Dieback due to Lasiodiplodia theobromae, a new constraint to cocoa production in Cameroon

Since the introduction of cocoa (Theobroma cacao) in Cameroon in 1886, the main disease has been phytophthora pod rot. However, since the late 1980s, cocoa orchards have been increasingly affected by an unusual dieback disease. Dieback has been recorded in all of the cocoa producing areas of Cameroon, affecting 100% of cocoa trees at some farms.

Irrespective of age, affected cocoa trees manifest typical dieback symptoms. Leaves on the outer twigs yellow first (Fig. 1) and the damage may then extend along the whole branch, reaching the main trunk, eventually resulting in tree death (Fig. 2). The twigs and branches of diseased trees show internal discolouration with brown streaks in the vascular tissue (Fig. 3). White or yellowish exudate from trunks has also been reported. Although sudden widespread wilting and death may occur, affected trees more typically decline over several months, during which time flushes of new growth may develop at the collar of declining trees. Desiccated leaves and mummified fruits remain attached to declining trees for several weeks. Although tree mortality occurs throughout the year, symptoms are more severe during the dry season, especially for trees with nil or slight shade.

The fungus Lasiodiplodia theobromae (Pat.) Griffon & Maubl. (syn. Botryodiplodia theobromae Pat.), was most commonly associated with dieback and was consistently isolated from various tissues (twigs, bark, vascular tissue and fruits) of symptomatic plants. L. theobromae was identified based on morphological characters as described by Punithalingam (1976). These included a rapidly spreading dark green or black colony and conidia that were dark brown, striate, ellipsoidal, uniseptate and produced in ascostromatic pycnidia on potato dextrose agar (PDA). Cultures were deposited at the Centraalbureau voor Schimmelcultures (CBS), Utrecht, The Netherlands. Mycelial plugs from isolates CBS 120395 and 120396 on PDA, were inoculated separately on to wounded stems of twenty 6-month old cocoa seedlings in a greenhouse, to test pathogenicity. After 4 weeks of regular watering followed by 3 weeks water stress, 60% of the inoculated test plants withered and became irreversibly desiccated; 30% recovered as watering resumed but declined a few weeks later, reproducing the symptoms observed in the field (Fig. 4). In contrast, the uninoculated control plants (PDA plugs only) recovered and remained healthy. L. theobromae was re-isolated from all infected plants.

L. theobromae is a common, widespread pathogen of tropical woody trees, causing shoot blight and dieback of trees and shrubs and blue stain in timber (Mohali et al., 2005). L. theobromae was first reported on cocoa in Cameroon in 1895, and has since caused minor symptoms of charcoal pod rot. However, with this new development of severe dieback, similar to that described recently on other tree crops such as mango and kumquat in other countries (Khanzada et al., 2004; Ko et al., 2004), L. theobromae is becoming a major constraint to cocoa production in Cameroon.