Studies of the Clove Tree: Sudden-Death Diseases and its Epidemiology

Abstract

Sudden-death disease of cloves has been present and steadily increasing in both Zanzibar and Pemba for many years. The only premonitory symptom is a slight chlorosis followed by thinning of the foliage and reduction of the absorbing system. Death follows after a period, which may vary from only a few days to many months. Death occurs from lack of water caused by the disorganization of the absorbing roots. The outbreaks fall into three classes (1) the sporadic, which ceases to spread after killing a few trees; (2) the 'Pemba' type showing clear peripheral spread; (3) the diffuse epidemic type.In Pemba some 500 small outbreaks are scattered through the clove areas and some seem to be passing from type (2) to type (3). The total number of trees affected there is less than in Zanzibar, where the situation approximates to a single out- break involving half of the clove-growing area of the island. The rate of spread of the disease varies, but it is accelerating. Various causes, physical, physiological, and pathological, have been suggested to account for the condition. The epidemiology suggests that all but a pathogenic hypothesis can be discarded. Of the possible pathogens, a virus carried by a lethargic vector is the most probable. Suspicion is attached to a scale insect which is tended and transported by the red tree-ant, Oecophylla longinoda.