Pruning Masks Pygmy Date Palm Problem
If palms are pruned before identifying the cause of decline, sometimes the symptoms are removed that indicate a nutritional or disease problem. In the case of this recently pruned pygmy date palm, a few leaves with a disease called Pestalotiopsis remained. Pestalotiopsis is a fungus that causes diseases of both the palm leaf petiole and leaf blade, often at the same time. Palm leaf diseases caused by Pestalotia are the same as those caused by Pestalotiopsis.
Many palms are hosts to these fungi. However, the disease is often observed in Florida on Phoenix roebelenii (pygmy date palm) where it can cause a fatal bud rot. The fungus Pestalotiopsis causes leaf spots, petiole/rachis blights and sometimes a bud rot of palms. Unlike the other leaf spot and petiole blight pathogens, which attack either the leaf blade or the leaf petiole, Pestalotiopsis attacks all parts of the leaf from base to tip. Leaf spots will begin as very small, yellow, brown or black spots that enlarge in size. The spot usually turns gray with a black outline. Lesions on the petiole and rachis are similar. Symptoms may occur on multiple leaves at once, especially on juvenile palms.
Pestalotiopsis produces abundant spores that can be easily dispersed by wind and water (splashing from rain or irrigation), so sanitation and water management are critical. Injury prevention and good palm nutrition are part of the overall management strategy. In the landscape where nutrient deficiencies are often problematic, pruning diseased leaves may make the deficiency worse. The question is: which will debilitate the palm most—the disease or the nutrient deficiency? In many situations, the nutrient deficiency causes chlorosis (yellowing) and necrosis (death) of the leaf tissue creating the wound necessary for Pestalotiopsis infection. By eliminating the deficiency symptoms, one prevents disease development also. Thus, proper nutrient management is critical for disease prevention and management.
While fungicides may be useful to prevent further spread of the disease, they are merely a supplement to water management, sanitation, injury prevention and good palm nutrition. Fungicides alone will not solve the problem. It is critical to understand that fungicides do not cure the leaf spot or petiole blight already present. Once a leaf spot or petiole lesion occurs, it will remain for the duration of the life of that leaf. Fungicides are used to prevent further spread of the disease by protecting leaf tissue that has not been infected by the fungal pathogen.
The UF/IFAS publication of Pestalotiopsis is available here.