Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Inking of peaches and nectarines is the appearance of dark colored, blue-black spots on fruit surfaces. While these spots are superficial, they cause the fruits to be unmarketable or be a second grade. Inking is due to damage to skin cells and the death of these cells but not the tissue below. Susceptibility varies with variety. Inking may appear when the fruit is on the tree but most often occurs after picking when abrasion occurs during harvest and handling.
Research in California has shown this disorder is caused by abrasion in combination with high concentration of certain metals such as aluminum, copper or iron. These metals may come from water or applied products such as fungicides, insecticides, or foliar fertilizers. For example, Imidan and Delegate insecticides have high levels of aluminum; the miticides Vendex, Acramite, and Omite have high levels of aluminum and some also have high levels of iron; the fungicide Elite has a high level of aluminum. Wash water high in iron or low in pH can increase inking problems
Another cause can be excess rain at harvest causing swelling and some cell damage along with associated storm damage and abrasion. This, in combination with metals, can increase the incidence of inking.
Control of inking includes reducing the amount of metals that go on fruit, reducing abrasion damage, and avoiding late applications of products that can increase inking (do not use during the 3 weeks before harvest).
Captan fungicide can also cause spotting, streaking, and bronzing on peaches if applied too near to harvest and at too high of concentrations. It has also been implicated in inking damage.