Update on Peach Skin Disorders

Phil Brannen1 , John Mark Lawton2, Juan Carlos Melgar2, Brian Lawrence2, and Guido Schnabel2

1Department of Plant Pathology, University of Georgia

2Department of Plant and Environmental Sciences, Clemson University

Inking has been an issue in southeastern peaches for years, but it has been difficult to determine the major causes of inking as it has been so sporadic.  Captan has always been one of the suspected culprits for inking, but even with captan, the evidence has been rare.  Kathy Taylor, previous UGA peach specialist, conducted research with captan in 2006, 2007, and 2008.  The 2006 and 2007 data demonstrated that there was no difference among the 30, 15 and 5 days before harvest treatments with regard to inking, with none having inking increase over the untreated control.  However, in 2008, fruit that received hand gun application of captan close to harvest, 5 days before harvest, were more prone to inking than fruit that received captan treatment at 30 days before harvest.  This finding was different from that in the two previous years in which treatment as close as 4 days before harvest caused only minimal or no inking.  This increased inking with closeness to harvest was generally consistent across all of the varieties she looked at in 2008, except for ‘Big Red’.  A look at the weather data from the weather station in Fort Valley, GA indicated that all varieties except ‘Big Red’ received rainfall in the period between the late captan application and harvest.  She concluded that possibly the later application along with rainfall just after the late application favored such inking.  This rainfall connection has been further noted by growers and extension professionals in the past few years.

Additionally, Kathy determined that implementation of daily brush cleaning on packing lines reduced the level of powdered pesticide formulations like captan, which tended to build-up on packing line brushes, contaminating fruit with unduly high levels during packing.  Because captan is a particularly abrasive material, she tested the hypothesis that the primary source of captan injury was physical and exacerbated by brushing during fruit packing.  Her study demonstrated that contamination of packing line brushes with captan caused substantial inking.

In 2019, Clemson research confirmed that captan applied at 10 and 1 days prior to harvest increases “bronzing,” which may be just a variant of inking on cultivars with less red blush. Inking and bronzing can co-exist, with inking being more pronounced in red-blush areas. In addition, a rain event just before harvest significantly increased bronzing in captan-treated fruit – more so than captan alone.  This further indicates an interaction between late applications of captan and rainfall as contributors to peach skin coloration disorders. The 2019 study further determined another possible and important aspect of bronzing – excess potassium in the soil.  As with captan, the bronzing increases with high soil potassium levels but only in combination with high tree transpiration rates (induced by excessive irrigation prior to harvest in the 2019 study). There are several theories as to the role of potassium in bronzing development, but all are speculative at this time.

Bottom line, here are some important things to consider:

(1) Clean your brushes in the packing line.

(2) Do not utilize captan just before harvest.  We feel relatively comfortable with using captan at 21 days prior to harvest, but that would be as close as we would risk.  It is a good resistance-management tool for brown rot, scab, and anthracnose, so captan should be one component in spray programs – especially for early and late cover sprays in wet years.  However, the inking and bronzing issues associated with captan, especially when applications are followed by rainfall just prior to harvest, make late use ill-advised.

(3) Maintain balanced fertility in the soil.  If high soil potassium levels are conclusively shown to increase bronzing/inking, then we really need to determine the current “optimal range” for potassium (based on leaf analysis).

The Clemson study suggests nutrient imbalance could be a predisposing factor for bronzing. When combined with high transpiration, the cells in the fruit skin may become more susceptible to additional stress factors. Those include the already mentioned preharvest abrasion of captan, but also various postharvest activities that can affect weakened fruit skin. For now, we break down bronzing by inducers/exacerbators and alleviators.  The potential inducers/exacerbators are nutritional imbalances in the soil, heavy metals preharvest (e.g. captan and maybe others), preharvest rainfall combined with high temperatures (high transpiration rate), the ripening season, and post-harvest handling. The potential alleviators are well-buffered and nutritionally balanced soils, post-harvest hydrocooling (with proper pH and chlorine levels), and some adjuvants capable of reducing transpiration prior to harvest.  There clearly is more to do, but there has now been some progress that led to the development of this evidence-based hypothesis. With additional research, connections such as captan and rainfall are no longer as theoretical.


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