Phytophthora megakarya

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Phytophthora megakarya
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Clade: Diaphoretickes
Clade: SAR
Clade: Stramenopiles
Phylum: Oomycota
Order: Peronosporales
Family: Peronosporaceae
Genus: Phytophthora
Species:
P. megakarya
Binomial name
Phytophthora megakarya

Phytophthora megakarya is an oomycete plant pathogen that causes black pod disease in cocoa trees in west and central Africa. [1] This pathogen can cause detrimental loss of yield in the economically important cocoa industry, worth approximately $70 billion annually. [2] It can damage any part of the tree, causing total yield losses which can easily reach 20-25%. [3] A mixture of chemical and cultural controls, as well as choosing resistant plant varieties, are often necessary to control this pathogen.

Contents

Hosts and symptoms

Phytophthora megakarya's only known host is Theobroma cacao , or the cocoa tree, located in West and Central Africa. It is considered to be the most virulent species of Phytophthora which infects T. cacao, causing the greatest percentage of yield loss. [3] 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. [4] Bark wounds, called cankers, can form on stems and branches as a result of disease. [4] Infected cocoa pods rot and turn black, providing insight on the origin of the disease name. [4] Other symptoms that may occur include damping off, dieback, lesions, mummification, premature drop, soft rot, and shriveling. [5] 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. [4] Whitish spores can be seen growing on these spots 3 to 5 days following their emergence. [4] The spots will grow and darken in color until eventually the entire pod turns black and becomes mummified. [6] 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. [1]

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. [7]

Disease cycle

P. megakarya is an oomycete that has a polycyclic disease cycle, producing three asexual spore types: sporangia, zoospores, and chlamydospores. [4] 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. [7] 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. [4] Zoospores are produced from these sporangia as secondary inoculum. They may infect the plant either directly or indirectly depending on the availability of water. [4] 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. [4] Chlamydospores serve as survival structures for P. megakarya, in some instances surviving as long as 18 months. [4] An indirect infection results in the formation of encysted spores in the absence of water; mycelium production occurs after germination of these spores. [4]

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. [4] Under humid conditions a single pod may produce up to 4 million sporangia. [4] These sporangia can be dispersed by rain, movement of planting materials, insects, rodents, and contaminated harvesting tools. [4]

Pathogenesis

Not much is known about the pathogenesis of P. megakarya. Like all oomycetes, zoospores produced by P. megakarya need free water on plant surfaces in order to encyst, germinate, and penetrate host tissues. [6] For the pathogen to enter a plant cell, the effector protein of the pathogen must attach itself to the binding protein of the plant, thereby getting carried into the cell. [8] A germ tube is formed during germination of the zoospore which typically gives rise to an appressorium. [9] These structures penetrate the epidermal cells of the plant’s tissue and form haustoria. Haustoria invade the plant intracellularly to retrieve nutrients while further dispersing the pathogen within the host. [9] Chlamydospores that survive in the soil produce mycelia that can also infect plant structures. [4] Infections of stems and branches lead to the formation of cankers while infections on cocoa pods cause pod rot. [4] The development of cankers has also been associated with insects that burrow into the bark of cocoa trees. [6]

Environment

Phytophthora spp. occur wherever cocoa is grown and is the most economically detrimental pathogen of cocoa in West Africa where the two species P. megakarya and P. palmivora dominate. [4] [10] West Africa’s environment is characterized by a dry season from November until February. [11] During this time, moisture-laden air from the equator moves in, providing ideal moisture conditions for the growth of both cocoa trees and P. megakarya. [11] Cocoa is known to grow well in countries that occur in the subhumid zone, primarily in the forested areas of the countries. [11] The forested regions of West Africa near the equator receive 1500mm to 2000mm of rain; cocoa is planted in these areas where the forest is cleared. [11] Since P. megakarya favors wet conditions, these forested regions are ideal during the wet seasons. [4] P. megakarya survives in the soil during elongated dry periods. [10] When the conditions are right, zoospores swim toward the surface of the soil where a fine aerosol is produced in the presence of water to transport the spores to the pods. [11] In comparison to P. palmivora, P. megakarya is able to produce greater quantities of inoculum more quickly and can distribute it earlier in the season; thus it may infect more pods in a shorter time than P. palmivora. [1]

