Black Pod Research Articles

Strains of Phytophthora palmivora (butl.) butl. from Theobroma cacao L. : II. Isolates from non-african countries

Examination of a further forty isolates of Phytophthora palmivora , from fourteen countries outside West Africa, has shown that isolates of the ‘cacao’ strain in West Africa are similar to those found elsewhere in development and appearance of lesions in cacao pods, sporangial dimensions, and oospore production with sexually complementary strains. West African isolates of the ‘rubber’ group were unlike isolates from cacao from any other country except in that they formed oospores with ‘cacao’ group isolates. Most isolates of the cacao black pod pathogen were of the ‘cacao’ group, although ‘rubber’ group isolates did occur, and isolates of the ‘cacao’ group were remarkably uniform, with few exceptions. A survey of 239 isolates of the pathogen from cacao has shown the need for a new method of subdividing the species, and of a fresh complete survey of the fungus.

A Die-back of Cacao Seedlings in Nigeria caused by a Species of Phytophthora

IN September 1954 a die-back disease was seen on cacao seedlings growing in Agricultural Department nurseries established in various localities of the western region of Nigeria. There is no previous record of this disease in Nigeria, but it has reappeared in the nurseries in subsequent years. The disease caused considerable losses in these years, and in a particular nursery as many as 70 per cent of the young seedlings were killed. A species of Phytophthora has been established as the causal agent, and when inoculated into healthy cacao pods the symptoms produced were identical to those of black-pod disease caused by Phytophthora palmivora (Butl.) Butl.

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

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.

Airborne dispersal of Phytophthora palmivora, causing blackpod disease of Theobroma cacao.

AT the 1951 Cocoa Conference, Mr. G. F. Bowman, chief of the American Cacao Research Centre, Turrialba1, referred to work by Salazar2 which shows that the sporangia of P. polmivora are almost certainly not airborne, except in drops of water. Contrary evidence was not apparently produced at the Conference ; therefore it is desirable to direct attention to certain observations in Nigeria which indicate that P. palmivora sporangia may be airborne and that infection apparently results thereby. In this respect, the species in question would appear to be comparable with P. infestans in temperate regions, the airborne dispersion of which has been discussed by Gregory3 and by Van der Plank4. It seems necessary to postulate long-distance dispersal to explain the epidemiology of black-pod disease. Observations have therefore been made to test this hypothesis, and also to determine whether free moisture is essential for take-off and dispersal of sporangia in Nigeria.

The Epiphytes of Theobroma Cacao in Nigeria in Relation to the Incidence of Black-Pod Disease (Phytophthora Palmivora)

The Epiphytes of Theobroma Cacao in Nigeria in Relation to the Incidence of Black-Pod Disease (Phytophthora Palmivora)

Umdhlebi Tree of Zululand

There are two species, in both the leaf is lanceolate, dark green, glossy, hard, and brittle, and from both a thick milky juice exudes, while the fruit is like a long black pod, red at the end. One species is a tree with large leaves, and peculiar looking stem, the bark hanging down in large flakes, showing a fresh growth of bark underneath: in the words of my informant, “What a villainous-looking tree! nasty, rough, ugly!” The other species is a shrub, with smaller leaves, and the bark not peeling off the stem. Both species are said to possess the power of poisoning any living creature which approaches it; the symptoms of poisoning by it being severe headache, blood-shot eyes, and delirium, ending in death. The person affected dies either in delirium, or instantaneously without any delirium. A superstition is connected with this plant. Only a few persons in Zululand are supposed to be able to collect the fruits of the Umdhlebi, and these dare nut approach the tree except from the windward side. They also sacrifice a goat or a sheep to the demon of the tree, tying the animal to, or near the tree. The fruit is collected for the purpose of being used as the antidote to the poisonous effects of the tree from which they fall—for only the fallen fruit may be collected. As regards habitat, these trees grow on all kinds of soil, not specially on that which exudes carbonic acid gas, but the tree-like species prefers barren and rocky ground. In consequence of this superstition, the country around one of these trees is always uninhabited, althongh often fertile.