When a palm tree develops moldy black spots on its leaves, a fungal disease is usually the culprit. Palm trees tend to grow in warm, and sometimes wet or humid, climates that foster fungal spore growth. Many common types of palm trees, such as Mexican blue palm (Brahea armata) and Guadalupe palm (Brahea edulis), grow in U.S. Department of Agriculture plant hardiness zones 9a to 11a. Others, such as triangle palm (Neodypsis decaryi), grow in the warmer USDA zones 10b to 11a.
Many different types of fungi can cause leaf spot diseases in palm trees, but the spots and symptoms themselves often look the same, no matter the fungus type. Some common palm fungal diseases include several leaf spot diseases, Phytophthora fungal diseases, false smut, diamond scale and bud rot. Prevention and treatment techniques are often similar for different types of fungi. You will not need to send the spores into a lab to identify exactly what kind of fungus your tree has unless you have extreme difficulty getting rid of it.
Fungal spores grow best in damp environments, so one of the best ways to avoid leaf spots is to avoid overhead watering of palm trees. It is also a good idea to plant multiple palm trees with enough space so the leaves do not touch each other. Plenty of space prevents fungal spores from spreading and improves air circulation. Improved air circulation reduces humidity and allows the leaves to dry out quickly after rain.
It can help to water palm trees in the morning to keep them from being wet overnight. It is also helpful to prune away severely diseased palm tree leaves. Another way to reduce leaf spot and blight diseases is to make sure a palm is getting enough nutrients. You can find fertilizers specifically formulated for palm trees, such as 8-2-12 ratio fertilizers. University of Florida IFAS Extension recommends applying 1 1/2 pounds of fertilizer for every 100 square feet of ground space. You may also want to try an at-home soil testing kit to check the soil for nutrient deficiencies.
While fungicides do not cure existing leaf spots, they do prevent fungal spores from spreading and causing more leaf spots. Fungicides alone will not solve a leaf spot problem, but they help when you also provide proper watering, growing conditions and fertilizer. Copper fungicides work well to help manage fungal diseases in palm trees. Make sure to choose a fungicide with a label indicating it is safe use with palms or other plants in your yard. Spray fungicides evenly on palm foliage. Fungicides can contain potentially dangerous chemicals, so it is important to carefully follow any instructions on the label.
- University of Florida IFAS Extension: Leaf Spots and Leaf Blights of Palm
- Clemson Cooperative Extension: Palm Diseases & Nutritional Problems
- The American Phytopathological Society: Diseases and Disorders of Ornamental Palms
- University of Florida IFAS Extension: Graphiola Leaf Spot (False Smut) of Palm
- University of California Statewide Integrated Pest Management Program: Palm Diseases in the Landscape
- National Gardening Association: Brahea Armata
- National Gardening Association: Brahea Edulis
- National Gardening Association: Neodypsis Decaryi
- University of Florida Sumter County Extension: Fertilizing Palm Trees
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