Fruit Physiological Disorders

FIELD INKING OR BLACK STAINING: SKIN DISCOLORATION OF PEACH AND NECTARINE.

C. H. Crisosto and E. J. Mitcham. UC Davis Postharvest Specialists.

Photo by Carlos Crisosto

Importance: Field inking or black staining is a type of skin discoloration and a frequent problem in California, Washington, Georgia, South Carolina, New Jersey, and Colorado, as well as in other production areas in the world such as Italy, Spain, Greece, New Zealand, Australia, Argentina, and Chile. Rejections due to skin discoloration may reach up to 50%.

Symptoms: Field inking symptoms appear as brown and/or black spots or stripes that are restricted to the skin. Symptoms become evident within 48 hours after harvest. However, inking symptoms are triggered during harvest and during transportation to the packinghouse. Field inking usually begins in the field, although symptoms may take more time to become apparent.

Causes: Abrasion damage in combination with heavy metal contamination is required for field inking development. The skin cells, where the anthocyanin/phenolic pigments are located, collapse and their contents react with heavy metals turning their color dark brown/black. Iron, copper and aluminum are the most deleterious heavy metal compounds that can combine with anthocyanin/phenolic pigments (metallo-polyphenol complex). Only 5-10 ppm of iron (Fe) is enough to induce inking at the physiological fruit pH (~ 3.5). This contamination can occur in the field within 15-20 days before harvest or during harvesting operations. When applied close to harvest, pre-harvest foliar nutrient, fungicide and insecticide sprays that contain the above-mentioned metals have the capacity to induce field inking on peach and nectarine fruit when followed by abrasion damage.

Control of field inking:

• Reduce fruit abrasion damage by treating fruit gently, use air-ride suspension on trailers, and avoid long hauling.
• Reduce fruit contamination by keeping picking containers dirt free and clean; avoid dust contamination on fruits.
• Check your water quality for contamination with heavy metals {Fe, copper (Cu) & aluminum (Al)}.
• Test your pesticides for presence of heavy metals (Fe, Cu & Al) early in the season (5-10 ppm Fe is the critical level).
• Do not spray foliar nutrients or preharvest fungicides containing Fe, Cu, or Al within 21 days of predicted harvest.
• Chemical manufacturing companies should attempt to identify and remove from their. Products any potential sources of contaminants that may contribute to inking formation, and to develop safe pre-harvest spray intervals (PHI) for foliar nutrients, fungicides, miticides, and insecticides.
• Growers need to know the composition of the chemicals commonly used in their tree fruit pre-harvest and postharvest operations and understand how they may affect inking incidence.
• In orchards where inking is a problem, delay packing for ~48 hours so you will be able to remove fruit with field inking before placing fruit in the box.
• Fine tune your post-harvest fungicide application to assure that your residues are above the effective minimum recommended, but well below the maximum residue limit  MRL) or tolerance.
• As a long-term solution, it is suggested that chemical manufacturers attempt to identify and remove the possible sources of contamination from their products that may cause skin discoloration before distributing them.

References

Cheng, G.W. and Crisosto, C.H. 1994. Development of dark discoloration on peach and nectarine fruit in response to exogenous contaminations. Journal of the American Society for Horticultural Science 119, 529-533.

Cheng, G.W. and Crisosto, C.H. 1995. Browning potential, phenolic composition, and polyphenoloxidase activity of buffer extracts of peach and nectarine skin tissue. Journal of the American Society for Horticultural Science 120, 835-838.

Cheng, G.W. and Crisosto, C.H. 1997. Iron-polyphenol complex formation and skin discoloration in peaches and nectarines. Journal of the American Society for Horticultural Science 122, 95-99.

Crisosto, C.H. 2009. Updating peach, nectarine and plum inking (skin discoloration) development information. Central Valley Postharvest Newsletter 18(1): 1-4.

Crisosto, C.H., Johnson, R.S., Luza, J. and Day, K. 1993. Incidence of physical damage on peach and nectarine skin discoloration development: Anatomical studies. Journal of the American Society for Horticultural Science 118, 796-800.

Crisosto, C.H., Johnson, R.S., Day, K.R., Beede, B. and Andris, H. 1999. Contaminants and injury induce inking on peaches and nectarines. California Agriculture 53, 19-23.

2020