Black spots appearing on apples are a common frustration for home growers and commercial producers alike. These blemishes can signal a range of issues, from purely cosmetic damage to serious infections that could threaten the entire tree. Accurately diagnosing the cause is the first step toward effective treatment and protecting future harvests. The spots can be the result of aggressive fungal pathogens or non-infectious physiological issues related to nutrient uptake. Understanding the specific appearance of the lesion, along with the environmental conditions present, offers the most reliable way to determine the underlying problem.
Identifying Common Fungal Diseases
Two common and destructive fungal infections frequently cause black spots on apple fruit: Apple Scab and Black Rot. Apple Scab, caused by the fungus Venturia inaequalis, is characterized by spots that begin as small, olive-green, velvety lesions on the fruit’s surface. These spots mature into rough, corky, dark brown to black scabs, often with an irregular outline. Early infection can cause the fruit to become deformed, cracked, and drop prematurely.
This fungus thrives in cool, wet spring weather, requiring prolonged periods of moisture for the spores to germinate and infect the tissue. The primary source of infection in the spring comes from spores overwintering in the fallen, infected leaves from the previous season. Secondary spores are produced on the developing scabs, which are then spread by splashing rain to cause new infections throughout the summer.
Black Rot, caused by the fungus Botryosphaeria obtusa, presents a different and more aggressive type of fruit damage. On the apple, this infection usually starts as a small, dark, reddish-brown spot, often near the calyx or blossom end of the fruit. As the rot expands, it commonly develops a distinct pattern of concentric brown and black rings, which can help distinguish it from other diseases.
The infected tissue remains firm and leathery as the rot progresses, eventually causing the entire apple to shrivel into a hard, black, wrinkled “mummy” that often remains attached to the tree. This fungus frequently enters the fruit through an existing wound, such as an insect injury or a crack in the skin. Botryosphaeria obtusa also infects the tree’s woody tissue, creating sunken cankers on branches and surviving the winter in both these cankers and the mummified fruit.
Non-Infectious Causes
Not all dark spots on apples are caused by infectious pathogens; some are the result of physiological disorders or physical damage. The most common non-infectious cause that mimics a disease is Bitter Pit, which is a localized calcium deficiency within the fruit tissue. These spots appear as small, sunken lesions on the skin, often concentrated near the calyx end of the apple. When the fruit is sliced open, the tissue directly beneath these spots is brown, spongy, and distinctly bitter-tasting.
Bitter Pit is a nutritional imbalance, not a fungal or bacterial infection. It occurs when calcium, which is transported with water via the xylem, is drawn away from the fruit and towards the leaves in high-vigor trees. This imbalance is often exacerbated by environmental stress, like irregular watering, or by an excess of competing nutrients such as potassium and magnesium in the soil.
Other non-infectious spots can simply be the result of environmental factors or injury. Sunscald, which is damage from intense, direct sunlight, can create dark, discolored patches on the exposed side of the fruit. Simple bruising from handling or wind-whipped branches can also cause localized discoloration that turns brown or black over time. These physical injuries typically lack the distinct, structured patterns of fungal infections or the bitter, internal corkiness of Bitter Pit.
Management and Prevention Strategies
Preventing black spots on apples requires an integrated approach that targets both fungal and nutritional issues. For managing infectious diseases like Apple Scab and Black Rot, sanitation is the most important cultural practice. Removing and destroying fallen leaves in the autumn prevents the Venturia inaequalis fungus from overwintering and releasing primary spores in the spring.
Pruning the tree annually to ensure good air circulation within the canopy helps the foliage and fruit dry quickly after rain, which discourages spore germination and infection. For serious fungal issues, a preventative fungicide spray program is necessary, starting in the early spring as buds break and continuing until petal fall. These applications protect the new, susceptible growth before the spores can cause infection.
Managing Bitter Pit requires focusing on the tree’s nutrition and water balance to ensure sufficient calcium reaches the developing fruit. This often involves applying foliar sprays of calcium chloride or calcium nitrate, which are directly absorbed by the fruit, starting shortly after petal fall.
Growers should also avoid excessive use of nitrogen, potassium, and magnesium fertilizers, as high levels of these nutrients can interfere with calcium uptake and distribution. Maintaining consistent soil moisture through the growing season is important, as irregular watering can disrupt calcium movement and exacerbate the disorder.