Green spot in 'WA 38': growth dynamics, vascular integrity, and underlying mechanism(s)

Authors: CARRA, B - Washington State University; SERRA, S - Washington State University; KNOCHE, M - Washington State University; Rudell Jr, David; MUSACCHI, S - Washington State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: ‘WA 38’ (Cosmic Crisp™) is a new apple variety coming into production with the expectation that it will be a mainstay of US production in the near future. As with most apple varieties, ‘WA 38’ production has very specific challenges. In this case, a peel defect called “green spot”, with symptoms including large green patches on the surface of this dark red apple and corky flesh immediately below patches, can develop in the orchard during the middle portion of fruit development leading to significant product loss undercutting the viability of the variety. As part of ongoing work, we sought to determine events during fruit development that may lead to green spot symptoms with the intent of developing production strategies that reduce or eliminate green spot. Conditions that lead to green spot are largely unknown so, in this work, we sought to determine if fruit growth patterns specific to this variety or its parents are linked with disorder risk. Results indicate symptoms typically develop towards the top of the apple where fruit were growing the most at the time when symptoms first appear. Due to this growth, water transport to the cells that are altered or eventually die to form the symptoms are cut off. This may contribute to symptom development. Using this information, further work is determining whether orchard conditions may be adjusted to modify fruit growth and control green spot.

Technical Abstract: 'WA 38' is a cross of 'Enterprise' and 'Honeycrisp'. It is susceptible to the surface disorder known as green spot (GS), which reduces its marketability. This study aimed to characterize fruit growth in 'WA 38' and explore potential links to vascular integrity. Among the three cultivars, 'WA 38' exhibited the most rapid fruit growth, followed by 'Enterprise' and 'Honeycrisp'. While fruit diameter was similar across cultivars, 'WA 38' apples were significantly longer. Allometric analysis revealed hyperallometric growth in the height of the calyx lobes and pedicel end region relative to diameter in all cultivars. Conversely, core height growth was hypoallometric. 'WA 38' showed faster calyx lobe elongation than either parent cultivar. However, in the pedicel end region, 'Honeycrisp' exhibited more rapid growth than ‘WA 38’. The position of vascular bundles was unusual across the three cultivars. Petal bundles were more peripheral than sepal bundles in 'WA 38', followed by 'Enterprise' and 'Honeycrisp'. Acid fuchsin feeding pedicels of immature 'WA 38' fruit stained sepal and petal bundles and a dense network of minor veins. In mature fruit, staining was limited to petal and sepal bundles near the seed cavity. More petal bundles were stained in immature fruit than mature fruit, while sepal bundle staining remained constant. Notably, rupture of petal bundles occurred near the seed cavity. Green spot disorder did not consistently affect the calcium-to-dry-mass ratio. However, bagged fruit showed lower nitrogen and calcium but higher potassium dry mass ratios than non-bagged fruit. The xylem of 'WA 38' may become dysfunctional probably because of hyperallometric growth in height at the pedicel end and the unusual peripheral position of petal bundles. Although the total Ca/dry mass ratio of 'WA 38' appears sufficiently high, imbalances will likely arise between different Ca pools within the fruit.