Management

Chemical control

Chemical control is often necessary to conserve yield, although it can come at a very high price. [12] Chemical control targets the initial inoculum, reducing the amount of mycelium and sporangia present. In west Africa chemicals are applied using spray tanks. [1] Fungicides such as copper oxychloride, Mancozeb, and Metalaxyl+Mancozeb are typically used. [13] Mixtures of these chemicals can be used to prevent the pathogen from acquiring resistance to certain chemical compounds. [13] Many of these fungicides contain heavy metals which can be damaging to the environment. [12] Spraying is recommended 4-6 times a year from the last week of May to the end of the growing period. [13] [12] Overuse of chemicals will result in increasing amounts of heavy metals being absorbed into the environment. [14]

Cultural control

Cultural practices not only reduce the incidence of disease but also allow for better use of chemical control. [1] If the disease is not causing much damage, cultural control alone may be sufficient. [13] Removal of infected plant parts reduces sporangial inoculum and is an effective cultural control. Weeding reduces the humidity of the surrounding air which is unfavorable for the production of sporangia. Frequently harvesting the pods reduces the amount of sporangial inoculum. [1] Another form of cultural control is the burning of the pod husk piles to destroy additional sources of chlamydospores and sporangia. [1] These cultural practices can limit disease caused by P. megakarya significantly.

Biological control

In certain regions of Africa, Trichoderma asperellum is used as a biological control agent. [15] It is applied as a wettable powder, containing cassava flour and T. asperellum. [15] Though this method has shown positive results, the formulation is not practical for such a wet, tropical region. [15] Research is being done into examining different formulations that can more effectively be applied. [15]

Genetic resistance

Choosing a resistant variety of plant is an effective method of minimizing the damage of P. megakarya. Breeding for resistance for either P. palmivora or P. megakarya will increase the plant's resistance to both pathogens simultaneously. [1] Much of the current research into P. megakarya and black pod disease is looking at the cellular processes involved in infection. [8] Through the examination and understanding of these processes, researchers can alter the genetic makeup of T. cacao trees in hopes of more effectively controlling the spread of black pod disease. [8] Growing a cocoa variety with resistance can also increase the effectiveness of chemical applications. [1] Resistance will also reduce the quantity of infectious plants, thereby reducing the amount of cultural control required. [8]

Importance

Phytophthora megakarya is the most important cocoa pathogen in central and west Africa. It is endemic to Cameroon, Nigeria, and Ghana and is present as an invasive pathogen in Côte d'Ivoire. [16] These countries account for four of the top ten world producers of cocoa, Côte d'Ivoire being the number one producer worldwide. [17] In some cases, when left untreated, the pathogen has led to an 80% loss of cocoa pods. [18] However, the infection may not always be localized on the pods. Cankers formed on the bark of the cocoa trees lead to a reduction of tree vigor and total yield, and in extreme cases, result in a 10% loss of trees annually. [16] As the trees age pod production decreases; warnings of chocolate shortages as soon as 2020 have been predicted based on the combination of these factors. [19]

Related Research Articles

<i>Phytophthora</i> Genus of single-celled organisms

Phytophthora is a genus of plant-damaging oomycetes, whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems. The cell wall of Phytophthora is made up of cellulose. The genus was first described by Heinrich Anton de Bary in 1875. Approximately 210 species have been described, although 100–500 undiscovered Phytophthora species are estimated to exist.

<i>Phytophthora cinnamomi</i> Species of single-celled organism

Phytophthora cinnamomi, also known as cinnamon fungus, is a soil-borne water mould that produces an infection which causes a condition in plants variously called "dieback", "root rot", or, "ink disease".

Phytophthora sojae is an oomycete and a soil-borne plant pathogen that causes stem and root rot of soybean. This is a prevalent disease in most soybean growing regions, and a major cause of crop loss. In wet conditions the pathogen produces zoospores that move in water and are attracted to soybean roots. Zoospores can attach to roots, germinate, and infect the plant tissues. Diseased roots develop lesions that may spread up the stem and eventually kill the entire plant. Phytophthora sojae also produces oospores that can remain dormant in the soil over the winter, or longer, and germinate when conditions are favourable. Oospores may also be spread by animals or machinery.

<i>Phytophthora palmivora</i> Species of single-celled organism

Phytophthora palmivora is an oomycete that causes bud-rot of palms, fruit-rot or kole-roga of coconut and areca nut. These are among the most serious diseases caused by fungi and moulds in South India. It occurs almost every year in Malnad, Mysore, North & South Kanara, Malabar and other areas. Similar diseases of palms are also known to occur in Sri Lanka, Mauritius, and Sumatra. The causative organism was first identified as P. palmivora by Edwin John Butler in 1917.

