Brown spots on apples are a common issue for both commercial growers and home gardeners. These blemishes are symptoms pointing to various underlying problems, ranging from microscopic fungal infections to nutritional imbalances or improper storage conditions. Understanding the distinct characteristics of these spots is the first step in accurate diagnosis. The source of the blemish dictates the correct course of action, whether the solution involves pre-harvest orchard management or post-harvest storage adjustments. This guide identifies the origin of the browning and offers specific strategies for prevention and control.
External Spots Caused by Fungal and Bacterial Infections
Brown spots appearing on the surface of an apple while still on the tree are often caused by living pathogens, such as fungi. One prevalent example is Apple Scab, caused by the fungus Venturia inaequalis, which first appears as olive-green or dark, velvety patches on the skin. As these lesions mature, they become dark brown to black, scabby, and can cause the underlying fruit tissue to become corky and cracked.
This disease is favored by high humidity and prolonged wet periods during the early growing season. Other blemishes come from rot fungi, such as Black Rot (Physalospora obtusa) or Bitter Rot (Glomerella cingulata), which frequently appear after a minor injury.
Black Rot lesions start as light brown spots that darken and are often marked by distinct, concentric dark bands, resulting in a firm rot. Bitter Rot spots are typically circular and slightly sunken, enlarging into a saucer shape, and they can penetrate the fruit without an initial wound, especially in hot, rainy conditions.
Physiological Disorders and Environmental Damage
Browning not caused by a pathogen is often the result of physiological disorders related to nutrient availability or environmental stress. Bitter Pit is a common non-infectious disorder caused by a localized calcium deficiency within the developing fruit. Symptoms usually appear near the calyx as small, sunken, dark spots beneath the skin, which become brown and spongy in the underlying flesh. This condition is aggravated by factors such as low soil moisture, heavy pruning, or excessive tree vigor, which divert calcium away from the fruit toward the rapidly growing leaves.
Environmental factors can also directly damage the fruit, leading to brown lesions, such as those caused by Sunscald. This damage occurs when apples are exposed to intense, direct sunlight, causing large, bleached or dark, necrotic patches on the exposed side.
Physical trauma during handling can cause internal browning, appearing as soft, discolored areas beneath the skin due to cell damage and oxidation. Another internal issue is watercore, where water-soaked, translucent areas appear in the core or surrounding flesh, indicating a disruption in the fruit’s sugar and water balance.
Post-Harvest Handling and Storage Solutions
Many brown spot issues develop after the apple has been harvested, often due to improper temperature or atmospheric conditions during storage. Superficial Scald, also known as Storage Scald, is a chilling injury that develops on the skin after prolonged cold storage, typically beyond three months. This disorder is characterized by diffuse, irregular light-to-dark brown patches on the skin, often more severe on the greener side of the apple. It results from the oxidation of a naturally occurring volatile compound called alpha-farnesene.
Another cold-related problem is Chilling Injury, which causes internal browning or breakdown of the flesh when cold-sensitive varieties are stored below their optimal temperature range. This appears as a slight radial browning just under the skin, which can progress deeper into the flesh, making the tissue spongy.
For home storage, maintaining a temperature near 0°C (32°F) is recommended for most varieties, paired with high humidity to prevent shriveling. Apples should be cooled gradually after harvest, and susceptible fruit should be consumed quickly to avoid symptom development.
Integrated Prevention Strategies
Preventing brown spots requires an integrated strategy that addresses sanitation, nutrition, and environmental management throughout the growing season. Orchard sanitation is a fundamental step, involving the removal of fallen leaves and mummified fruit from the previous season. These materials harbor the fungal spores responsible for diseases like Apple Scab and Black Rot. Reducing the source of the initial infection minimizes the need for later chemical intervention.
Proper tree structure and air movement are maintained through dormant and summer pruning. This opens the canopy to sunlight and promotes faster drying of the fruit and foliage. This cultural practice helps to suppress the growth of moisture-dependent fungi, such as those causing Sooty Blotch and Flyspeck.
Nutrient management is a focus, particularly ensuring adequate calcium supply through multiple foliar sprays starting early in the growing season to minimize Bitter Pit incidence. Consistent irrigation is necessary, as fluctuating soil moisture levels can interfere with the tree’s ability to transport calcium effectively.