Peach Bronzing: Insights into a Troublesome Skin Disorder
What is peach bronzing?
Bronzing is a skin disorder that affects the skin color of peaches, primarily increasing in severity after a few days of storage, in mid to late-ripening peach cultivars, and rendering the fruit unmarketable. It manifests as single or multiple irregularly shaped blotches on the fruit skin, which can be bronzed or black depending on their location. On a yellow background, they appear bronzed, while on the red-blush side, they appear black (Fig. 1). These patches typically exhibit a rough texture, often resembling scaly or corky tissue. Moderate cases tend to have blotches around the equator, stem end, and tip (Fig. 2). While it does not pose direct health risks to consumers, it can result in reduced consumer acceptance and, consequently, economic losses. Unlike similar disorders, like inking (identifiable by the emergence of dark, brown-black blemishes or streaks on the fruit's exterior, specifically limited to the skin) or skin burning (manifesting as brown and/or black regions on the fruit's skin, which generally arises during the packinghouse during the grading process at the brushing and washing stage), bronzing has not been extensively studied, and most reports mainly focus on postharvest influences for the disorder development.
Bronzing has been observed in both preharvest (when fruit is still on the tree) and postharvest (after a few days of cold storage at 1°C). It involves a complex interplay of multiple factors, and thus it is challenging to pinpoint the exact causes. However, reports have suggested that peach bronzing is originally triggered in the field rather than during handling or storage and therefore, swift cooling and storage intensify the issue but do not instigate bronzing. Factors such as unusually hot weather in May, with temperatures exceeding 98°F, have also been linked to bronzing. Other factors such as nutritional management and irrigation practices are also suspected contributors. Conventional growers often apply fertilizer annually, regardless of soil and tree needs, potentially leading to nutrient imbalances. Additionally, bronzing appears more severe following significant rain events, suggesting a possible role for nutritional imbalance and water relations in this skin disorder.
What factors cause or contribute to peach bronzing?
Regarding genetic background several cultivars, including 'Scarletprince', 'Julyprince', 'Sweet Dream', 'PF23', 'Messina', and 'Redglobe', have exhibited bronzing. These all correspond to cultivars that ripen around July in the Eastern United States.
It has been reported that several abiotic factors work to exacerbate the development of peach bronzing throughout the growing season and/or during storage. Work by researchers at Clemson University hypothesized based on preliminary findings that a potential link between rainfall and the development of peach bronzing existed. The study suggests that rain occurring during periods of high transpiration may induce stress in the fruit's fastest-growing cells, particularly those around the fruit's equator, as well as at the top and bottom. This stress could result from water and nutrient imbalances, potentially leading to cell collapse. This aligns with the observation of increased bronzing incidence in years with rainfall at the time of harvest. Bronzing experiences a significant surge in ripening peaches within the initial days following a rainfall event. For instance, if rain falls before the initial harvest, the occurrence and severity of bronzing would be notably high during the first pick. However, by the second or third pick, which typically occurs around 3 to 6 days later, there might be a notable absence of bronzed peaches.
Clemson researchers have further identified another crucial aspect of bronzing: excess potassium in the soil. Bronzing is amplified by elevated soil potassium levels but only when coupled with high tree transpiration rates induced by excessive pre-harvest irrigation. Several theories exist regarding the role of potassium in bronzing development, though all remain speculative at this time.
Furthermore, Captan fungicide, commonly used to control brown rot, has been suspected of contributing to these bronzing issues. In a two-year study, applying Captan 80WDG at different rates of preharvest-induced bronzing. Interestingly, increased rainfall during the preharvest season correlated with higher incidence and severity of the bronzing disorder.
These findings collectively suggest that peach bronzing may be the result of a complex interplay of multiple factors contributing to skin integrity weakening at the time of harvest.
What are the practices that can be implemented to prevent peach bronzing?
Avoid planting cultivars that have a high susceptibility to this disorder. If grown, caution should be taken during preharvest management and at harvest.
Avoid using Captan just before commercial harvest particularly when followed by rainfall. While captan is a valuable tool for managing brown rot, scab, and anthracnose resistance, it should be used cautiously. Additionally, the increased presence of potassium in peach skin cells may outcompete calcium and other cations, potentially leading to the predisposition of peach skin cells to bronzing. Exploring the possibility of calcium foliar applications may provide a potential solution to counteract the adverse effects of excessive potassium. Research has demonstrated that such applications can lead to an increase in calcium levels in peach skin. Increased calcium levels should offset the effects of potassium-rich soil. Maintaining well-buffered and nutritionally balanced soils is crucial to creating an environment where the fruit can thrive and minimize the risk of bronzing.
Using cold water baths, such as postharvest hydrocooling (conducted with the correct pH and chlorine levels) for peaches can effectively reduce the incidence of bronzing. Lastly, ensuring gentle treatment of peaches postharvest can also help mitigate the development of peach bronzing.
This article appears in April 2024, Volume 15, Issue 1 of the Vegetable and Fruit News.