Institute of Food and Agricultural Sciences
Hendry County Horticulture News
Palm Diseases - Ganoderma is Incurable
Palm trees symbolize the tropics in the public imagination. Coconut palms swaying in the breeze adorn advertisements for Florida's beaches and resorts. Palms are a dominant feature in the landscape in housing developments throughout Florida. As landscape plants, palms are valued for their unique shape and the intricate texture and form of their leaves and stems. Their variety, beauty, and adaptability have made them some of the most prized landscape plants in the warmer areas of the state.
Despite their durability and adaptability, a number of diseases do affect palms in Florida. Lethal yellowing, an incurable disease, has greatly reduced the population of coconut palms on the lower east and west coasts of the state. Research into this disease has identified several resistant varieties of coconut, which are being used to rebuild coconut populations in affected areas. Control of the disease can also be obtained with antibiotics, although this method is costly and temporary at best.
Another serious disease of palms has recently made it's presence known in south Florida, where palms are a dominant part of the landscape. Ganoderma butt rot of palms is a lethal and incurable disease which affects mature palms. The causal organism Ganoderma zonatum, a type of shelf or bracket fungus, was only identified by scientists at the University of Florida, Fort Lauderdale Research and Education Center, in 1994. This disease is particularly insidious in that it attacks only mature trees. In addition, it seems that very few if any palms are resistant to this disease.
The symptoms of Ganoderma begin with the older fronds withering, drooping and turning brown. The leaflets often roll back along the petioles. The fronds then droop parallel to the trunk. The fronds do not break off but are retained on the trunk. New growth slows, decreases in size and becomes pale green or yellow.
As older fronds continue to die, younger leaves may show nutrient deficiencies. They may wilt periodically and the tips may turn brown. Death of the tree usually occurs within 6 to 12 months after symptoms develop, although in some instances they may hang on for several years after the first conk is produced. Such trees should be removed upon identification to prevent possible contaimination of nearby palms.
Additional symptoms include bleeding or a reddish exudate which stains the trunk and the formation of a conk or bracket fungus on the lower trunk. The conk is often present on the lower trunk soon after the symptoms of decline begin. The presence of a conk is proof that the tree has Ganoderma. Sometimes conks are not produced so that absence of a conk does not mean that a palm may not be infected. The conk is the reproductive body of the fungus. In our area, conks may be produced at any time. Initially, the conk is nothing more than a soft, white circular blob on the tree about an inch in diameter. It starts out flat against the tree. As it develops, it extends outward as a shelf, but is still soft and white. Older conks are kidney shaped, usually woody, somewhat shiny, with colored bands of reddish brown and lighter shades.
At maturity conks become swollen along the outer edge, revealing a white lower surface where spores are produced. Millions of spores may be released from a single conk. The spores act like tiny seeds and may be easily spread by the wind to healthy palms.
At present, it is assumed that all palms are susceptible to Ganoderma butt rot. There is no treatment for the disease. Symptoms of decline with the presence of a conk is positive proof that a palm has the disease. Infected palms should be cut down immediately. The best method of disposal of the trunk is burning to destroy the fungus. Palms should not be left in the landscape after cutting. This will only result in the production of infectious spores.
If possible the stump should also be removed and burned. If not it should be watched for the production of conks which should be removed as soon as they start to form. These can be burned or placed in a plastic bag and put in the garbage.
The diseased palm should not be replaced with another palm as fungus present in the soil and roots of the diseased tree will probably infect the new tree. Trees other than palms are not susceptible to the disease. If you must replant a new palm, you can try to remove all the old soil and roots and bring in fresh soil. This may or may not work in the long run. Soil fumigation has not been shown to have any effect in eliminating the fungus from the soil, as it can survive within bits of wood or decayed roots in the soil.
It is important to avoid injuries to the roots and trunks of palms to avert the possibility of creating a wound which may permit spores to infect a new palm tree. Periodic observation and quick removal and proper disposal of diseased palms are the major methods of fighting this devastating disease.
Gene McAvoy is the horticulture agent with the Hendry County Extension Service. Direct your horticulture questions to PO Box 68, LaBelle, FL 33975, e-mail - firstname.lastname@example.org or phone 863-674-4092 or 863-983-1598. You are also welcome to visit the Hendry County Extension Office at 225 Pratt Blvd., LaBelle. Office hours are from 8:00 - 5:00.
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