Phytophthora megakarya (black pod of cocoa)
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Seedborne Aspects
- Plant Trade
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Phytophthora megakarya Brasier & M.J. Griffin
Preferred Common Name
- black pod of cocoa
International Common Names
- English: seedling blight of cocoa; trunk canker of cocoa
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Chromista
- Phylum: Oomycota
- Class: Oomycetes
- Order: Peronosporales
- Family: Peronosporaceae
- Genus: Phytophthora
- Species: Phytophthora megakarya
Notes on Taxonomy and NomenclatureTop of page Phytophthora megakarya was first described by Brasier and Griffin (1979). It had been previously identified as Phytophthora palmivora, but can now be readily distinguished from P. palmivora on several grounds, namely: large nuclei of the gametangia containing 5-6 large chromosomes, compared with P. palmivora with 9-12 much smaller chromosomes (Sansome et al., 1975, 1979); caducous sporangia, with medium-length stalks (10-30 µm), versus P. palmivora with short (1-5 µm) stalks and P. capsici with long stalks (30->200 µm) (Zentmeyer, 1987); different RFLP, protein and isoenzyme patterns (Erselius and Shaw, 1982; Forster et al., 1990; Forster and Coffey, 1991).
A fourth species with papillate sporangia also causes disease in cocoa (Phytophthora citrophthora); however, this species has persistent sporangia and can thus be readily distinguished from the other species.
Lee and Taylor (1992) used rDNA variation to study evolutionary relationships among these species. The ITS I and ITS II regions showed low or undetectable intraspecific variability. However, interspecific nucleotide difference was 0.3-14.6%. A common lineage was proposed for P. palmivora and P. megakarya, and P. capsici and P. citrophthora also appear to show a close relationship.
DescriptionTop of page (From Brasier and Griffin, 1979).
P. megakarya is a member of Group II of Phytophthora, as defined by Stamps et al. (1990). It produces caducous papillate sporangia, is heterothallic and antheridia are amphigynous.
Sporangia are limoniform, obpyriform or ellipsoid with rounded bases, varying from 20-60 x 13-41 µm, with a length-breadth ratio of 1.2-1.6, and are formed in a sympodium. Sporangia are caducous, with pedicels ranging from 10-30 µm long.
Oogonia are produced in paired cultures of A1 and A2 compatibility types only. The A1 compatibility type is most frequently isolated. Oogonia range in size from 19-37 (av. 27) µm, and taper to a funnel-shaped base at the oogonial stalk. Antheridia are amphigymous, spherical, averaging 13 µm long. Oospores are plerotic 23-28 µm diam. with a wall thickness ranging from 1.5-3 µm.
DistributionTop of page P. megakarya appears to be confined to West Africa (Zentmeyer, 1987), where it is the most common species of Phytophthora causing black pod disease. Theobroma cacao (cocoa) is indigenous to the New World and ranges from southern Mexico in the north to Brazil and Bolivia in the south. The centre of origin is considered to be the basin of the Upper Amazon (Wood, 1975). This would indicate that if P. megakarya is only present in West Africa, it must have another host that is as yet undetermined and cocoa is a new host. Alternatively, P. megakarya may have co-evolved with cocoa in South America, and subsequently been introduced to West Africa. No records appear to exist for P. megakarya outside West Africa and the only host on which it has been recorded is cocoa.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Cameroon||Widespread||Despreaux et al., 1987|
|Côte d'Ivoire||Present||Kebe et al., 2002|
|Equatorial Guinea||Widespread||Zentmeyer, 1987|
|Guinea||Widespread||Oudemans and Coffey, 1991|
|Togo||Widespread||Djiekpor et al., 1981|
Hosts/Species AffectedTop of page There do not appear to be any hosts other than cocoa (Theobroma cacao) for P. megakarya. Despreaux et al. (1987) reported that P. megakarya is not saprophytically active in the absence of the host. They found the infection potential of the soil decreased rapidly with time outside epidemic periods.
Host Plants and Other Plants AffectedTop of page
|Ananas comosus (pineapple)||Bromeliaceae||Habitat/association|
|Carica papaya (pawpaw)||Caricaceae||Habitat/association|
|Colocasia esculenta (taro)||Araceae||Habitat/association|
|Elaeis guineensis (African oil palm)||Arecaceae||Habitat/association|
|Mangifera indica (mango)||Anacardiaceae||Habitat/association|
|Musa x paradisiaca (plantain)||Musaceae||Habitat/association|
|Persea americana (avocado)||Lauraceae||Habitat/association|
|Theobroma cacao (cocoa)||Malvaceae||Main|
|Xanthosoma sagittifolium (elephant ear)||Araceae||Habitat/association|
Growth StagesTop of page Flowering stage, Fruiting stage, Pre-emergence, Seedling stage, Vegetative growing stage
SymptomsTop of page 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.
