Localisation of coconut foliar decay virus in coconut palm

Coconut foliar decay virus (CFDV) occurs at a very low concentration in coconut palm. A 1203 nucleotide segment of the sequenced encapsidated circular single-stranded 1291 nucleotide CFDV-DNA has been amplified and transcribed for use as a ³²P cDNA probe for the virus. A rapid method for the extraction of DNA from coconut palm has been devised for a dot-blot hybridisation assay using this probe. An alternative non-radioactive probe has also been developed for future use in CFDV diagnosis. CFDV-DNA was shown to be distributed unevenly in a range of infected palms, necessitating the use of multiple sampling to reliably detect infection in diagnostic tests. Viral DNA was detected in symptomatic and asymptomatic palms of both high and low susceptibility, in disease-free tolerant cultivars, and in palms in remission from disease. Within the same palm, detectability of viral DNA varied little within leaflets, but varied more within and between fronds. CFDV-DNA was detected 6–8 months after insect-mediated inoculation, and symptoms generally appeared after another 1–4 months. In situ hybridisation of rachis tissue showed localisation of DNA within the phloem, but its distribution in the phloem was uneven. CFDV-DNA was detected in tissue adjacent to and within necrotic zones which develop into the petiolar lesions associated with the disease-specific collapse of fronds. Virus was detected in the body of the insect vector, and, where its distribution could be resolved, in the abdomen rather than the head.