Observations on black-pod disease ( Phytophthora palmivora) of cacao in Nigeria

Abstract

The rotting of cacao pods caused by Phytophthora palmivora is referred to as black-pod disease. Typical disease symptoms were reproduced by artificial inoculation, vising a pure culture of the fungus, but in general it was convenient to use sporangia from naturally infected pods as inoculum material. In Nigeria, as contrasted with the Gold Coast, relatively few pods are infected at the base, in comparison with numbers infected laterally or distally. This circumstance is considered in relation to the anatomy of the pod and in connexion with the possibility of infection from the subtending cushion, via the pod stalk. Cushion infection is liable to occur when diseased pods are left on the trees, but such cushion infection is unimportant, because relatively few cushions (either ‘healthy’ or ‘diseased’) bear pods in the succeeding season. Some sporangia take-off in ‘dry’ air, but more take-off under rain-splash conditions. Dispersion gradients were observed over both short (3 m.) and long (180 m.) distances. There was ‘under-dispersion’ at short distances from a source, but ‘over-dispersion’ due to extraneous sources occurred with longdistance dispersion. There is evidence that sporangia from the soil cause pod infection, and this phenomenon is offered as an explanation of ‘over-dispersion’. Black-pod incidence is mainly determined by numbers of pods available and climatic conditions. The temperature and humidity regimes in the months when the disease is prevalent were found to be in accordance with information on conditions favourable for germination of sporangia. However, an epidemic may not start, in spite of apparently favourable climatic conditions, if insufficient pods are present.