physiological disorders account for significant losses for producers of apple. These disorders include bitter pit in cultivars like 'Honeycrisp' and green spot in 'WA 38'. These disorders are more present when elemental ratios of potassium, magnesium, and nitrogen are higher relative to calcium in the fruit. Crop load has been shown to be a strong factor contributing to nutrient ratios but is difficult to control in traditional rootstock evaluations due to bienniality and vigor. Here, we used a sampling approach to gather fruit quality, yield, growth, and management data across 40 orchards in 2020 and 2021 (20 'Honeycrisp' and 20 'WA 38' orchards). We then used a mixed model approach to account for variation in continuous variables like crop load and vigor as well as discrete variables like management strategies, orchard, age, planting density, and rootstock. Bitter pit ranged from 0% to as above 100% in 2020 and 2021. Green spot in 'WA 38' ranged from 0% to almost 70%. Bitter pit incidence in 'Honeycrisp' was significantly higher across the two years compared to green spot in 'WA 38'. Even when accounting for crop load, rootstock strongly contributed to nutrient-related disorder incidence. Rootstock similarly affected nutrient ratios in the fruit peel and overall disorder incidence was significantly correlated with bitter pit and green spot incidence in 'Honeycrisp' and 'WA 38' cultivars, respectively. Through a state-wide sampling approach, we are able to better identify management strategies that help growers manage at-risk orchards and management strategies to mitigate them.