About 18 months ago, I wrote an article about a disease of our Florida palms called TPPD, or Texas Phoenix Palm Decline. TPPD is named as such because it was originally discovered in the coastal region of Texas and primarily attacks palms in the genus Phoenix, which include date palms. The disease has been renamed lethal bronzing because of the coloration of the infected fronds as they decline.
At the time of my last article, the primary palms affected include Canary Island Date Palm, Phoenix canariensis; Edible Date Palm, Phoenix dactylifera; Wild Date Palm, Phoenix sylvestris; Queen Palm, Syragus romanzoffianum; and our state tree, Cabbage or Sabal Palm, Sabal palmetto. However, lethal bronzing is now suspected of infecting other species, including Jelly or Pindo Palm, Butia capitata; Chinese Fan Palm, Livistonia chinensis; Coconut palm, Cocos nucifera; Adonidia palm, Adonidia merrillii; Bismark palm, Bismarkia nobilis; Foxtail palm, Wodyetia bifurcata; Windmill palm, Trachycarpus fortunei; and Areca palm, Dypsis lutescens. Several of these species are found only in the southern part of Florida; however, Queen Palm, Canary Island Date Palm, Wild Date Palm, Jelly or Pindo Palm, and Cabbage or Sabal Palm are common palms in our area. The latter, Cabbage Palm, is our state tree, and is found throughout Florida.
Also, at my last writing, lethal bronzing was noted in 22 counties. That list has now been expanded to 31 counties from Duval (Jacksonville) west to Jefferson County (east of Tallahassee) down to extreme South Florida. I detected lethal bronzing in early 2018 here in Citrus County. Considering the disease was initially detected in Florida in 2006 in Hillsborough, Manatee and Sarasota Counties, this rate of spread is remarkable and alarming. Of particular concern is the potential decline of Cabbage Palms, which are prevalent in natural areas in Florida as well as planted in many cities throughout Florida. In some cities, such as St. Augustine Beach, Cabbage Palms make up over 50% of the urban tree population. If lethal bronzing were to spread through cities such as this, the effect would be devastating.
Lethal bronzing is spread by an insect known as a leafhopper, which extracts plant juices through its piercing/sucking mouthparts. It was initially suspected that the disease was transported via imported palms from Texas and spread by native planthoppers. However, Dr. Brian Bahder, assistant professor at the University of Florida Research and Education Center in Ft. Lauderdale, now suspects that the leafhopper was spread into Florida from the Yucatan Peninsula of Mexico in 2005, where Hurricane Wilma first made landfall before continuing on to Florida. Research is ongoing to try to isolate the particular leafhopper which spreads the disease.
The first obvious lethal bronzing symptom is the premature dropping of the fruit. Most or all of the fruits will drop at one time rather than spread out over a period of time as is normal. Flower necrosis (death) follows. However, these symptoms cannot be detected if the tree is not mature enough to produce fruits and flowers, it is not flowering or fruiting season, or if they have already been trimmed from the tree.
The next symptom to follow is the discoloration of the fronds, starting with the oldest fronds. Rather than turn yellow and then brown, the discoloration is a reddish-brown to dark brown or gray and occurs in a relatively short period of time. The discoloration begins at the tips and works downward toward the trunk. During this time, the spear leaf or terminal frond dies. Because this is the only growing portion of a palm tree, when the spear or terminal leaf (frond) dies, no new fronds will develop and the tree will die. This terminal or leaf spear will die when approximately one-third of the other fronds have turned brown in Phoenix or Date Palms and approximately two-thirds of the other fronds in Cabbage Palms.
Lethal bronzing is caused by an organism known as a mycoplasma, which is similar to a bacteria but has no cell walls. The lethal yellowing disease of coconut palms in South Florida is also caused by a mycoplasma. While these organisms are similar to one another, they are genetically distinct. Lethal bronzing can only be identified with certainty through genetic testing in a laboratory. Sawdust from the infected tree must be collected by drilling into the trunk and the sawdust sent to a laboratory for positive identification.
The disease can be prevented using regular injections of OTC, or oxytetracycline. This is the same prevention used for lethal yellowing of Coconut Palms in South Florida. This treatment must be repeated every 120 days and can be done with a single injection of OTC into the trunk. Once the symptoms have appeared in the tree, however, treatment may not be successful. If a single tree is suspected of having the disease, it is recommended that nearby palms be injected as a preventative. OTC has been utilized successfully for many years against lethal yellowing.
If you witness yellowing and browning of fronds on the above-mentioned species, pay attention to the pattern of the disease progression. If you suspect lethal bronzing, consult a certified arborist. The disease can be tested by sampling and sending to a laboratory for confirmation and can be cured if caught early enough or prevented through injections.
Eric H. Hoyer is a certified arborist, a certified forester, a registered consulting arborist and a qualified tree risk assessor with Natural Resource Planning Services Inc. He can be contacted at email@example.com.