We have to understand the cause of bronzing in order to design management strategies. Therefore our foremost objective is to reproduce the symptoms in the field, which has never been done before. Based on previous observations we believe that bronzed areas of the fruit are not caused by outside factors such as insects or pesticide damage. This has been established in preliminary work. Obviously we are dealing with a disorder that is coming from ‘inside’ the tree. It likely is a physiological disorder related to tree stress due to inappropriate horticultural practices. The field trials will be conducted in orchards that historically had bronzing issues and will be designed to test the hypotheses. The results of this study will be used to make the first ever science-based bronzing management recommendations.
-survey cultivars for bronzing incidence and fruit for patterns
-reproduce bronzing symptoms in the field
-determine the presence or absence of viroids in affected peaches
Preharvest identification of bronzing on peach fruit and postharvest assessment of severity. The study focused on four mid-season peach varieties ripening primarily in July under South Carolina conditions: ‘Julyprince’, ‘Scarletprince’, ‘Sweet Dream’, and ‘Redglobe’. Twelve blocks were selected at four commercial farms: Titan Farms, Dixie Bell Farms, Watsonia Farms, and Cotton Hope. In each of the blocks six trees were tagged and 50 fruit were monitored for four weeks prior to harvest and eventually harvested at the end of the commercial growing season. The 50 observed fruit were from one limb or adjacent limbs. The limbs with experimental fruit were flagged to make sure that the same area of the tree was monitored weekly. Fruit with atypical coloration was tagged, numbered, and photographed. After harvest, the occurrence, severity, and distribution of blotches was recorded. Specifically, the area of the fruit where bronzing occurred (top, center, or bottom), and the pattern that the bronzing took on the fruit (connected or scattered). The experiment was repeated in 2016, however, none of the fruit were bronzed. This data is not shown in this report.
Screening for viroids known to cause skin disorders on other fruits. Peaches with and without bronzed skin were primarily collected from Titan Farms, Ridge Spring SC, and some were from the Clemson University Musser Fruit Research Center, Seneca SC. For DNA and RNA extractions, peach skin (about 2 cm2 in size) was cut from bronzed and non-bronzed peaches using a razor blade. A total of 100 mg of skin was used for each treatment and RNA extraction was done using TRI Reagent® Solution according to the manufacture’s instruction. About 200 mg peach skin was used for DNA extraction and DNA was extracted using Qiagen Genomic DNA Extraction Kit according to the instruction. To investigate PLMVd, ASSVd and DAV in bronzed peach skin, specific primer pairs PLMVd_compl / PLMVd_homol (Hadidi et al., 1997) and p833 / p834 (Marini, 2007) were chosen to detect the PLMVd in bronzed peach skin. In addition to the PLMVd detection, primer pairs ASSV_F1 / ASSV_R2, ASSV_F2 / ASSV_R2, ASSV_F1 / ASSV_R1, and ASSV_F2 / ASSV_R1 were designed based on the conserved regions in the genomes of ASSVd and DAV (GenBank nos. EU825613; FJ974095; EU031461; KP765432; FM178283; FN376409; X71599; HG764197; M36646) for ASSV and DAV detection in bronzed peaches. cDNA synthesis was conducted using 500 ng total RNA from non-bronzed and bronzed peach skin with random hexamer primer or specific primer, and Superscript III reverse transcriptase (Invitrogen) in a final volume of 10 ml according to the manufacturer’s recommendations. PCR amplification reaction mixtures each contained 2.5 μl of 10× PCR buffer, 0.8 μM each primer, 200 μM each dNTP, and 1 U of Taq DNA polymerase (New England Biolabs, Ipswich, MA). The amplification parameters for PLMVd and ASSV detetction both consisted of an initial denaturation at 94°C for 4 min followed by 34 cycles of 94°C for 50 s, 55°C for 50 s, and 72 °C for 1 min. A final elongation step of 5 min at 72°C was always incorporated.
Field Experiment. A field experiment was conducted at Titan Farms to investigate the influence of crop load, irrigation system and calcium applications on bronzing incidence. Treatments were applied in three replicates in a complete block design. Each replicate consisted of 5 trees. Peaches were rated three times (first, second, and third harvest)
Over the last two years we made significant progress in our understanding of the cause of bronzing. The first breakthrough was the discovery that bronzing is exacerbated but not caused by postharvest handling, cooling, or storage. We obtained visual evidence that the damage was initiated in the field rather than during handling or storage. We found that rapid cooling and storage exacerbate but do not cause bronzing, which again is in contrast to a disorder called ‘skin burning’ described for California peaches (Cantin et al., 2011).
Bronzing is initiated in the field prior to harvest, but years of monitoring have not yielded evidence for fungal pathogens (despite multiple isolation attempts) or insect damage (mites, thrips, etc). Furthermore, from an entomology perspective, insect damage would not be expected to be increased after rain events. We did not detect phytoplasma using qPCR 23S rRNA real-time PCR (Christensen et al., 2004; Hodget et al., 2009), and synthetic pesticide damage was excluded when blotches were found in organic orchards. Additional attempts to induce bronzing by overdosing fruit all season long with pesticides also failed.
