Armillaria on cacao in São Tomé


Outbreaks of root disease caused by Armillaria generally originated from infection of shade trees, some species of which were very susceptible. Serious attacks often developed after shade tree had been felled or otherwise killed, even if they were resistant to Armillaria whilst living. The fungus spread rapidly, up to 5 m/year in cacao growing in areas of high rainfall. Mortality had reached nine per cent locally, but was usually much lower; considerable financial loss often resulted. The absence of rhizomorphs in soil may partly be explained by high soil temperatures and adverse conditions of soil aeration but other inhibitory effect were probably also involved. Suitably situated trenches generally prevented further spread of the fungus in cacao, but further losses sometimes occurred within trenched areas. Other possible control measures are discussed and attention is drawn to the possibility of new foci arising through infection by air-borne spores.