Phytophthora megakarya

Phytophthora megakarya
Scientific classification
Domain:	Eukaryota
(unranked):	SAR
Superphylum:	Heterokonta
Class:	Oomycetes
Subclass:	Incertae sedis
Order:	Pythiales
Family:	Pythiaceae
Genus:	Phytophthora
Species:	P. megakarya
Binomial name
Phytophthora megakarya Brasier & M.J. Griffin (1979)

Phytophthora megakarya is a plant pathogen that causes black pod disease in cocoa trees in West and Central Africa. This pathogen can cause detrimental loss of yield in the economically important cocoa industry. It can damage any part of the tree, causing a wide range in the reduction of yield. A mixture of chemical and cultural controls are often necessary to control this pathogen.

Phytophthora megakarya's only known host is Theobroma cacao, or the cocoa tree, located in West and Central Africa. This pathogen causes Black pod disease which produces an array of symptoms throughout the host’s life cycle. P. megakarya is a facultative parasite that can infect any part of the cacao tree at any time under optimal environmental conditions. Trees as young as seedlings may show symptoms of infection such as blight or root rot. Bark wounds, called cankers, can form on stems and branches as a result of disease. Infected cocoa pods rot and turn black, providing insight on the origin of the disease name. Other symptoms that may occur include damping off, dieback, lesions, mummification, premature drop, soft rot, and shriveling. The process of infection by P. megakarya is quite rapid. Small translucent spots on the pod can be seen 2 to 3 days after infection. Whitish spores can be seen growing on these spots 3 to 5 days following their emergence. The spots will grow and darken in color until eventually the entire pod turns black and becomes mummified. In more advanced stages, the pathogen may take over the internal tissues and cause the cocoa beans to become warped. P. megakarya also readily forms stem cankers that are usually confined to the lower part of the tree, but may be present anywhere on the tree. Canker lesions may extend beneath the soil surface, providing a source of primary inoculum.P. megakarya can be distinguished from other Phytophthora species by its production of gametangia. These gametangia have large nuclei containing five to six chromosomes and sporangia growing off of medium-length stalks.

P. megakarya is an oomycete that has a polycyclic disease cycle, producing three asexual spore types: sporangia, zoospores, and chlamydospores. Although it is rare, P. megakarya can also produce sexual oospores through heterothallic mating which requires two different mating types; so far none have been observed. Mycelium plays an important role in the infection of the cocoa trees; mycelium found in the soil and in cankers on the bark develops into sporangia, which can then germinate. Zoospores are produced from these sporangia as secondary inoculum. They may infect the plant either directly or indirectly depending on the availability of water. A direct infection by the zoospore results in the production of more mycelia, which may develop into sporangia capable of releasing more inoculum or chlamydospores. Chlamydospores serve as survival structures for P. megakarya, in some instances surviving as long as 18 months. An indirect infection results in the formation of encysted spores in the absence of water; mycelium production occurs after germination of these spores.P. megakarya depends heavily on the correct environmental conditions to cause disease. Primary infections usually occur in June, however, disease peaks between August and October. Under humid conditions a single pod may produce up to 4 million sporangia. These sporangia can be dispersed by rain, movement of planting materials, insects, rodents, and contaminated harvesting tools.

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