In our field experiment, postharvest bronzing incidence varied between varieties and seemed to be more prevalent the later the fruit was picked, with the highest incidence occurring in ‘Sweet Dream’. Bronzing incidence indicates how many fruit has any sign of bronzing whereas severity indicates how much of the surface area on each fruit was affected by bronzing. Titan Farms ‘Sweet Dream’ was represented at two locations and had the highest incidence of blotches. ‘Scarletprince’ blocks showed no bronzing at any of the three locations. The highest severity was found in ‘Sweet Dream’ at Watsonia Farms.
Fruit skin can be affected by viroids and viruses. Blotches on plum fruit of cvs. Taijo and Oishi-Wase Sumomo, were first detected in Japan (Sano et al., 1989; Terai, 1985) and symptoms were associated with the plum dapple disease caused by the hop stunt viroid (Sano et al., 1989). Discolored spots found on apple fruit of cv. Starking Delicious, caused by apple dimple fruit viroid (ADFVd) is another example of viroids affecting fruit skin. Apple scar skin viroid (ASSVd) and its closely related dapple apple viroid (DAV) were detected in apple fruit and developed small to large brown circular spots. Apple scar skin has also been reported in peach and apricot (Zhao and Niu 2006; Zhao and Niu 2008). Peach latent mosaic viroid (PLMVd) has been reported on peach to cause several small, circular, discolored areas on the skin (Albanese et al., 1991; Desvignes, 1979). Our analysis revealed that peaches with bronzing tested negative for either PLMVd, ASSVd, or DAV (this study; Hu and Schnabel, 2016). That does not, however, rule out the involvement of undiscovered viroids.
New evidence based on symptom patterns suggests that, perhaps in addition to as yet unknown viruses or viroids, physiological factors may be involved. The evidence is based on the here reported, multi-farm (Titan Farms, Dixie Bell Farms, Watsonia Farms, and Cotton Hope) and multi-cultivar (‘Julyprince’, ‘Scarletprince’, ‘Sweet Dream’, and ‘Redglobe’) study conducted in South Carolina in 2015. When bronzing incidence was of medium severity (more than just a single blotch but not so much as to covering large portions of the fruit), patches centered around the equatorial plane of the fruit as well as in circles around the very top and the very bottom (Boatwright and Schnabel, 2016). Those areas of the fruit expand the fastest during maturation. In some circumstances the entire equatorial plane became sunken.
We hypothesize based on our preliminary findings that rainfall during periods of high transpiration may cause stress in the form of water and/or nutrient imbalance to the fastest growing cells of the fruit (equator) as well as to the top and bottom of fruit leading to cell collapse. This hypothesis fits with increased bronzing incidence in years with rainfall at harvest. An integrated approach is required to examine potential associations between orchard conditions, peach physiology, cellular physiology, and perhaps, yet to be discovered viroids or viruses in bronzing. Further research conducted in organic and conventional orchards in South Carolina using paper bags to reduce pesticide residues and protect from pathogens and pests provided further evidence for abiotic factors involved. Bags were placed prior to pit hardening and removed at harvest. Bagged and non-bagged fruit had the same amount (5-10% depending on variety and location) of bronzing.
Field experiments indicated no impact of crop load, irrigation practice or calcium application on bronzing incidence. These results indicate again that bronzing may be caused by a number of different effects working together to weaken peach skin integrity at harvest.
Educational & Outreach Activities
Schnabel, G., J. Allran, MJ Hu, and JC Melgar 01/2017. Update on peach skin disorders bronzing and streaking. Fruit and Vegetable Conference, Savannah, GA. Invited Presentation.
Schnabel, G., J. Allran, MJ Hu, and JC Melgar 12/2016. Mystery blotches on peach; what we know does not cause bronzing. Shenandoah Fruit Workers Meeting, Winchester, VA.
Schnabel, G. J. Allran, and JC Melgar 2016. Progress in our understanding of peach skin bronzing. Southeastern Professional Fruit Workers, Conference, Gainesville, FL.
Hu, M.J. and G. Schnabel 2016. Peach Skin Bronzing: Could a virus or viroid be involved? South Carolina Peach Council Research Reports 16:60-66.
Boatwright, H. and G. Schnabel 2016. Investigation of bronzing on mid-season peach varieties. South Carolina Peach Council Research Reports 16:67-70.
Occurrance, Causes, and Management of peach bronzing. Workshop for growers, agents and specialists at the Madren Center, Clemson University, January 2016
The following progress has been made and conclusions were drawn from this study:
-we were unable to link bronzing to irrigation practices, Calcium deficiency, or crop load
-peach skin bronzing affects peaches of multiple varieties across U.S. East Coast state lines and is initiated prior to harvest.
-rainfall during harvest increases bronzing incidence.
-pests, fungi, pesticides, known viroids, and phytoplasmas were not linked to bronzing.
-The data obtained through this SARE project was used to submit a NIFA SCRI proposal for $3 million
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