Disease
The main disease caused by P. megakarya is black pod disease of cacao. P. megakarya is primarily known for the damage it causes to the cacao pod, but it is known to subsist on roots (Gregory et al., 1984) and can cause dieback of seedlings (Bowers et al., 2001). Since all Phytophthora spp. causing black pod disease of cacao in Africa were classified as P. palmivora prior to 1979 (Brasier and Griffin, 1979), it is unknown when P. megakarya was first “discovered.” The center of origin is believed to be primary forests of Central and West Africa (Nyasse et al., 1999). To date, P. megakarya has only been reported in this geographical region.
Symptoms of black pod disease on cacao.
The majority of cacao is grown on small parcels of land within a forested setting; impact on cacao production can result in losses of 50-80% in nontreated areas (Despréaux, 2004). The disease spreads primarily through dissemination of sporangia via rain and wind, insects, and human transmission. Treatment with fungicides is the recommended management technique (Opoku et al., 2007). Cultural practices, such as tree pruning, cleaning equipment during harvest, and keeping cull piles away from the trees, are known to reduce the spread of this pathogen (Akrofi et al., 2003; Ndoumbe-Nkeng et al., 2004; Opoku et al., 2006).
Black pod of cacao caused by P. megakarya and P. palmivora can be distinguished in the field 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.
The only economic host of importance is Theobroma cacao. P. megakarya primarily causes a pod rot but is also known to cause cankers and seedling blight. This species is only known in West and Central Africa. Several other forest species are known to become infected and to maintain the pathogen at low levels but the impact on black pod disease itself is not fully understood.
| Host Latin Name | Host Common Name | Symptoms | Habitat | Region | |
|---|---|---|---|---|---|
| Cola nitida | Cola tree | Fruit rot | Forest | Africa | |
| Dracaena mannii | Asparagus tree, Soap tree | None - found on roots | Forest | Ghana - Africa | |
| Funtumia elastica | Silkrubber | None - found on roots | Forest | Africa | |
| Irvingia sp. | African mango | Fruit rot | Forest | Africa | |
| Ricinodendron heudelotii | African nut tree | None - found on roots | Forest | Africa | |
| Sterculia tragacantha | African tragacanth | None - found on roots | Forest | Africa | |
| Theobroma cacao | Cacao | Blight, Canker, Fruit rot | Agricultural setting, Forest | Africa, West Africa |