Written by Bernardita Sallato, WSU Extension, and Garrett Bishop, GS Long
Green spot refers to a discoloration identified on WA 38 fruit that could indicate a potential disorder. The cause(s) for green spot are unknown and we have limited experience with it because there are only a few mature WA 38 orchards evaluated. Initially, the symptoms were only in the peel without a clear impact in the flesh (Figure 1).
Visible symptoms of green spot in WA 38 apples can generally appear two to four weeks before harvest with variable degrees. Usually first appearing as a green russet or flecking affecting only the peel, and in some cases developing into distinct green halos and necrotic tissue that can also affect the flesh. Under severe cases, it can develop splits (Figure 2).
This disorder appears to sometimes affect both the peel and the fruit flesh immediately beneath the peel, with a ‘sandy’ appearance and a green to brown color (Figure 3).
To better understand the disorder and its cause, in 2018 a joint effort between Washington State University, the WA Tree Fruit Research Commission (WTFRC) and GS Long, fruit were evaluated from four mature WA 38 blocks focusing on the relationship between incidence of green spot and fruit mineral composition. Apple fruit with and without green spot symptoms were collected at harvest from four different Washington orchards – one in Prosser, one at WSU Roza orchard near Prosser, one in Quincy, and one at WSU Sunrise orchard near Wenatchee. Prosser and Quincy orchards are on M9-337 rootstock, while WSU Roza and WSU Sunrise have two rootstocks: M9-Nic29 and G41 rootstock. After 4 weeks of storage at 33 ºF, fruit with weight between 300 and 350 grams from each site and rootstock were categorized according to their green spot severity. Clean fruit (no green spot) and green spot severity between 4 and 5 (Figure 4), were then divided into peel and flesh for nutrient analyses. The peel samples correspond to the skin of the apple obtained with a handheld peeler. The flesh samples correspond to the hypanthium of the fruit, obtained by coring the fruit including peel and pulp while removing the seed core out of the sample. Total nitrogen (N), potassium (K), calcium (Ca) and magnesium (Mg) were determined following standard nutrient analyses. Soils samples were collected from each location and evaluated for pH, organic matter, cation exchange capacity (CEC), calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), nitrate (NO3) and ammonium (NH4). Results were compared between sites and the standard values recommended for tree fruit (See more at https://treefruit.wsu.edu/orchard-management/soils-nutrition/fruit-tree-nutrition/). In our study, all soils were within adequate levels of nutrient and there were no indication that nutrient availability in the soil had a relation with green spot condition.
Nutritional concentration of fruit. Fruit exhibiting green spot symptoms had more nitrogen than clean fruit (Table 1). Calcium concentration ranged from 3.8 to 5.2 mg/100g, and green spot afflicted fruit were consistently 10 to 22% less. Rather than absolute concentrations of individual nutrients, the ratios between key nutrients have been extensively utilized as indicators for fruit quality and nutrient diagnostics (Ferguson and Watkins 1989, Miqueloto et al 2014, Sallato et al, 2017). In our evaluation of the N:Ca ratio we observed a similar response to that obtained with nitrogen analyses alone, but the differences were greater. Potassium and magnesium concentration were equivalent between green spot and clean fruit, although K:Ca and (K+Mg):Ca ratios were higher in green spot fruit compared to the clean fruit. Across all sites and rootstocks, N:Ca ratios of clean fruit ranges from 4.7 to 8.8 and between 9.9 to 12.7 on green spot fruit (Table 1).
Table 1. Nutrient concentration in fruit flesh expressed as mg/100g of fresh fruit, and nutrient ratios from four WA 38 orchards, and presence or absence of green spot.
