Alleviation of Postharvest Skin Dimpling of MN55 Apple Fruit and A
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HORTSCIENCE 56(7):803–808. 2021. https://doi.org/10.21273/HORTSCI15847-21 All three are single-stranded RNA viruses, typically latent (symptomless), and spread through vegetative propagation by grafting, Alleviation of Postharvest Skin and top-working on infected rootstocks or in- terstems. ACLSV can elicit diverse symptoms Dimpling of MN55 Apple Fruit and a of apple fruit, including sunken spots on fruit skin, and is often detected in coinfection with ASPV and ASGV (Cieniewicz and Fuchs, Possible Association with Apple stem 2016a). In apple, no fruit symptoms of ASPV infection have been reported, but pitting and pitting virus deformity can occur in fruit of susceptible Cindy B.S. Tong, Hsueh-Yuan Chang, James J. Luby, and pear cultivars (Cieniewicz and Fuchs, 2016b). Similarly, no reports of apple fruit disorder David Bedford have been reported with ASGV infection, al- Department of Horticultural Science, University of Minnesota, 1970 though swelling of pale areas of mandarin fruit Folwell Avenue, Saint Paul, MN 55108 rind (intumescence) has been associated with this virus (Lovisolo et al., 2003). Benham E.L. Lockhart and Roy G. Kiambi The project described in this report was Department of Plant Pathology, University of Minnesota, 1991 Upper undertaken to characterize the postharvest Buford Circle, Saint Paul, MN 55108 performance of MN55 apple fruit, and inves- tigate the skin dimpling disorder, the extent Dimitre Mollov to which postharvest treatments can alleviate National Germplasm Resources Laboratory, U.S. Department of its development, and its possible associations Agriculture—Agricultural Research Service, 10300 Baltimore Avenue, with apple viruses. Building. 004 BARC-West, Beltsville, MD 20705 Materials and Methods Additional index words. cold storage, Rave™, RNA virus Plant material. MN55 trees harvested for Abstract. MN55 is an apple (Malus 3domestica Borkh.) cultivar recently released by these studies were all grown at the University the University of Minnesota apple breeding program, with fruit marketed in the U.S. of Minnesota Horticultural Research Center as RaveVR . When stored for 4 months at 0 to 4 C, MN55 fruit can develop several in Chanhassen (lat. 445103000 N, long. storage disorders, including skin dimpling. Skin dimpling incidence was greater for 933904100 W) in 2016–20. The types of trees fruit harvested 1 week later than those harvested earlier. Dimpling was not alleviated used were 1) original seedling grafted onto by prestorage treatments of 1-methylcyclopropene or diphenylamine or by holding B9 rootstock 6 months after germination, fruit at room temperature for 1 day before long-term cold storage. However, dim- 2) second test trees in the breeding program’s pling incidence was very low when fruit were stored at 6 to 7 C. Because viruses evaluation orchard that were grafted onto B9 have been implicated in other fruit dimpling disorders, the presence of viruses in or M.26 rootstocks, 3) top-worked trees, and MN55 leaves and fruit was studied. Apple stem pitting virus (ASPV) was detected by 4) rootstock trial trees in a block, all of which microscopy, reverse transcriptase polymerase chain reaction (RT-PCR) methodology, had been grafted onto B.118 rootstock (Table and high throughput sequencing (HTS) in peel of fruit from MN55 trees that exhib- 1). Second test trees had been grafted from ited skin dimpling after 4 months of storage at 0 to 1 C. ASPV was also detected in the original tree started from seed (original supermarket-purchased fruit of other cultivars with noticeable skin dimpling. Al- seedling), had passed initial quality evalua- though ASPV was not conclusively demonstrated to cause skin dimpling in our work, tions, and were selected for further evaluation its prevalence indicates that further investigations are warranted to determine the re- before commercial testing. Top-worked trees lationship between viruses and skin deformities in stored apples. have scion from the original MN55 seedling tree grafted onto existing trees of NY 66305- 139 grafted onto M.26 rootstock. Apple (Malus Âdomestica Borkh.) fruit are and AA44, a cultivar from the University of To investigate harvest maturity effects on susceptible to a variety of postharvest disorders Arkansas, and is sold in the United States as skin dimpling development, fruit were har- (Meheriuk et al., 1994; Plagge et al., 1935) that RaveVR . In preliminary studies at the University vested from five or more rootstock trial trees may or may not be present at harvest but are of Minnesota, it was found to be susceptible to on 12, 17, and 23 Aug. 2016; and 17 and 24 typically manifested after several months of superficial scald, soft scald, and skin dimpling Aug. 2017 after fruit background peel color cold storage (Smock, 1977). Common exam- when stored at 4 C. had turned yellow-green. Fruit