What Causes Black Spots on Tangerines?

Types

• Tangerines with infestations of honeydew-producing insects may develop sooty mold. Primarily a cosmetic condition, sooty mold is caused by fungi that adhere to honeydew on tangerines and cause black or dark gray fungi to grow on tangerines and leaves. Greasy spot is a fungal disease that affects many citrus fruits, including lemons, tangerines and tangelos. The fungus causing greasy spot enters trees through stomata on leaves and eventually spreads to fruit. Greasy spot fungi is most active when temperatures range from 77 to 86 degrees degrees Fahrenheit. Water and high humidity are also necessary for this fungus to be active.

Effects

• Tangerines with sooty mold fungi have fungus splattered across the surface of the fruit and leaves. This fungi can be fuzzy, crusty or velvety, and is black or very dark charcoal gray in color. According to the University of Florida, sooty mold is a superficial fungal growth, but it can cause reduced vigor and off-color tangerines in trees when coverings of fungus are heavy enough to block sunlight. Greasy spot is characterized by black lesions on the leaves and fruit of tangerines. Infected leaves may become mottled and light yellow in color, and black, greasy lesions form on leaf surfaces. These greasy lesions also appear on tangerines and are raised and black. Greasy spot may appear in November, but some trees do not show symptoms until later in the winter.

Cultural Control

• Reducing and controlling sooty mold in tangerines requires the control of honeydew-producing insects. Whiteflies, mealybugs, scales and aphids are the most common cause of honeydew production and sooty mold. When sooty mold is present, it usually means large numbers of insects are also present. Spraying your tangerine tree with hard jets of water can help reduce the number of insects feeding on your tree. Concentrate on the undersides of leaves where these insects feed for the best results. Greasy spot is best controlled with fungicide applications.

Chemical Control

• Honeydew-producing insects on tangerine trees can be controlled by applying soap solutions or horticultural oils. These products are available at your local gardening center and work on contact to kill insects, reducing honeydew production and controlling sooty mold. Soap solutions and oil products may cause damage to sensitive trees, so test these products on a small area of your tangerine tree before applying them to the entire tree. If your tree shows no signs of damage after seven days, it is safe to apply to the entire tree. Horticultural oils will loosen sooty mold fungi, making it easier to wash from your tree. Greasy spot is usually treated with applications of petroleum oils or fungicides. However, these treatments are costly and are not recommended for home gardeners unless the disease is severe or widespread.