date palm lethal decline, Texas Phoenix decline
‘Candidatus Phytoplasma palmae’subgroup 16SrIV, strain D: Kingdom Bacteria, Division Firmicutes
Phoenix canariensis, Phoenix dactylifera, Phoenix reclinata, Phoenix roebelenii, Phoenix sylvestris, Pseudopheoenix sargentii, Sabal mexicana, Sabal palmetto, Syagrus romanzoffiana, Thrinax radiata
Mexico (Yucatan), USA (Florida and Texas). In Florida, the pathogen has been detected in Broward, DeSoto, Hardee, Highlands, Hillsborough, Indian River, Manatee, Orange, Palm Beach, Pasco, Pinellas, Polk, Sarasota, and Sumter counties. In Texas, the disease has been confirmed in Bexar, Cameron, Hidalgo, Kleberg, Nueces, Tarrant, and Willacy counties.
Field diagnosis is based on a series of symptoms. No single symptom is diagnostic of Texas Phoenix palm decline. Rather, it is the appearance and chronological progression of symptoms that accurately identifies the disease.
The first obvious symptom of Texas Phoenix palm decline on mature palms is premature drop of most or all fruits and inflorescence necrosis. The fruit drop occurs within a few days; it is not spread out over a prolonged period of time (Fig. 1). However, these two symptoms will only be observed if the palm is mature enough to produce fruit, if it is the season for flowering and fruiting, and if the flowers or fruits have not been trimmed from the palm.
The next symptom is discoloration of the foliage, beginning with the oldest leaves. The discoloration begins at leaf tips. While portions of the leaf turn yellow, they do not remain yellow for very long. Instead, the leaves quickly turn varying shades of reddish-brown to dark brown or gray (Fig. 2). Unless the palm is being monitored closely, the onset of leaf discoloration is usually first recognized as a greater number of dead older leaves than is normal for natural senescence (Figs. 3-5). However, if the dying or dead leaves are being regularly removed, even this symptom of foliage discoloration may not be obvious
Death of the spear leaf is the next symptom observed. This appears to occur quite early in the disease process for Phoenix spp. (less than 1/3 of the leaves are discolored or necrotic (Fig. 6 and 7), but later in the process for Sabal palmetto (2/3 of the leaves are discolored or necrotic) (Fig. 8).
Once the spear leaf has died, no new leaves will develop, and the remaining leaves will continue to become discolored or necrotic from the oldest to the youngest leaves. In some instances, by the time the spear leaf dies, palm roots at or near the soil surface are soft in texture and easily broken. Such palms can be easily rocked back and forth in the ground because the root system is decaying.
Phytoplasma diseases: Lethal yellowing causes the same symptoms as Texas Phoenix palm decline in Phoenix species. It would not be possible to determine which pathogen was affecting the palm without the proper molecular diagnostic test.
Fungal diseases: Ganoderma butt rot will cause lower leaves in the canopy to die prematurely or cause an overall wilt of the canopy, but it will not cause premature death of the spear leaf. Bud rot causes premature death of the spear leaf, but will not cause premature death of the lowest leaves.
Disorders: Potassium deficiency will cause discoloration and premature death of the lowest leaves, but does not cause premature death of the spear leaf or fruit drop.
Death of the spear leaf may not always be obvious. Both Phoenix species and Sabal palmetto have numerous young leaves surrounding the spear leaf. Unless you see the spear leaf is dead or find it hanging from the canopy or on the ground, you will probably need to physically examine the canopy closely to determine whether a healthy spear leaf is present. Also, the young spear leaf of a healthy Phoenix species is enclosed in a thin, brown sheath (which tears like paper). Be careful not to confuse the normally occurring brown sheath for a dead spear leaf.>