Rootstock and Nutrient Imbalance Leads to ‘‘Green Spot’’ Development in ‘WA 38’ Apples

Abstract

‘WA 38’ is a new apple (Malus domestica Borkh.) cultivar, released by Washington State University (WSU) in 2017. An unknown disorder, ‘‘green spot’’ (GS), dark green halos in the epidermis, with necrotic, corky, and oxidated cortical tissue underneath the damaged epidermis, leads to unmarketable fruit and has become a threat to the adoption and profitability of ‘WA 38’, with young and mature orchards exhibiting up to 60% incidence in 2020. Given the apparent susceptibility of ‘WA 38’ to GS, this research investigated GS relation with nutrient levels in fruit. Research was carried out in 2018 and 2019 in a ‘WA 38’ apple block planted in 2013, on ‘Geneva 41’ (‘G.41’) and ‘M.9-Nic 29’ (‘M.9’) rootstocks. In both years, fruit number per tree, fruit weight, and fruit diameter were evaluated in 18 trees per treatment, from both rootstocks. From each tree, fruit were classified for presence or absence of GS, and subsequently analyzed for nutrient concentration in the peel and in the flesh, nutrient extraction, and total nutrient content, on an individual apple basis. Apples with GS had higher nitrogen (N) and magnesium (Mg) levels in the peel, regardless of year and rootstock. Apples grown on ‘G.41’ rootstock exhibited higher GS incidence and reduced crop load in both years; reduced size and fruit diameter were exhibited only in 2018. Fruit on ‘G.41’ had higher N, potassium (K), and Mg in the flesh and higher N and Mg in the peel, with lower levels of calcium (Ca) in the flesh and peel; however, only in 2018, with no differences in 2019. GS in ‘WA 38’ apples appears to be another Ca-related disorder in which excessive vigor, rootstock, and N and Mg excess are predisposing factors for its development.

Investigations of Multiple Approaches to Reduce Green Spot Incidence in ‘WA 38’ Apple

“…Symptoms of GS emerge approximately three months after full bloom and worsen until harvest. Unlike other cork-like physiological disorders such as bitter pit and Jonathan spot [4,6], GS symptoms on 'WA 38' apples seem to develop exclusively preharvest; worsening of severity during storage has not been observed [46].…”

“…In the second year, we also evaluated whether the application of Parka ® [48], a phospholipid-and polysaccharide-based coating designed to promote plant cuticle health, would alter GS incidence or severity in 'WA 38'. Additionally, to further supplement the previous work of Sallato et al [46], we investigated the effect of rootstock selection and the role of mineral composition of peel on GS in 'WA 38' apples.…”

“…Although Sallato et al [46] found that GS in 'WA 38' was associated with fruit nutrient profiles similar to bitter pit-afflicted fruit in other cultivars, as well as differential susceptibility risk by rootstock selection, the specific cause(s) of the disorder and processes to mitigate it remains unclear. We hypothesized that GS in 'WA 38' may be triggered by conditions in the orchard, such as extreme light, heat, or humidity.…”

“…Outstanding fruit quality traits of this cultivar were reported [44]. A preharvest physiological disorder affecting 'WA 38' apples called "green spot" (GS, for symptom appearance at its onset) was identified in some experimental and commercial orchard plantings [29,45,46]. Symptoms typically occur near the pedicel end of the fruit as superficial regions of altered peel color without an apparent impact on the underlying cortex tissue, or as sunken discolored peel underlain by darkened, cork-like areas of necrosis in the affected flesh.…”

“…These affected areas may still be evident late in the season, but mild cases are often masked on apples that achieve intense red coloration by harvest. Symptoms affecting both peel and cortex typically appear as spots with regular margins, previously described as "dark green halos in the epidermis, with necrotic, corky, and [oxidized] cortical tissue underneath the damaged epidermis" [46]. In some cases, spots may darken along the season and appear brown by harvest.…”