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Symptoms and Signs

No single symptom is diagnostic of lethal yellowing. Symptoms are variable among palm genera and, in the case of coconuts, among cultivars. It is the pattern of appearance and chronological progression of symptoms that accurately identifies the disease. Confirmation of lethal yellowing is based on a molecular diagnostic assay using the polymerase chain reaction (PCR). At least 36 palm species (Table 1) have been documented as susceptible to lethal yellowing, but coconut palm (Cocos nucifera) is most vulnerable to the disease, followed by Pritchardia species, Christmas palm (Adonidia merrillii), and date palm (Phoenix dactylifera).

Table 1. Palm species susceptible to lethal yellowing disease.
Adonidia merrillii	Dictyosperma album	Phoenix dactylifera
Aiphanes lindeniana	Dypsis cabadae	Phoenix reclinata
Allagoptera arenaria	Dypsis decaryi	Phoenix rupicola
Arenga engleri	Gaussia attenuata	Phoenix sylvestris
Borassus flabellifer	Howea belmoreana	Pritchardia affinis
Caryota mitis	Howea forsteriana	Pritchardia pacifica
Caryota rumphiana	Hyophorbe verschaffeltii	Pritchardia remota
Chelyocarpus chuco	Latania lontaroides	Pritchardia thurstonii
Cocos nucifera	Livistona chinensis	Ravenea hildebrantii
Corypha utan	Livistona rotundifolia	Syagrus schizophylla
Crysophila warsecewiczii	Nannorrhops ritchiana	Trachycarpus fortunei
Cyphophoenix nucele	Phoenix canariensis	Veitchia arecina

The first obvious symptom on mature palms (those able to produce fruit) is a premature drop of most or all fruits. For coconuts, the calyx end of the nut (fruit) will usually develop a brown to black, water-soaked appearance (Figure 1). Nut or fruit-fall is accompanied or followed by flower necrosis. This symptom is most readily observed on newly mature flowers as they emerge from the spathe (Figure 2). Male flowers abscise, and no fruit is set.

Figure 1

Figure 2

The next symptom observed on mature palms (the first symptom for immature palms or non-fruit bearing palms) is foliar discoloration. This symptom varies markedly among coconut cultivars and other palm genera.

For tall-type coconut cultivars (e.g., ‘Jamaica Tall’), the foliage turns yellow, beginning with the lowermost (oldest) leaves and progressing until the entire crown is affected (Figure 3). In some cases, this symptom is first seen as a solitary, yellowed leaf (“flag leaf”) in the middle of the leaf canopy (Figure 4). Typically, yellowed leaves remain turgid, but eventually turn brown, desiccate, and hang down to form a skirt around the trunk for several weeks before falling. As leaf yellowing advances, the spear (youngest) leaf collapses and hangs down in the crown. Death of the apical meristem (bud) usually occurs when one-half to two-thirds of the crown has yellowed. Eventually, the entire crown of the palm withers and topples, leaving a bare trunk standing (Figure 5). Infected palms usually die within 3 to 5 months after the first appearance of symptoms.

Figure 3	Figure 4
Figure 5

For dwarf-type coconut cultivars (e.g., ‘Malayan Green Dwarf’), leaves generally turn a reddish to grayish-brown rather than yellow (Figure 6). Leaflets on the green form of the Malayan Dwarf cultivar may be folded around the midvein (Figure 7). Sometimes, affected leaves appear flaccid, giving an overall wilted appearance to the palm canopy.

Figure 6

Figure 7

Foliar yellowing also develops on Caryota mitis (clustering fishtail palm), C. rumphiana (Figure 8), Chelyocarpus chuco (Figure 9), Corypha utan, Dictyosperma album (hurricane or princess palm) (Figure 10), Livistona chinensis (Chinese fan palm), Pritchardia spp., and Trachycarpus fortunei (windmill palm). In contrast, successively younger leaves turn varying shades of reddish-brown to dark brown or gray in other palm species, such as Adonidia merrillii (Christmas palm), Borassus flabellifer (palmyra palm) (Figure 11), Dypsis cabadae (cabada palm), Phoenix spp. (date palm, Canary Island date palm, wild date palm) (Figure 12), and Veitchia arecina (Montgomery palm). Differences may occur in the stage at which spear leaf collapse and necrosis appears on these species. For date palms and palmyra palm, death of the spear leaf often precedes foliar discoloration. For Adonidia and Veitchia spp., the spear leaf is usually not affected until after all other leaves have died.

Figure 8	Figure 9	Figure 10
Figure 11	Figure 12

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