Phytophthora megakarya

Overview

The only economically important host of Phytophthora megakarya Brasier & Griffin is Theobroma cacao. The pathogen is one of several Phytophthora species causing black pod disease of cacao. Isolates of P. megakarya were grouped taxonomically with P. palmivora isolates until 1979, when it was recognized as a new species (Brasier and Griffin, 1979). Found only in Central and West Africa, losses due to black pod disease caused by P. megakarya can be 50-80% in nontreated areas (Despréaux, 2004).

Etymology: Derived from ‘large’ (mega) ‘karyotype’ (karya) due to the larger chromosomes compared to P. palmivora.

Morphology

P. megakarya is heterothallic with amphigynous antheridia and distinctly pyriform oogonia. The caducous sporangia are papillate, varying in shape from limoniform, obpyriform, and ellipsoid. Terminal chlamydospores have an average diameter of 30 µm.

Genetics

P. megakarya is in phylogenetic clade 4, closely related to P. arecae, P. palmivora, P. quercina, and P. quercetorum (Balci et al., 2008).

http://phytophthora-id.org/ is a good web-based, searchable database for rapid identification of Phytophthora species based on sequencing of the internal transcribed spacer (ITS) or the cytochrome oxidase (cox) 1 and 2 spacer regions followed by BLAST searching the database.  Both databases are highly selective and include only sequences that are associated with published Phytophthora species descriptions or classic Phytophthora phylogenetics references.

Growth

Colonies on V8 agar grow in a stellate pattern with aerial mycelium; hyphae are coralloid. Growth on CMA is sparse with no aerial mycelium.

Distinguishing Characteristics for Identification

P. megakarya differs from P. palmivora morphologically by its narrow, medium length sporangium pedicels compared to broad, short, and occluded pedicels of P. palmivora and the pyriform, tapering downward oogonia compared to the spherical, narrow stalk oogonia of P. palmivora (Erwin and Ribeiro, 1996). P. quercetorum (Balci et al., 2008) and P. quercina (Jung et al., 1999), both in Clade 4, are homothallic in culture while P. megakarya is heterothallic.

Disease History

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.

Impacts in the Forest

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.

Forest and Wildland Hosts and Symptoms

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.

Educational Materials

• STCP: Farmer Training Guide on ICPM Cocoa
• Organic Cocoa Production: A guide for Farmer Field Schools in Sierra Leone
• African Organic Agriculture Training Manual 09-14