He was born in the South Birmingham area and educated at Cheltenham Grammar School. He entered Christ's College, Cambridge to study zoology but then accepted a Colonial Agricultural Service research studentship in plant genetics. After studying at the Cambridge University School of Agriculture, he moved to the Imperial College of Tropical Agriculture in Trinidad to carry out a research project into the pollination of the cocoa tree. In 1937 he moved to the Gold Coast as a plant pathologist at a time when the local cocoa trees were dying from an unknown cause, which was assumed to be the result of climate change. Posnette discovered that in fact they were suffering from a virus spread by mealy bugs. The following year he established a Cocoa Experiment Station at Tafo to carry out research into appropriate control methods, but came to the conclusion that the best solution was to breed a resistant variety of cocoa tree. However, nearly all the trees in the area were of one type originally imported from Brazil, so in 1943 he returned to Trinidad and carried out a project of cross-fertilisation between local trees of the local Trinitario variety and trees from the Upper Amazon. When planted in the Gold Coast, some of the seedlings showed hybrid vigour, producing earlier crops with higher-than-normal yields and, importantly, were more tolerant of the swollen shoot virus. The introduction of his Upper Amazon hybrids played a large part in saving the West African cocoa industry from extinction and the hybrids now form the basis of most cocoa improvement programmes worldwide. In 1949 he left Africa to work at the East Malling Research Station on viruses of berry fruits, and later on diseases of various fruit trees. He contributed to the international research that led to the development of new virus-free fruit tree varieties and became head of the Pathology Department in 1957. He then served as deputy director from 1969 and as director from 1972 until his retirement in 1979.
He lived until 1970 at Little Ashurst Farm, Chart, Kent, where he and his wife built up a successful fruit farm, where Posnette planted every tree himself. He had married in 1937 Isabelle La Roche, with whom he had a son and two daughters,Jane Marion, Suzanne Lucy and John LaRoche.
Legacy
The cocoa research station which he founded in the 1930s became the West African Cocoa Research Institute.
