Black Mildew on an Orange Tree

Black Mildew

• If you see black mildew growing on an orange tree, then it is infested with black spot, a fungus known to target all types of citrus trees. It begins as spots of black mildew growing on the tree’s leaves. As the infection worsens, the spots become lesions that penetrate deeply into a leaf’s tissues. Over time, the black mildew can also spread to orange fruit, where it can go on to become penetrating lesions that eat through a fruit’s rind.

Issues

• Black spot is unsightly; mildew growing on an orange tree makes it look unhealthy. If it goes untreated, the deepening lesions can damage leaves so much that they die and drop early. Losing enough leaves interferes with a tree’s ability to nourish itself. If this occurs before fruiting, you could see a reduced or non-existent harvest. If the infection occurs after oranges have begun to grow, they could be ruined by direct infection by black mildew. Those that aren’t directly infected can be smaller and less flavorful that normal.

Solutions

• Treat black mildew by spraying infected orange trees with a copper-based fungicide. Available for purchase at nurseries or garden supply stores, these fungicides kill most fungal infections and are effective against black spot. Apply the spray according to the manufacturer’s suggested schedule. When spraying infected portions of the tree, also spray a small surrounding border of healthy leaves and fruit. This helps isolate the mildew and prevent it from spreading to other areas of the tree.

Considerations

• While the black mildew won’t spread from your orange tree to your grass, it can easily spread to other citrus trees. Orange trees usually recover from infection with treatment, but lemon trees can be badly damaged by it. Black spot is easiest to treat when it is caught early and treatment is begun as soon as the diagnosis is made. You could need to apply fungicide over a period of weeks before the infection clears completely.