By: Molly Giesbrecht and Kevin Ong
In the late 1970s, a lethal disease of palm trees first appeared in the Rio Grande Valley. The symptoms resembled those of the phytoplasma-caused lethal yellowing disease already well-documented in Africa, the Caribbean, and Florida. Electron microscopy showed phytoplasma cells in the phloem (vascular) tissues of the infected palms, which led to the presumption that the disease was caused by the lethal yellowing phytoplasma.
In 2001, many palms in the Corpus Christi area exhibited the same symptoms and began declining. In 2002, the disease was identified as a phytoplasma distinct from, but related to, the lethal yellowing phytoplasma. Now known as date palm lethal decline (DPLD), Texas Phoenix palm decline, or lethal decline, it is the only known phytoplasma disease of palms in Texas. It affects Phoenix spp., Syagrus romanzoffiana, and Sabal palmetto palms and is present in Bexar, Cameron, Harris, Hidalgo, Kleburg, Nueces, and Willacy counties.
Phytoplasmas are a specialized group of bacteria that infect only plants. They belong to the class Mollicutes, have no cell wall, and cannot survive outside of a living host. Phytoplasmas cause disease on a wide range of plant hosts and are typically transmitted by leafhopper and planthopper insects. The specific vector (the insect that transmits the bacteria) or vectors for this disease have not been determined.
The disease symptoms progress in a chronological order. Both the combination of symptoms and their progression are important in evaluating the possible presence of the disease, since other diseases and stress factors can cause many of the same symptoms.
- Typically, the first symptom on mature, fruit-bearing trees is the dropping of most or all of their fruit within a few days (Fig. 1).
- A red to brown necrosis (death of the plant tissue) of the oldest leaves can occur simultaneously or follow next, progressing from leaf tips to the base of the leaves.
- On Phoenix palms, after the first set of oldest leaves die, the central spear leaf dies. This is the most indicative symptom associated with this disease
- Although it may often go unnoticed, moderate to extensive root necrosis and decay typically occurs by this time, sometimes to the extent that the tree can be rocked back and forth by pushing on the trunk. Trees affected by this pathogen have more extensive root decay than trees affected by the lethal yellowing phytoplasma.
- The remaining leaves will continue to die progressively from the lower crown upward (older leaves to younger leaves) (Fig. 2).
- The tree will die completely within a year.
Phoenix canariensis, P. dactylifera, P. sylvestris,P. reclinata, P. roebelenii, Sabal palmetto, and Syagrus romanzoffiana palms can host this disease. In Texas, the most frequently affected species is P. canariensis because it is widely used in landscape plantings and is susceptible to this disease.
Nurseries can monitor quarantined palms grown in Texas and coming from out of state. Sample suspected trees according to the protocol TPDDL WI 2.47 Sampling Palms for Phytoplasma Detection and send the sample to the Texas Plant Disease Diagnostic Lab for testing. The laboratory uses molecular methods to determine the presence of phytoplasma.*
Sample and test symptomatic host trees. If a tree is positive, report the results to the Texas Department of Agriculture and remove the tree. To prevent infection in areas known to have had the disease, healthy trees can be treated with trunk injections of oxytetracycline HCl. To remain effective, repeat the applications every few months for the remaining life of the tree or as long as there is a threat of infection.
Quarantines are established in areas where the disease is prevalent and around confirmed infected trees to restrict moving susceptible host tree species out of these areas. As of May 2014, all of Cameron, Hidalgo, Nueces, and Willacy counties are under quarantine as well as sections of Bexar, Harris, and Kleburg counties.
Quarantine regulations require that
- Susceptible trees within a 1-mile radius of an infected tree remain stationary for 6 months and treated with insecticides for vector control for 3 months after the quarantine begins.
- Susceptible host trees within 2 miles of a confirmed infected tree must also be treated with insecticides for vector control for 3 months before the tree can be moved. They also must be treated within 48 hours before being moved and inspected for disease symptoms within 24 hours after being moved.
- Susceptible host trees outside of the 2-mile radius around the infected tree(s) but within the established quarantine zone must be treated for vector control for 6 weeks before being moved, treated within 48 hours of being moved, and inspected within 24 hours after being moved.
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