LEAF AND FRUIT ABSCISSION IN SAFOU, DACRYODES EDULIS (G. DON), H. J. LAM, IN THE HUMID FOREST ZONE OF CAMEROON: ASSOCIATED PARASITES, PESTS AND SEASONALITY OF THEIR DAMAGE

Abstract

ABSTRACT Early leaf and fruit abscission is a major problem affecting the production of safou in the humid forest zone of Cameroon. The identification of possible causes of early leaf and fruit abscission in safou and how parasite and or pest damage change with season formed the basis of this work. Between January and November 2004, data were collected daily from safou trees selected from farmer's field and grouped on the basis of the presence or absence of insect spots, brownish spots, whitish spots or no spots. The data collected included mean daily temperature, daily rainfall, Leaf Abscission Density (LAD), Fruit Abscission Density (FAD), number of leaves abscised with insect spots, whitish spots, brownish spots, and month when fruiting began. Leaves samples collected from the study population of safou trees and bearing galls revealed that these trees were invaded by insects from two families: Oligotrophus sp. of the family Cecidomyiidae (Diptera) and Brachymeria sp. of the family Chalcididae (Hymenoptera). From the brownish spots on leaves and brownish sunken spots on fruits, Colletotrichum sp. was consistently isolated while Pestalotia sp. was obtained from whitish spots on leaves and brownish slightly raised spots on fruits. Leaf abscission in safou was associated with the presence of Oligotrophus sp. And Colletotrichum sp. while fruit abscission was associated with the presence of Colletotrichum sp., Pestalotia sp. and early fruiting trees. The mean LAD was higher when safou trees where invaded by parasites/pest than when these trees are not invaded (1.71 as against 0.30 leaves/m2/day, F=29.57, DF=1, P<0.0001). On the other hand, the mean FAD was higher in parasite/pest invaded trees than those not invaded (0.14 as against 0.04 fruits/itt/day) but the difference was not significant (F=1.52, DF=1, P=0.21). Instead, the mean FAD was higher (0.31 fruit/m2/day) in early fruiting trees (Those that start fruiting in December) than in normal fruiting trees (those that start fruiting in February) (0.14 fruit/m-/day). The monthly distribution of the number of fruits abscised per day, number of leaves abscised per day with Oligotrophus sp. spots, Colletotrichum sp. spots, and Pestalotia sp. spots change considerably with season. Brachymeria sp. was suspected to be a parasitoid of Oligotrophus sp. and thus reduces the activity of the latter in causing leaf abscission. This work is innovative on this tree crop and the phenomenon of early leaf and fruit abscission may be influenced by many factors. That not withstanding, the seasonal dynamic observed will serve as a useful guide for future research on this tree crop.