from the dif- ples of these disorders include superficial scald, While superficial and soft scald are known ferent trees were pooled, and then randomly soft scald, bitter pit, soggy breakdown, water to be chilling injuries, there are few data on divided into four lots of 8 to 15 fruit each. core, and various types of flesh browning. Pres- the effects of storage temperature on skin dim- For experiments testing various postharvest torage treatment, such as diphenylamine (DPA) pling. Al Shoffe et al. (2016) documented a treatments, fruit were harvested from root- dips, 1-methylcyclopropene (1-MCP) fogs, heat chilling disorder that they labeled “wrinkly stock trial trees on 24 Aug. 2016; 17 and 24 treatment (Lurie et al., 1991), or precondition- skin” in ‘Honeycrisp’ apple fruit, which may Aug. 2017, 26 Aug. 2019; and 20 and 27 ing at a warm temperature, can alleviate devel- be the same disorder as skin dimpling. In pre- Aug. 2020, also when fruit background peel opment of superficial scald (Johnson et al., vious studies, skin dimpling was associated color was yellow-green. Fruit were sprayed 1980; Jung and Watkins, 2008), senescent with several viruses and viroids, such as apple twice with 10 to 18 ppm naphthaleneacetic breakdown (Nock and Watkins, 2013), and soft dimple fruit viroid (He et al., 2010), Apple acid (Fruitone L, Valent Biosciences, Liber- scald (Moran et al., 2010), respectively. Sus- chlorotic leafspot virus (ACLSV), Asian Pru- tyville, IL) about 7 d before each harvest oc- ceptibility to these disorders varies by cultivar, nus virus, Peach latent mosaic viroid, Nectar- curring after 17 Aug. of each year to prevent and modern cultivars are no less susceptible ine stem-pitting–associated virus (Xu et al., preharvest fruit drop. than older ones (Howard et al., 2018). 2019), Zucchini yellow mosaic virus (Coutts For virus testing via RT-PCR, five leaves MN55 is a recently released variety from et al., 2011), and Pepino mosaic virus (Moerk- per tree were harvested from one original the University of Minnesota apple breeding ens et al., 2015). ACLSV, Apple stem groov- seedling, two top-worked, and three rootstock program (Bedford and Luby, 2014). It was de- ing virus (ASGV), and ASPV are widely trial trees on 20 July 2018. Fruit were har- veloped from a cross between ‘Honeycrisp’ distributed and commonly occurring in apples. vested from the original seedling and the HORTSCIENCE VOL. 56(7) JULY 2021 803 Table 1. Trees harvested for experiments in this study. using a razor blade, placed on a glass slide, and viewed with a Nikon SMZ1500 stereo Type of tree Years harvested Tissue sampled Experimental objective microscope (Nikon Instruments Inc., Mel- Original seedling 2018 Leaves and fruit RT-PCR for virus detection ville, NY) equipped with an Olympus Evolt Fruit HTS for virus detection E-330 digital camera (Olympus America 2020 Leaves HTS for virus detection  fi Top-worked 2018 Leaves RT-PCR for virus detection Inc., Center Valley, PA) at 1 magni cation. 2020 Leaves HTS for virus detection Virus particles in MN55 fruit were im- Second test 2020 Leaves HTS for virus detection aged using transmission electron microscopy Rootstock trial 2016, 2017 Fruit Harvest date effects (TEM). Virus particles were partially purified 2018 Leaves and fruit RT-PCR for virus detection from apple skin tissue and leaf tissue as de- Leaves HTS for virus detection scribed by Lockhart et al. (1997). Briefly, ap- 2019, 2020 Fruit Storage temperature effects ple skin or leaves were powdered in a mortar HTS = high throughput sequencing; RT-PCR = reverse transcriptase polymerase chain reaction. with liquid nitrogen, and the powder ex- tracted 3:1 (v/w) with 250 mM Tris-HCl, pH 7.4, containing 1% (w/v) Na2SO3 and 0.5% three rootstock trial trees (same trees used for and sent overnight to Beltsville, MD, via (v/v) 2-mercaptoethanol. The extract was fil- leaf harvests) on 20 July (five fruit per tree), overnight shipment. tered through cheesecloth and clarified by 9 Aug. (three fruit per tree), and 27 Aug. Maturity assessments. At every harvest, low-speed centrifugation at 18,000 gn (max) (five fruit per tree). To determine whether vi- 10 fruit were assessed for fresh weight, firm- for 15 min. Triton X-100 was added to the rus could be detected in fruit of other culti- ness, soluble solids content, percentage of red supernatant to a final concentration of 2% (v/ vars showing varying levels of skin dimpling, peel coloration, and starch index. Fruit dry v), the mixture was shaken for 1 min, layered five fruit of three commercial apple cultivars matter content was only measured in 2016 over 6 mL of 30% (w/v) sucrose in extraction were purchased from a local grocery store on and 2017. Fruit were weighed the day of har- buffer, and centrifuged for 2 h at 148,000 gn 6 Dec. 2019. vest after transport from the orchard to the (max). The resulting pellets were resus- For HTS, five fruit from the original seed- laboratory.