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Young coconut palms easily succumb to R. cocophilus attack. There is no record of any tree, once affected, having recovered. The disease occurs more commonly in trees 2.5-10 years old, with greatest incidence in those 4-7 years old. Occasionally, a palm as young as 1.5 or as old as 20 years or more may be attacked.
The symptoms described are those for palms of the tall cultivar of coconuts or 'typica' which grow in the West Indian islands. These symptoms differ somewhat in the dwarf variety 'nana' and also some panama talls. Chlorosis first appears at the tips of the oldest leaves and spreads towards their bases but, occasionally, the younger leaves may be affected first. The brown lower leaves may break across the petiole of the lower part of the rachis, or they may become partly dislodged at the base and hang down. Nuts are shed prematurely, either simultaneously with the development of leaf symptoms, or slightly before. The crown often topples over, about 4-6 weeks after symptoms first appear, due to associated severe damage caused internally by the larvae of the palm weevil. However, the trunk remains standing in the field for several months until it decays. At the onset of symptoms, the chlorotic yellow appearance of the leaves around the stem is sometimes indistinguishable from those of trees growing under conditions of poor drainage or during intense drought.
The most characteristic symptoms are the internal lesions. In a cross-section of the stem, they appear as an orange to brick-red coloured ring, 2-4 cm wide, and at a distance of 3-5 cm from the periphery. In longitudinal section, the reddened tissue may appear as two united bands joined in the bole forming a 'U'-shape. Lesions at the upper end of the stem in the vicinity of the crown are discrete, appearing first as streaks and then as dots. The meristematic tissue in the bud remains white and apparently healthy. There is no putrefaction of the bud associated with R. cocophilus attack. In the roots, the normally white soft cortex becomes orange to faint red in colour, and has a dry and flaky texture when diseased. In the leaves, a solid core of mottled tissue, dull red to brown in colour, extends from the leaf-base up to 75 cm in the petioles.
The disease is not recognizable externally in its very early stages. The roots, stems and leaf petioles are already infested and there is full development of internal symptoms before the first external symptoms become visible. In the dwarf cultivars, the red colour gives way to shades of brown. Thus, instead of a red ring internally, there is a brown band. The discrete spots are also brown and the yellow discoloration of the leaves is not often apparent. Generally, the leaves become dried and brown, beginning at the tips of the leaflets and progressing downwards. The yellow dwarf cultivars respond in the same way as the green and the crosses between talls and dwarfs, or between Panama tall and any dwarf. They show a browning instead of a characteristic reddening of the leaves and stem tissue.
The heaviest losses due to R. cocophilus occur at the end of the wet season and in the first 2-3 months of the dry season (December to March) in Trinidad.
Red ring disease in new groves generally begins by infection of a 4-10 year old palm by the palm weevil, Rhynchophorus palmarum, carrying R. cocophilus. Effective patterns of control may be employed during several phases of the development of the epidemic.
The dispersal rate from the primary infector plant depends upon the development of R. palmarum within the diseased tree. Parasitism by the nematodes may limit the number of developing vectors and reduce the size, fecundity and longevity of the vector adults. Three months after infection, a new tree can be infected by a vector (female) emerging from the infector plant. If the insect is unmated and infertile, no vector will develop from this infection and red ring can die out when the diseased palm dies. This diseased tree, however, forms a source of inoculum as it becomes chemically attractive to all palm weevils including potential vectors. Phytosanitary measures of control are most effective at this time since disease symptoms are apparent before the progeny of the newly invaded insects emerge after 3 months.
Control in Coconut
There are no simple means of controlling R. cocophilus and no effective measures are available for control of the nematode in living palms. Control is based on prevention rather than cure either by the destruction of infested palm material by cutting and burning, or by the injection of herbicides and burning, or by trapping and killing of the weevil vectors before they spread the nematodes.
Many trees show yellowing and browning of leaves which may not be due to R. cocophilus attack. To prevent unnecessary destruction of trees, a core sample of the trunk should be taken with a 2-cm pipe (see Detection section) to determine the presence of R. cocophilus before control measures are employed.
Traps or guard baskets are designed to protect plantations from frequent outbreaks of R. cocophilus. They do so by attracting and killing palm weevils which may enter the plantations from nearby diseased trees. Guard baskets are made of 2-cm-mesh wire. They are cylindrical, 1 m high and 0.3 m in diameter. These baskets are filled with chunks of fresh tissue from diseased coconut trees to attract the weevil. If such trees are not available, chunks of palmiste or 'gru-gru' trees may be used. The guard baskets are sprayed with pesticide and distributed on the ground in the plantations at one basket per acre (2.5 baskets/ha) of young coconut trees. This procedure is especially recommended in the dry season when the weevils are most active in the cool nights. Guard baskets remain for about 2 weeks, after which the tissue and insecticide in the basket should be burnt. Fresh tissue should be placed in the basket and treated as previously described. Several variations are used in practice with different types of tissue, such as pineapple and papaya (Griffith and Koshy, 1990).
R. cocophilus causes major crop loss of coconut plantations in its restricted area of distribution. It can also seriously damage oil palms. The percentage loss can vary from a few percent to complete destruction of young coconuts. Young coconut palms easily succumb to R. cocophilus attack. There is no record of any tree, once affected, having recovered. The heaviest losses due to R. cocophilus occur at the end of the wet season and in the first 2-3 months of the dry season (December to March) in Trinidad.