Getting to the Bottom of ‘Bronzing’, A Peach Skin Disorder Causing Severe Losses for Organic and Conventional Peach Growers

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.