Written by Manoella Mendoza, Ines Hanrahan, and Gabriela Bolaños, Washington Tree Fruit Research Commission (WTFRC), July 2022.
- The lowest incidence of good flavor in WA 38 was from apples harvested at starch 1.5 (45%) and stored in RA for up to six months. Under the same conditions fruit harvested at starch levels from 2.0 to 4.5 had better flavor (83 to 100% good flavor).
- WA 38 apples had a higher percentage of good flavor when not treated with 1-MCP and stored in CA, except at ten months of storage
- For WA 38 stored in CA, the average fruit firmness was above 17.0 lb., regardless of treatment and storage length
- Diphenylamine (DPA) at 2100 ppm did not cause phytotoxicity on WA 38 apples
Based on questions received during the 2021-22 harvest and storage season, our team has done additional work to answer some of the most frequently asked questions. This article is a compilation of two experiments which the goals are:
- Evaluate the influence of 1-MCP treatment on fruit flavor considering the starch level at harvest.
- Examine DPA phytotoxicity on WA 38 apples.
The compound 1-methylcyclopropene (1-MCP) has been used as a postharvest storage treatment to slow fruit ripening. The 1-MCP binds to ethylene receptors and hinders ethylene-depended reactions, such as fruit maturation. It can improve firmness retention and reduce the incidence of storage disorders like superficial scald. However, 1-MCP efficacy is highly related to maturity at harvest and time of application, and when applied to unripe fruit, it may inhibit flavor development. Diphenylamine (DPA) is an antioxidant compound used for postharvest control of superficial scald on apples. DPA application is very common on Granny Smith apples due to their high superficial scald susceptibility. DPA is also known to cause phytotoxicity on apples, making them unmarketable. The treatment is often applied by drenching with a mix of DPA and a postharvest fungicide but can also be applied via aerosol or fogging within the storage room. Since no evidence of superficial scald has been seen on WA 38 apple, DPA application is not recommended, but a postharvest fungicide application is advised to decrease postharvest losses from decay. Because Granny Smith and WA 38 harvest timing overlap, a warehouse may face logistics challenges in applying a fungicide alone while avoiding DPA treatment on WA 38.
Experimental Design and Set-Up
1. Starch levels at harvest and flavor development during cold storage
Data was collected from the WSU apple breeding program phase 3 (P3) orchards in Quincy and Prosser from 2010 to 2016. The trees were planted in 2008 on M9 337 rootstock in both locations.
After harvest, a sample of 20 to 40 apples was collected and transported to the WTFRC laboratory in Wenatchee. Starch degradation, internal and external quality analyses were performed within 24 hours of harvest. Storage samples, in 30lb. crates, were drenched with a postharvest fungicide at one of the Stemilt facilities and stored in their research storage unit in Wenatchee. Half of each batch was treated with SmartFresh (1-MCP, 1000ppb) within one week of harvest, and samples of both 1-MCP treated and untreated fruit were stored in refrigerated air (RA, 33°F) and controlled atmosphere (CA, 34°F, 1% CO2, 2% O2) to mimic standard commercial storage conditions. WA 38 was stored in RA for 2 to 6 months and in CA for 4 to 10 months.
Quality analysis of storage samples occurred at the WTFRC laboratory after seven days at room temperature to closely mimic fruit quality as sampled by the consumer after transport and handling. Flavor assessment was conducted on 20 apples for each storage sampling combination during quality analysis. The flavor was classified as good, bland (no flavor), or off-flavor. We evaluated a total of 4,230 apples combining locations and years.
2. DPA phytotoxicity trial
WA 38 apples were harvested from an orchard near Rock Island and another near Quincy. The apples were drenched with DPA at 2100 ppm mixed with the postharvest fungicide Academy (fludioxonil and difenoconazole). The apples were stored in refrigerated air (RA, 33°F) and evaluated every other week for four months. Phytotoxicity was assessed visually and recorded as absent or present. The sample size for Rock Island and Quincy were 165 and 183 apples, respectively.