List of Symptoms/SignsTop of page
|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|
Biology and EcologyTop of page The capacity of P. megakarya to cause root infections is equivocal. Gregory et al. (1984) considered that root infection maintained a reservoir of inoculum, allowing zoospores to be released into the soil surface water. From there, the zoospores were spread up the plant by small splash droplets in convection currents into the leaf canopy. On pods, the first macroscopic sign of infection is observed about 2 days after initial infection, and is manifested as a minute translucent spot on the pod surface. Insects, particularly the small black ant (Crematogaster striatula), are responsible for moving inoculum from the soil to the canopy (Evans, 1973). These ants also use old infected pods to construct tents around the pod peduncle, and this can lead to infection from the peduncle region.
Despreaux et al. (1987) reported that the inoculum potential of soil declined rapidly with time outside epidemic periods, principally because it had low saprophytic capability and was not pathogenic on cocoa roots. They reported that the fungus could survive, however, between the bark and sapwood on cocoa stems for several months.
The fungus is dispersed by caducous sporangia, and the disease is clearly polycyclic. Sporangia form on the surface of infected pods at relative humidities in the range 60-80% RH and temperatures between 20-30°C (Gregory, 1969). Sporangia can germinate directly via a germ tube, or indirectly to release about 30 zoospores.
Wet, showery conditions are essential for infection and spread. Wood (1974) has shown that long periods of relative humidity at saturation point are required for the rapid spread of disease. The theory that relative humidity is the most important climatic factor helps to explain the higher incidence in Nigeria than in Ghana, and the almost complete absence of black pod disease in Malaysia.
The role of sexuality in nature in producing oospores is unclear. Erselius and Shaw (1982) found a low fecundity of A1 x A2 P. megakarya crosses, in terms of both number of well-formed oospores produced and percentage germination. This is surprising for intraspecific crosses. In nature, the A1 compatibility type is most common, whereas in the work of Erselius and Shaw (1982), the A2 compatibility type was most commonly generated in the laboratory crosses.
Seedborne AspectsTop of page Although P. megakarya infects the pods and seeds, it is unlikely to be seedborne. Richardson (1979) reports one equivocal record for P. palmivora being seedborne in cocoa. Infected seeds would not germinate, because they have been made non-viable, thus seed transmission through this means would be unlikely. Cocoa is also propagated clonally from rooted cuttings (Wood, 1975), and P. megakarya could be transmitted through the use of infested potting mix and/or water in the nursery, resulting in infected planting material. Nursery hygiene is essential to ensure disease-free planting material is used.
Since P. megakarya appears to be confined to West Africa, it is essential to implement strategies to limit its spread, particularly in nursery material.
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Bark||hyphae; spores||Yes||Pest or symptoms usually visible to the naked eye|
|Fruits (inc. pods)||hyphae; spores||Yes||Yes||Pest or symptoms usually visible to the naked eye|
|Leaves||hyphae; spores||Yes||Pest or symptoms usually invisible|
|Roots||hyphae; spores||Yes||Yes||Pest or symptoms usually invisible|
|Stems (above ground)/Shoots/Trunks/Branches||hyphae; spores||Yes||Yes||Pest or symptoms usually visible to the naked eye|
|Wood||hyphae; spores||Yes||Yes||Pest or symptoms usually visible to the naked eye|
|Plant parts not known to carry the pest in trade/transport|
ImpactTop of page 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.
DiagnosisTop of page P. megakarya can be readily isolated from diseased pods through plating onto V-8 agar medium containing antibiotics such as P10VP medium (Eckert and Tsao, 1962; Tsao, 1983). Tissue for plating should be selected from the advancing margin of the lesion. If the fungus is sporulating on the surface of the pod, sporangia can be picked off with fine forceps and plated directly.
Detection and InspectionTop of page The disease is readily recognized through the presence of a brown to black spot on the pod, which eventually spreads to encompass the entire pod. Under conditions of high humidity, a white bloom comprising fungal mycelia and sporangia may be present on the surface. In advanced stages, the fungus invades the internal tissues of the pod, including the seeds, causing shrivelling. Diseased pods eventually mummify.
Similarities to Other Species/ConditionsTop of page Four papillate Phytophthora species are commonly isolated from cocoa: P. palmivora, P. megakarya, P. capsici and P. citrophthora. Erwin and Ribeiro (1996) provide a direct comparison of the morphological features of each of these except P. citrophthora. This species can be distinguished from the other three through having persistent sporangia. Sporangial pedicels of P. palmivora are short (2-5 µm), those of P. megakarya intermediate (10-30 µm) and those of P. capsici long (40-150 µm). The karyotype of P. megakarya also is n=5-6 large chromosomes, whereas P. capsici and P. palmivora have n=9-12 small chromosomes. Blaha (1983) suggested that P. megakarya could be readily separated from P. palmivora because P. palmivora is stimulated by white light, whereas vegetative growth of P. megakarya is inhibited. The differences were even more marked after exposure to green light.
Prevention and ControlTop of page Introduction
Control of P. megakarya revolves around three strategies; cultural methods, chemical control and disease resistance. At present, control relies upon cultural methods.
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.
Fungicides have been extensively used for black pod control. In Nigeria, commercial applications began in 1953, using carbide Bordeaux mixture (Thorold, 1953). Copper derivatives are still commonly used in West Africa, and in Zambia and Zaire (Mabbett, 1997). In Togo, the use of metalaxyl and red copper oxide reduced losses from 80% to 3% and 19%, respectively (Djiekpor et al., 1981). These workers proposed integrated control based on cultural and chemical treatments, plus the use of resistant planting material.
