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Species Page

black pod of cocoa

Phytophthora megakarya
This information is part of a full datasheet available in the Crop Protection Compendium (CPC). Find out more information on how to access the CPC.
©CAB International. Published under a CC-BY-NC-SA 4.0 licence.

Distribution

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Extent
Invasive
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Host plants / species affected

Main hosts

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Theobroma cacao (cocoa)

List of symptoms / signs

Fruit - lesions: black or brown
Fruit - mummification
Fruit - premature drop
Roots - soft rot of cortex
Seeds - rot
Seeds - shrivelled
Stems - canker on woody stem
Whole plant - damping off
Whole plant - plant dead; dieback

Symptoms

Symptoms in cocoa caused by P. megakarya can be easily confused with those caused by the three other species of Phytophthora which also cause black pod and include: P. palmivora, P. capsici and P. citrophthora. P. palmivora is the dominant species on cocoa in Nigeria and Cameroon (Gregory and Maddison, 1981; Zentmeyer, 1987).

Black pod caused by P. megakarya and P. palmivora can be distinguished because P. megakarya produces lesions with irregular edges on the fruit whereas lesions caused by P. palmivora have regular borders and are generally smaller (Erwin and Ribeiro, 1996). Pods are susceptible at all stages of development and may be infected at any place on the surface. The first symptom is a brown to black spot on the pod, which spreads rapidly in all directions and eventually covers the whole pod. The beans become infected internally about 15 days after the initial infection and are soon of no commercial value.

Generally, pods closest to the ground are first infected, with the disease rapidly spreading to affect fruit on the entire tree. P. megakarya can also cause seedling blight and trunk cankers (Zentmeyer, 1987), but its capacity to cause root rot is equivocal. Luz and Mitchell (1994) reported that even at high inoculum levels P. megakarya caused little damage to roots and no seedling mortality. Despreaux et al. (1987) also indicated that P. megakarya is not pathogenic to cocoa roots. Gregory et al. (1984), however, stated that P. megakarya is primarily a root-infecting pathogen.

Prevention and control

Introduction

Control of P. megakarya revolves around three strategies; cultural methods, chemical control and disease resistance. At present, control relies upon cultural methods.

Cultural Control

Inoculum levels of P. megakarya are rapidly reduced in the absence of the host, and cocoa is the only known host. This affords opportunities to limit the spread through ensuring that disease-free nursery material is planted when clonal material is used for propagation. Spread can also be restricted by surface disinfestation of harvesting implements before moving from one tree to another.

Improved control is also obtained by avoiding bare earth (thus reducing spore splash) within the plantation (Waller and Holderness, 1997). Management of the amount of light entering the canopy is also critical, to ensure improved aeration and to promote the drying of the pod surface. Planting under thinned jungle is commonly employed in West Africa and, while cheap and simple, provides uneven shade which is difficult to regulate (Wood, 1975). Shade is critical in young trees to promote development of the most productive canopy shape. Clear felling of jungle, followed by planting of temporary and permanent shade trees, allows more effective regulation of light (Wood, 1975).

Other cultural control methods include improving sanitation by removing infected pods and pod husks. These need to be removed from the plantation to where they no longer provide an effective inoculum source. Also, ripe healthy pods should be regularly harvested, often daily (Thorold, 1959).

Soil tunnels built upon the trunk surface by ants are also responsible for moving inoculum of P. megakarya into the infection court (Gregory and Maddison, 1981). Sometimes tunnels are built onto the tops of pods as a shelter for tended mealy bugs, exacerbating the black pod disease problem. Ant management is a critical management issue.

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Impact

Black pod disease of cocoa in West Africa, caused almost exclusively by P. megakarya, still remains one of the most serious constraints on cocoa production. Surveys during the 1978 and 1979 harvest season in Togo revealed losses of up to 80%, when no control measures were taken (Djiekpor et al., 1981). Erwin and Ribeiro (1996) estimated a 20-30% loss of the world's cocoa crop to black pod, and in some areas they estimated that 90-95% of the crop is rendered unusable.