The windmill palm grows up to 20 feet in height with a 10-foot spread. The palm leaves are 18 to 36 inches. The windmill palm thrives in USDA hardiness zones 8 through 11 in the southern and western coastal states, but is not native to the United States. The windmill palm prefers full sun, but will grow in partial shade as long as the soil is well-drained and fertile. There are several diseases that affect the windmill palm.
Lethal yellowing is caused by a phytoplasma, which behaves like an early virus. Lethal yellowing is deadly to most palms, including the windmill palm. It is spread by the Myndus crudus, a leaf-hopping bug. The Myndus crudus feeds on infected palms and ingests phytoplasm, then infects other palms by carrying the disease from an infected windmill palm to an uninfected windmill palm. Symptoms include browning, then blackening in the cluster of flowers. There is a heavy coconut fall and the fronds brown and yellow.
Treatment of Lethal Yellowing
Lethal yellowing is controlled through inoculation of antibiotics--a hole is drilled into the truck of the tree, into which a hollow case is inserted. The hollow case has a silicone seal on one end of it. The antibiotic is injected into the case. Treatment is started as soon as the flowers show any darkening. The windmill palm must receive antibiotics throughout the rest of its life, or else the symptoms of the disease reappear.
A fungus, phytophthora, causes root rot. Windmill palms planted in cold, wet and soggy soil is highly susceptible to root rot. When the windmill palm contracts root rot, the fronds turn yellow. The palm might survive during the cooler months, but as soon as the weather turns hot, the root rot causes the plant to die. To prevent root rot, be sure the soil is well-draining. If water tends to stand in the area of windmill palms, make French drains to move excess water to another portion of the landscaping that needs the water.
Leaf spot is in infection that is caused by several fungi and bacteria. The leaf spot leaves are elongated and circular, and could possibly have an oily appearance. The spots are different sizes and colors, depending on the organism that caused the leaf spots. The color and size also differ from the stage of development of the disease.
Treatment for Windmills with Leaf Spot
If the windmill palm has leaf spots, do not water the palm from the top (if the palms are small). Leaf spot will not kill a windmill in most cases, but if the damage does become more severe, spray the windmill palms with the appropriate fungicide. Fungicide for leaf spot is available at your local nursery. If the windmill palms produce coconuts for food, only copper fungicides should be used.