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

C. A. Thorold
Journal of Ecology
Vol. 40, No. 1 (Feb., 1952), pp. 125-142
DOI: 10.2307/2258025
Stable URL: http://www.jstor.org/stable/2258025
Page Count: 18
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The Epiphytes of Theobroma Cacao in Nigeria in Relation to the Incidence of Black-Pod Disease (Phytophthora Palmivora)
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Abstract

1. The severity of black-pod disease is governed by humidity conditions which are probably related to rainfall differences. 2. Mean annual rainfalls in various parts of the cacao areas of Nigeria range between 45 in. (1143 mm.) and over 300 in. (7620 mm.). 3. All the epiphytes identified as occurring on Theobroma cacao in Nigeria are listed, but only the bryophytes and vascular plants are considered as possible climatic indicators. 4. Liverworts occur under relatively dry conditions, vascular plants require relatively wet conditions, and the mosses are intermediate are regards the degree of dryness they will tolerate. On the above basis, seven categories of possible epiphyte occurrences are defined and constitute `epiphyte ratings' numbered one to seven. 5. Mean epiphyte ratings are derived by observing about 100 cacao trees and averaging the individual ratings so obtained. 6. The frequencies of epiphyte ratings for individual trees are fairly symmetrically distributed, with a general mean of 3.5. The modal class (3) tends to be too comprehensive, and suggestions are made towards remedying this defect by subdivision, based on the occurrence of certain liverwort genera in particular. 7. Positive and significant correlations were found for the following pairs of variables: mean epiphyte rating and rainfall; mean epiphyte rating and mean number of black pods per tree; mean number of black pods per tree and mean number of total (healthy plus diseased) pods per tree. Mean epiphyte rating and mean number of total pods per tree are negatively and significantly correlated. 8. Other factors affecting mean epiphyte ratings are considered, namely: (a) variability of individual epiphyte ratings; (b) age of tree; (c) different spacings of trees; (d) shape of tree; (e) nature of bark; (f) temperature; (g) illumination; (h) soil. None of these factors is found to be of importance except illumination. 9. In the relatively dry areas, epiphytes are suppressed when natural shade reduces the illumination to 1/56 of full exposure illumination. In the wet areas, epiphytes are little affected by shade unless relative light intensity is as low as 1/300, such conditions being exceptional. 10. It is likely that further useful information on the relation between climatic conditions and the incidence of black-pod disease can be obtained by observations on particular epiphyte species, but such studies have not been undertaken.

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