|Location: Rootstock – Condition||Nutrient Concentration
mg/100g of fresh matter
|Prosser: 337 – Clean||33||3.9||114||5.3||8.6||29.3||30.7|
|Quincy: 337 – Clean||23||4.6||126||4.7||5.0||27.2||28.2|
|WSU Roza: G41 – Clean||43||5.2||154||5.8||8.3||29.9||31.0|
|WSU Roza: G41 – Green Spot||47||4.7||156||5.7||9.9||33.2||34.4|
|WSU Sunrise: G41 – Clean||43||4.9||140||5.8||8.8||28.6||29.8|
|WSU Sunrise: G41 – Green Spot||49||3.8||135||5.6||12.7||35.2||36.7|
|WSU Sunrise: M09 – Clean||22||4.8||117||4.7||4.7||24.6||25.6|
|WSU Sunrise: M09 – Green Spot||40||4.0||122||4.7||10.1||30.2||31.4|
Fruit peel concentrations were similar to that obtained in the flesh, except at the WSU Roza orchard. Results from WSU Roza orchard could have been affected by the sunburn spray program of coatings and calcium base products that remained in the fruit peel even after fruit were washed and scrubbed before analysis. At WSU Sunrise, green spot fruit had higher nitrogen and potassium, and lower calcium compared to the clean fruit. Again, the differences were highlighted when comparing N:Ca, K:Ca and (K+Mg):Ca ratios were fruit affected with green spot exhibited higher values compared to clean fruit (Table 2).
Table 2. Nutrient concentration in peel, expressed as percentage of fresh fruit, and nutrient ratios of fruit peel in fruit from different location, rootstock and green spot condition.
|Location: Rootstock – Condition||Nutrient Concentration
% dry matter
|Prosser: 337 – Clean||0.21||0.07||0.56||0.08||2.9||7.7||8.8|
|Quincy: 337 – Clean||0.33||0.12||0.68||0.09||2.7||5.5||6.3|
|WSU Roza: G41 – Clean||0.41||0.13||0.71||0.09||3.5||6.0||6.8|
|WSU Roza: G41 – Green Spot||0.36||0.14||0.79||0.10||2.7||5.8||6.5|
|WSU Sunrise: G41 – Clean||0.42||0.11||0.73||0.09||4.2||7.5||8.4|
|WSU Sunrise: G41 – Green Spot||0.49||0.07||0.81||0.11||6.7||11.0||12.5|
|WSU Sunrise: M09 – Clean||0.36||0.11||0.61||0.09||3.4||5.7||6.6|
|WSU Sunrise: M09 – Green Spot||0.43||0.08||0.74||0.11||5.3||9.1||10.4|
Note that samples with and without green spot were obtained from the same site, same tree and sometimes even the same spur (Figure 5), similar to the expression of bitter pit in Honeycrisp (Miqueloto et al 2014, Saure, 2014).
Nutrient concentration in green spot fruit were similar to what has been reported for varieties susceptible to bitter pit disorder (Baugher, et al. 2017, Cheng and Miranda, 2018, Kalcsits, 2016) and other calcium deficiency disorders (Marschner 2002, Sallato et al 2017). Several authors have pointed out that higher ratios of N:Ca, K:Ca, (Mg+K):Ca are indicative of calcium deficiency disorders incidence (de Freitas, et al 2010, Amarante, et al. 2011, Kalcsits, 2016) also observed in green spot fruit in this study.
Most recent reviews have indicated that a calcium deficiency in fruit should be considered the result rather than a cause (Saure, 2014). In this regard, calcium deficiency has been associated with xylem cell abundance and functionality (de Freitas et al 2011, Miqueloto, et al. 2014) as well as to many other abiotic external stresses such as water management, salinity, shoot and root temperature, light, etc. (Ho and White, 2005, de Freitas and Mitcham, 2012, Saure, 2014). Consequently, many growing factors can affect the development of the disorder, and the causes in one site can differ from another.
In this preliminary evaluation, the examination of each site independently gave us a better understanding of nutritional differences between apples with and without green spot, and its close relation with calcium deficiency disorders and bitter pit in apples.
- Green spot fruit had lower levels of calcium, combined with high levels of nitrogen and potassium, similar to the relations observed in bitter pit, a calcium deficiency disorder.
- Fruit nutrient concentration should be analyzed within homogeneous growing conditions for adequate interpretation, and absolute elemental values alone might not be appropriate for green spot diagnostic.
- Trees under the same management program and growing condition can develop fruit with or without green spot, suggesting an internal regulation for its development.
This article aims to inform growers about the presence and symptomatology of green spot and to serve as starting point for future research. This study was not replicated or validated through a scientific process. A replicated trial, that supports these preliminary results, is currently under evaluation for a second year of sampling. WSU and collaborators will continue to further investigate green spot.
We would like to thank Ines Hanrahan and the team at the Washington Tree Fruit Research Commission for their collaboration.
Regional Tree Fruit Extension Specialist
Washington State University, IAREC, Prosser.
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