Investigations of multiple approaches to reduce green spot incidence in ‘WA 38’ apple

Authors: SHEICK, RYAN - Washington State University; SERRA, SARA - Washington State University; Rudell, David; MUSACCHI, STEFANO - Washington State University

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/8/2022
Publication Date: 11/11/2022
Citation: Sheick, R., Serra, S., Rudell Jr, D.R., Musacchi, S. 2022. Investigations of multiple approaches to reduce green spot incidence in ‘WA 38’ apple. Agronomy. 12(11). Article 2822. https://doi.org/10.3390/agronomy12112822.
DOI: https://doi.org/10.3390/agronomy12112822

Interpretive Summary: ‘WA-38’ is a new apple variety that has recent seen widespread planting in Washington State. As this variety is coming into production, there is concern about a disorder or defect that develops on and immediately under the peel surface as corking of the flesh during the growing season. The disorder, called “green spot”, renders the fruit unmarketable. As many similar disorders have been historically associated with heat, drought, and light stress, we sought to reduce or eliminate the disorder by modifying the light environment. We found that nearly eliminating light using paper bags also eliminated green spot while merely reducing light using shade netting had mixed results. Another characteristic of similar apple corking disorders is an imbalance of mineral nutrition which we evaluated alongside altering the light environment. Bagging can influence a variety of factors aside from merely light including light, humidity, and air flow around the apple. Mineral nutrition was not clearly linked with symptoms or conditions that mitigated the disorder. While mineral nutrition may have a role in it does not appear to be a cause and, instead, environmental conditions may be the ultimate trigger of the disorder.

Technical Abstract: A new pre-harvest apple (Malus × domestica Borkh.) disorder named “green spot” (GS) was recently identified on ‘WA 38’ apples. Previous work indicated a tentative association between GS and fruit mineral imbalance and an influence of rootstock selection on GS frequency; however, the specific causes, risk factors, and mitigation steps have not been explored. In this study, the role of microclimate, modified using netting or fruit bagging, on GS incidence in ‘WA 38’ apples over two years on two different dwarfing rootstocks, ‘Geneva 41’ (‘G.41’) and ‘Malling 9-Nic29’ (‘M.9’), was investigated. Early season fruit bagging reduced GS appearance, but netting showed mixed results between year and rootstock. Rootstock selection influenced GS incidence, especially in the 2021 season. Fruit mineral analyses highlighted some differences of mineral composition between symptomatic and asymptomatic apples, which help to corroborate earlier reports; however, within the full context of these results, it is not clear that nutrient imbalance influences disorder incidence. Further work to understand the mechanisms behind GS emergence should consider environmental factors, including light, humidity, and wind, as well as anatomical features of ‘WA 38’ fruit, such as lenticel morphology and physiology.