Bronzing is a skin disorder in form of irregular shaped patches ranging from a single patch when incidence is low to almost the entire fruit being covered if incidence is very high in the field. The edges of the patches are irregular in shape and affected areas reveal a complete loss (perhaps due to oxidation of epidermal cells) of yellow and red pigments. Soon after cold storage the cells underneath the fruit skin start to decay and patches sink slightly into the fruit. Symptoms in the field are rather unambiguous and for the untrained eye of little concern. In some areas of the fruit the trichomes appear brighter and are more showy. These areas of the fruit skin transform into what we call bronzing.
Occurrence. Bronzing symptoms have been observed postharvest since the 1970s, but incidence varies between almost non-existent to causing major postharvest yield losses. In South Carolina, bronzing is observed in years with above average rainfall in July. Varieties mostly affected include mid-season varieties such as Julyprince, Scarletprince, Redglobe, and Sweet Dream.Â Bronzing has been reported in California, Michigan, Pennsylvania, Georgia, and South Carolina and I have seen it on peaches from Spain indicating this is not just a US problem.
Cause. There have been many speculations around the cause of bronzing of peach skin. Speculations included early season thrip damage, the use of captan too close to harvest, the use of oil-based EC formulations, the application of certain fungicides, generation of abrasion due to handling of fruit during harvest, and pH or chemical reactions with hydrocooling water. Over time, however, none of these speculations held up with observations. We have not been able to isolate a microorganism from the patches.
Our research shows that bronzing is initiated prior to harvest and rapid cooling accacerbates but does not cause bronzing. New evidence based on symptom patterns found in the field suggest a physiological cause. We observed at medium severity bronzing patches to center around the equatorial plane (see picture gallery) of the fruit as well as in circles around the very top and the very bottom. Those are the areas of the fruit that expand the fastest during maturation. In some circumstances the entire equatorial plane become sunken. We hypothesize that too much water during periods of high transpiration may cause stress in form of water (nutrient?) imbalance to the fastest growing cells of the fruit (equator, top and bottom of fruit) that lead to cell collapse: in years with above average rainfall virtually all growers have problems with bronzing. That is consistent with observations from California and Pennsylvania that bronzing was most often observed after rainfall following periods of drought. No management options are known.
Streaking can be observed in many years, but it typically occurs infrequently and symptoms are negligible. However, in some years incidence and severity are significant. Streaks may be up to 5 mm in diameter, increase slightly in intensity and diameter from top to bottom, and reach maximum intensity and width near the bottom of the fruit. Streaks end near bottom of fruit in club-shaped fashion. Streaked peaches can be found on all sides of the tree, top and bottom, as well as inside and outside the canopy. In some varieties additional symptoms may appear. In addition to the streaking, the top half of the peach can display phytotoxicity symptoms similar to russeting of apple and early symptoms of bacteriosis. We assume that this is the result of smaller drops that never congregated into bigger drops able to run off. The symptoms are consistent with water drops forming on the upper half of the fruit, which slowly run down the fruit to form a streak of 5 mm width. As the drops move down, more water is absorbed and the drop increases in size leading to wider streaking width. The increased intensity of the streaks may be due to longer exposure (slower drop movement?) or higher dose (increase in toxin concentration) in the drop. Drops appear to have dried out at the lower part of the fruit leaving the characteristic club-shaped, round end of maximum width and intensity. In some varieties water that did not run off was able to produce phytotoxicity on upper half of fruit.
Current hypothesis. The Schnabel lab has done observations and experiments suggesting that for streaks to form the fruit must be at a susceptible stage (within 1 to 3 weeks of harvest). During this time pigments are formed. Streaks most often form after weeks of no rain followed by a light drizzle of less than 0.3 in. We observed that streaks were formed by the following day morning dew. Something in the dew appears to prevent red pigment formation. Controlled experiments suggest that acids connected with acid rain do not cause streaking. Instead we were able to reproduce symptoms with very low concentrations of clorine dioxide and chlorine ions. Both are gases that easily dissolve in water. We hypothesize that during morning dew these gases accumulate and concentrate in the dew and, if the concentration is high enough, interfere with pigment formation. Chlorine dioxide and chlorine gases are released into the atmosphere by factories, including the textile industry. It is possible that weeks of no precipitation are required to accumulate enough of these gases in the air.
The role of pesticide residues cannot be ruled out but several observations imply a minor (if any) role.
1. Streaking was observed on fruit that were bagged and received no pesticides until June 22. If pesticides are a factor, they must have come from leaf run off.
2. Pesticides historically associated with fruit finish problems were not sprayed in blocks with symptoms (no copper on Contender in 2015, no sulfur at Titan Farm in 2015, and no captan or Pristine in 2003).
3. Dr. Phil Brannen from UGA recalled seeing the same symptoms on untreated fruit in his experimental blocks in 2003: “I have observed this in 2003 in a research trial, and I initially thought it might be associated with a new fungicide. However, in the end, all treatments were showing some degree of the same symptom, to include the untreated control.” Note that untreated fruit were sprayed with insecticides and sulfur during cover sprays but residues were most likely washed off for the most part (50 to 80 % based on literature) due to more than 2 in of rain 6 days after the last pesticide application.
4. Streaking was observed on organically grown fruit in South Carolina