What Are Black Spots on the Fronds of Queen Palm Trees?
Several fungal diseases cause leaf spot or, if large areas of the leaves are affected, leaf blight. Most of these diseases produce black spots on leaves or stems. Fungal diseases start out as small lesions that develop into colored or black spots. The spots are sometimes circled by a contrasting color, such as yellow rings around black spots. Graphiola leaf spot has a significantly different profile, as the Graphiola phoenicis fungus forms lesions with a distinctive shape. This fungus appears as tiny, black, cup-shaped spots on the lower leaves of queen palms. Stressed, injured and nutrient-deficient palms are susceptible to fungal diseases, as are trees in damp climates. Treat infected trees by spraying the foliage with a fungicide containing copper hydroxide or copper salts of fatty acids. Do not remove infected leaves unless the infection is severe, as leaf removal can further weaken the tree or aggravate a nutrient-deficient condition.
Potassium deficiency can cause black spots of dead leaf tissue. Before the tissue dies and turns black, it will appear as yellow or orange translucent spots. As the deficiency continues, the leaf tips become affected, and may appear burned or frizzled. Signs of potassium deficiency start on the oldest leaves before affecting the newer growth. Potassium deficiency can be fatal to queen palms if left untreated. This type of nutritional deficiency is more common in sandy soils and may be the result of using high-nitrogen lawn fertilizer around the queen palm. Correct the potassium deficiency by fertilizing the palm with both slow-release potassium and slow-release magnesium. Using both nutrients simultaneously prevents an imbalance of potassium to magnesium. The supplemental nutrition will not improve affected leaves, but the newer growth will be healthy.
Black spots on the leaves of queen palms may be small formations of black sooty mold. This type of mold grows on honeydew, a sticky substance excreted by soft-bodied scale insects. Scales are tiny insects that suck the sap from trees, causing wilted foliage, yellowed leaves and stunted growth. These pests appear as small, stationary bumps along leaf blades and stems. Newly hatched scales are usually orange or yellow, and look like tiny crawling bugs. Natural predators usually keep scale populations in check, but some insecticides, dusty conditions and the presence of ant colonies can prevent this biological control. Ants are attracted by the honeydew, and they protect their food source by protecting scale insects from natural predators. Prevent the ants from reaching the scales by placing ant stakes around the base of the palm. The black spots of sooty mold are eliminated with the extermination of the scale insects. Treat infestations by spraying the palm with a horticultural oil or soap; avoid using insecticides, as these can kill beneficial predatory insects.
Many queen palm problems can be avoided with good horticultural practices. Prevent fungal infections by fertilizing the palm regularly, avoiding overhead watering and protecting the tree from damage by lawn equipment. Prevent nutritional deficiencies by using fertilizers specifically formulated for palm trees; these types of fertilizers contain a higher percentage of potassium. Keep scale insects under control by rinsing dusty trees, avoiding broad-spectrum insecticides and using barriers to keep ants from traveling up the tree.