Viruses, viroids and vectors

On San Miguel, one of the smallest of the 7 000 islands of the Philippines, a few coconut palms were slowly dying in 1928. Ten years later, 50 000 palms on this island were dead or dying. The disease seemed to be limited to a certain area and affected only coconut palms. No bacteria or fungi were found to be associated with the dying palms. The disease, believed to be caused by a virus, began to spread onto the nearby mainland of Luzon. It became known as cadang-cadang (dying-dying, or slowly dying, in the Bicolano language). During the following decades plant pathologists and entomologists from the USA, India, Italy and Australia were assisting their Philippine colleagues in trying to find the vector of the cadang-cadang disease agent and attempting to devise prevention and control measures. In 1976 the disease agent was found not to be a virus, but a viroid, one of a group of plant pathogens composed entirely of a sequence of naked DNA. With the identification of the disease agent the search for vectors was abandoned. No resistant coconut palm varieties have been found. By now an estimated 50 million palms have died in and around the area of the original outbreak. I shall describe how the cadang-cadang viroid may have been transmitted in the past and how further spread could be prevented. Until now no means for implementing control measures have been established. Session III. DNA replication, recombination and repair