COPPICING EXPERIMENTS ON THE SPREAD AND CONTROL OF CACAO SWOLLEN‐SHOOT DISEASE IN NIGERIA

A coppicing technique was used to determine the incidence and distribution of latent and unrecognized infection around naturally occurring outbreaks of cacao swollen‐shoot virus disease in Western Nigeria. All the apparently healthy trees within 30 yards of eighty outbreaks of various sizes were coppiced and most of the infected stumps which regenerated showed symptoms within a year. The distribution of the infected stumps around thirty‐five outbreaks studied in detail is expressed by the equation log 10 I = a + bx. I is the estimated intensity of infection in the coppiced stumps at distance x from the nearest infected tree removed at the time of coppicing. The constant a determines the height of the peak of the infection gradient and increases with outbreak size, whereas the slope of the gradient, determined by the negative constant b , is similar around all outbreaks. The results are consistent with information on the movement of the mealybugs and spread of cacao swollen‐shoot virus in outbreaks. Moreover, they indicate that outbreaks are controlled most economically by removing all obviously infected trees and adjacent apparently healthy ones. Control does not require the destruction of all the trees around outbreaks up to a distance of 30 yards as done previously, but can be achieved by felling fewer trees, the actual number depending on the size of the outbreak.