Because at least four different species of Phytophthora cause black pod disease, control through the use of disease resistance is likely to be more challenging than when only one species is involved. It has been recognized that Amelonado types of cocoa are more resistant to black pod diseases than Amazon, Trinitario and Criollo types (Wood, 1975). Breeding for resistance is being actively pursued. Nyasee et al. (1995) advocated the use of a leaf disc inoculation method for testing the disease reactions of prospective parents in a breeding programme. Cocoa clones tested for resistance to black pod with leaf bioassays responded in the same rank order as those reported for fruit inoculation. Blaha (1987) indicated the potential usefulness of esterase bands as markers for resistance. More recent work has focused on the use of DNA markers for identifying black pod resistant phenotypes (Despreaux and Eskes, 1997). These workers found a poor correlation between reactions of clones in the field and those obtained with leaf disc inoculation. They generated a saturated linkage map, and identified two quantitative trait loci (QTLs) in the parents that accounted for 46% of the total variation. These results indicate that progress can be made through breeding to improve black pod resistance.
Berry and Cilas (1994) also reported differences in parental reactions to P. megakarya following laboratory and field evaluation. They found general combining abilities were the only significant factors influencing yield, and the superiority of one parent, UPA 134, was confirmed.
These results indicated that natural variability does exist for reaction to P. megakarya in cocoa, which can be effectively exploited in breeding programmes.
ReferencesTop of page
Blaha G, 1983. Effect of light on Phytophthora palmivora and Phytophthora megakarya, agents causing brown rot on cocoa pods. Café Cacao Thé (Paris), 27:91-112.
Blaha G, 1987. Enzymatic polymorphism of the Phytophthora causal agents of black pod disease: research into variability linked to host-parasite interaction in cocoa. Proceedings of the Tenth International Cocoa Research Conference, Santo Domingo, Dominican Republic, 17-23 May 1987. London, UK: Cocoa Producers' Alliance, 397-406
Despreaux D; Cambrony D; Clement D; Nyasse S; Partiot M, 1987. Study of cocoa black pod in Cameroon: description of new control methods. Proceedings of the Tenth International Cocoa Research Conference, Santo Domingo, Dominican Republic, 17-23 May 1987. London, UK: Cocoa Producers' Alliance, 407-412
Eckert JW; Tsao PH, 1962. A selective antibiotic medium for isolation of Phytophthora and Pythium from plant roots. Phytopathology, 52:771-777.
Erselius LJ; Shaw DS, 1982. Protein and enzyme differences between Phytophthora palmivora and P. megakarya: evidence for self-fertilization in pairings of the two species. Transactions of the British Mycological Society, 78(2):227-238
Erwin DC; Ribeiro OK, 1996. Phytophthora Diseases Worldwide. St Paul, Minnesota, USA: American Phytopathological Society Press.
Forster H; Coffey MD, 1991. Approaches to the taxonomy of Phytophthora using polymorphisms in mitochondrial and nuclear DNA. In: Lucas JA, Shattock RC, Shaw DS, Cooke LR, eds. Phytophthora. Cambridge, UK: Cambridge University Press.
Gregory PH, 1969. Black pod disease project report. London, UK: Cocoa, Chocolate and Confectionery Alliance.
Gregory PH, 1972. Black Pod Disease Project Report. London, UK: Cocoa Chocolate and Confectionary Alliance.
Lee SB; Taylor JW, 1992. Phylogeny of five fungus-like protoctistan Phytophthora species, inferred from the internal transcribed spacers of ribosomal DNA. Molecular Biology and Evolution, 9(4):636-653
NyaseT S; Cilas C; Herail C; Blaha G, 1995. Leaf inoculation as an early screening test for cocoa (Theobroma cacao L.) resistance to Phytophthora black pod disease. Crop Protection, 14(8):657-663; 16 ref.
Thorold CA, 1953. The control of black pod disease of cocoa in the western region of Nigeria. Report of the Cocoa Conference, 108-115.
Thorold CA, 1959. Methods of controlling black pod disease (caused by Phytophthora palmivora) of Theobroma cacao in Nigeria. Annals Applied Biology, 46:225-237.
Tsao PH, 1983. Factors affecting isolation and quantitation of Phytophthora from soil. In: Erwin DC, Bartnicki-Garcia S, Tsao PH, eds. Phytophthora: Its Biology, Taxonomy, Ecology and Pathology. St Paul, Minnesota, USA: American Phytopathological Society, 219-236.
Wood GAR, 1974. Black pod meteorological factors. In: Gregory PH, ed. Phytophthora Diseases of Cocoa. London, UK: Longman.
Zentmyer GA, 1987. Taxonomic relationships and distribution of species of Phytophthora causing black pod of cocoa. Proceedings of the Tenth International Cocoa Research Conference, Santo Domingo, Dominican Republic, 17-23 May 1987. London, UK: Cocoa Producers' Alliance, 391-395.
Distribution MapsTop of page
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