Results and Discussion
1. Starch levels at harvest and flavor development during cold storage
The treatment combinations corresponding to each pick date were grouped based on starch reading at harvest and associated with the flavor classification received during fruit quality analysis after storage. The results below are based on the combined results of fruit harvested at Prosser and Quincy from 2010 to 2016. The RA and CA samples are analyzed separately.
Most fruit stored in RA for up to six months did not receive 1-MCP treatment; thus, it was not possible to determine if 1-MCP had affected fruit flavor in this condition. When differentiating fruit by starch index at harvest, more than 80% of the WA 38 apples were classified as having good flavor, except for fruit harvested at 1.5 starch, of which less than half had good flavor (Figure 2.).
At six and eight months of CA storage, at least 90% of the apples harvested at 2.0 to 4.5 starch were classified as having good flavor regardless of 1-MCP treatment (Table 2). However, the fruit treated with 1-MCP scored lower than the untreated fruit at six months and equal to or lower at eight months of storage. Fruit harvested at 1.5 starch and stored in CA for six months had the lowest percentage of good flavor compared with fruit harvested in the 2.0 to 4.5 starch range. The apples treated with 1-MCP scored similarly to untreated fruit but achieved better flavor ratings at ten months in CA.
Table 2. Percentage of good flavor by starch degradation level at harvest for apples stored in CA for six, eight, and ten months with or without 1-MCP treatment. Data summary of fruit harvested at Prosser and Quincy from 2010 to 2016. Total fruit evaluated equal 3060.
The fruit firmness (lb.) of WA 38 apples stored for six, eight, and ten months in CA are summarized in Table 3. Because the dataset is a compilation of sampling sets, the sampling size for each storage length varies, allowing a direct comparison between treatments per time point, but comparing storage lengths is not appropriate.
The average fruit firmness was above 17.0 lb., regardless of treatment and storage length. Apples treated with 1-MCP typically had higher firmness than untreated apples. However, the treatment difference was usually less than 1.0 lb., except for apples harvested at starch level 3.0 and stored in CA for ten months (diff. 1.7lb.). Titratable acidity and soluble solids concentration were comparable between 1-MCP treated and untreated fruit (data not shown).
Table 3. Average firmness (lb.) for apples harvested at starch levels of 1.5 to 4.5 (WA 38 starch scale), with and without 1-MCP application, and stored for six, eight, and ten months in CA. Data summary of fruit harvested at Prosser and Quincy sites from 2010 to 2016.
1-MCP treatment has limited use in controlling apple greasiness during storage. WA 38 greasiness incidence during RA and CA was discussed in the Fruit Matters newsletter released in the August 2021 edition (see additional resources). In summary, greasiness is more prevalent on fruit harvested from 2–3-year-old trees. The apples stored in RA typically develop more greasiness than fruit stored in CA. 1-MCP treatment suppresses greasiness development during storage in mature orchards but is less effective on fruit from young trees (when greasiness is high).
Considering fruit flavor ratings, quality parameters, and greasiness incidence, applying 1-MCP might be beneficial only for the longest-term CA storage if apples are harvested at a 3.0 starch level. For six to eight months of storage, 1-MCP does not appear advantageous or cost-efficient as it may be detrimental to fruit flavor and has no effect on quality parameters.
2. DPA phytotoxicity
WA 38 apples treated with a mix of DPA and Academy at 2100 ppm did not develop phytotoxicity symptoms during the four months of observation. This result indicates that WA 38 could be drenched with a fungicide solution containing DPA.
Caution: we only tested two lots of fruit. Distinct lots may present different levels of sensitivity to a chemical burn. It is generally not recommended to use DPA on WA 38 because it does not develop superficial scald.
We want to thank Tom Auvil (former WTFRC employee) and Kate Evans (WSU Apple Breeding program breeder) for their collaboration and the following companies for their contributions: Stemilt, Legacy Fruit, and Agrofresh.
Contact & additional information
Please contact Manoella Mendoza if you have any further questions. More information regarding WA 38 management can be found at the WSU Tree Fruit Comprehensive Tree Fruit Site or Washington Tree Fruit Research Commission