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Coconut palm (Cocos nucifera)
For coconut lethal yellowing, the complete range of symptoms can be seen only in mature bearing palms and the sequence and intensity of symptoms may vary with variety and environment. The following syndrome is the most representative. The first symptom is premature dropping of most or all of the fruit at every stage of develo pment. This may sometimes begin with bunches on one side of the palm but all bunches follow suit in a short space of time. The next symptom is blackening and bending down of some of the individual flowering branches in the youngest freshly opened inflores c ence. If younger inflorescences are cut open at this time they also show the blackening. This symptom is highly diagnostic for LY. In the third stage of disease development, the fronds turn yellow, beginning with the older ones and advancing toward the yo u nger ones near the center of the crown. For very susceptible varieties the colour is a bright, butter yellow. For the less susceptible varieties the colour is bronze. Sometimes a younger frond will yellow early in the sequence. This is commonly called a " f lag leaf" and is a significant symptom for diagnosing LY, but is not consistently present. Fronds that have turned yellow desiccate and hang from the tree. The final stage is the death of the growing point, causing the youngest unfolded leaf (the "spear" l eaf) to collapse. When this stage is reached, the entire bud may rot within days. Sometimes this stage is reached when there are still some green leaves left on the palm, but death of the bud is the equivalent of death of the palm. The entire sequence of s ymptoms from initial fruit drop to death of the bud usually takes from 3-6 months, and the rapidity of these events is in itself a symptom. As many diseases and physiological conditions of coconut palms are characterized by frond yellowing and necrosis, t hose that do not kill the palm within 6 months can be suspected to be something other than LY.
In palms other than coconut, early symptoms are similar. Fruits are shed prematurely and new inflorescences tend to become dark, necrotic, dry and gnarled. Leaf symptoms vary. For example, in Corypha elata, Pritchardia spp. Syagrus schizophylla, Trachycarpus fortunei, Dictyosperma album, and Hyophorbe verschafeltii, the leaves turn yellow beginning with the lowest fronds, and a "flag leaf" may develop. The leaves eventually die and turn brown and may fall or cling to the palm. In Veitchia merrillii, Borassus flabellifer, Caryota mitis and Phoenix spp., necrosis begins along the leaflet margins as a brownish stain.
Sources of infection - vectors feeding on leaves of infected palms during incubation period or early stage symptoms
Non-sources - those palms or those parts of palms on which vectors do not feed - dying (late stage symptoms] or dead palms; flowers, incuding pollen; seed including embryos; copra; oil; juice (fresh or fermented); timber (or any other partly processed product).
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