Inking of peach

Inking Inking of peach is the appearance of dark blue to almost black superficial spots on the fruit surface. According to Dr. Carlos Crisosto (UC Davis), who has probably done the most work on this issue, the spots are caused by a combination of abbrasion and heavy metal contamination (Crisosto et al. 1999; pdf). Some heavy metals in combination with abbrasion (including the disturbance of the peach fuzz) will destroy peach cells of the peach skin containing pigments such as anthocyanines. That is what causes the dark color. His research shows that especially aluminum, copper and iron are culprits of inking. They can come vrom a variety of souces like dust (fine dirt particles that drift onto trees) overhead irrigation, hydrocooler water, fungicides, insecticides, etc. Contrary to the intuitive thought that water would wash off the metals, rainfall or overhead irrigation may increase inking, as it washes metals, dust, etc. to the fruit from the leaves. Iron (Fe) levels above 250 ppm in water will cause direct inking, but you will not likely see Fe levels this high in well water. With abrasion, Fe levels >10 ppm will cause inking. Well and hydrocooler water should be checked as part of any investigation of inking. The levels are not researched as well, but levels > 50 ppm for Aluminum (Al) and Copper (Cu) will also cause inking when abrasion is added.

Here are some commercially available materials that contain unusually high levels of inking-causing heavy metals (for a more detailed list of heavy metal contents in Ag products click here):
Foliar nutrients: Micro Plex (Fe)
Insecticides: Imidan (Al), Delegate (Al)
Miticides: Vendex (Fe and Al), Acramite (Fe and Al), Omite (Al)
Fungicides: Elite (AL)
Additives: none

Keep in mind that the availability of heavy metals is dependent on the pH of the spray solution, the type and concentration of ligands on which the metal could adsorb, and the oxidation stage of the heavy metal. In general, the lower the pH, the more available are heavy metals. Try to keep your spray water at a neutral pH and you should be fine.

Tips to reduce inking:

Remove products with high levels of contaminants (Fe, Cu, and Al) from your spray program. Ideally, test your pesticides for presence of these heavy metals early in the season.
Reduce abrasion damage, handle fruit gently, avoid long hauling distances and keep harvest containers free of dirt. However, gentle abrasion is all that is required, so the act of picking the fruit by hand is abrasion. Movement in bins is also abrasion. Best is to have no metals on fruit.
Stop spraying foliar nutrients 21 days before harvest
Contamination of fruit can be reduced by keeping equipment clean, avoiding dust contamination of fruit, and avoiding foliar nutrients sprays containing Fe, Al, or Cu.
In orchards where inking is a problem, delay packaging for 48 hours (inking usually comes out within 1-2 days) you will be able to remove inked fruit prior to placing fruit in the box.
Screen your preharvest, water quality, and postharvest chemicals for Fe, Cu, and Al.
Fine tune your postharvest fungicide application to assure that your residues are above the effective minimum recommended, but well below the maximum residue limit (MRL) or tolerance.

Other considerations

The ph of the hydrocoolant water matters. It should be around 7. Iron is only available at low pH and therefore may cause more inking if the pH is lower than 6.5.
Another factor may be the availability of water too close to harvest. Too much water in the fruit (rain close to or during harvest, irrigation too close to harvest) may cause increased turgor in the cells and combined with abbrasion cause burst of cells and inking
Chlorine in the hydrocoolant water is NOT believed to cause inking.
Captan sprayed too close to harvest has been associated with a bronce discoloration of the fruit surface and sometimes inking. But there is no good data to back this up.