<i>Phytophthora cactorum</i> Species of single-celled organism

Phytophthora cactorum is a fungal-like plant pathogen belonging to the Oomycota phylum. It is the causal agent of root rot on rhododendron and many other species, as well as leather rot of strawberries.

Phytophthora citrophthora, also known as brown rot of citrus, is a soil borne oomycete that infects several economically important citrus crops. A diagnostic symptom of P. citrophthora is gummosis, wherein lesions around the base of the tree exude sap. Other common symptoms include dark longitudinal lesions forming at the soil line, a sour smell, and eventual cracking of the bark. Advanced symptoms include yellowing and necrosis of the tree canopy. Girdling action caused by the pathogen around the trunk can often cause the collapse of the tree. Resistant lemon varieties have been developed and their implementation has been effective at controlling the spread of the disease. Fruits that have been infected with P. citrophthora exhibit symptoms of brown rot characterized by a distinct odor. This disease is most active in the moderate temperatures of spring, fall, and winter months, opposite of most other Phytophthora species.

<i>Phytophthora medicaginis</i> Species of single-celled organism

Phytophthora medicaginis is an oomycete plant pathogen that causes root rot in alfalfa and chickpea. It is a major disease of these plants and is found wherever they are grown. P. medicaginis causes failure of stand establishment because of seedling death. Phytophthora medicaginis is part of a species complex with Phytophthora megasperma.

Phytophthora nicotianae or black shank is an oomycete belonging to the order Peronosporales and family Peronosporaceae.

Pythium irregulare is a soil borne oomycete plant pathogen. Oomycetes, also known as "water molds", are fungal-like protists. They are fungal-like because of their similar life cycles, but differ in that the resting stage is diploid, they have coenocytic hyphae, a larger genome, cellulose in their cell walls instead of chitin, and contain zoospores and oospores.

<i>Phytophthora erythroseptica</i> Species of single-celled organism

Phytophthora erythroseptica—also known as pink rot along with several other species of Phytophthora—is a plant pathogen. It infects potatoes causing their tubers to turn pink and damages leaves. It also infects tulips (Tulipa) damaging their leaves and shoots.

<i>Phytophthora capsici</i> Species of single-celled organism

Phytophthora capsici is an oomycete plant pathogen that causes blight and fruit rot of peppers and other important commercial crops. It was first described by L. Leonian at the New Mexico State University Agricultural Experiment Station in Las Cruces in 1922 on a crop of chili peppers. In 1967, a study by M. M. Satour and E. E. Butler found 45 species of cultivated plants and weeds susceptible to P. capsici In Greek, Phytophthora capsici means "plant destroyer of capsicums". P. capsici has a wide range of hosts including members of the families Solanaceae and Cucurbitaceae as well as Fabaceae.

Phytophthora fragariae is a fungus-like (oomycete) plant pathogen that causes red stele, otherwise known as Lanarkshire disease, in strawberries. Symptoms of red stele can include a red core in the roots, wilting of leaves, reduced flowering, stunting, and bitter fruit. The pathogen is spread via zoospores swimming through water present in the soil, released from sporangia.

<i>Phytophthora kernoviae</i> Species of oomycete

Phytophthora kernoviae is a plant pathogen that mainly infects European beech and Rhododendron ponticum. It was first identified in 2003 in Cornwall, UK when scientists were surveying for the presence of Phytophthora ramorum. This made it the third new Phytophthora species to be found in the UK in a decade. It was named Phytophthora kernoviae after the ancient name for Cornwall, Kernow. It causes large stem lesions on beech and necrosis of stems and leaves of Rhododendron ponticum. It is self-fertile. It has also been isolated from Quercus robur and Liriodendron tulipifera. The original paper describing the species, stated it can infect Magnolia and Camellia species, Pieris formosa, Gevuina avellana, Michelia doltsopa and Quercus ilex. Since then many other plants have been identified as natural hosts of the pathogen. Molecular analysis has revealed that an infection on Pinus radiata, recorded in New Zealand in 1950, was caused by P. kernoviae. The pathogen was also noted on Drimys winteri, Gevuina avellana, Ilex aquifolium, Quercus ilex, Vaccinium myrtillus, Hedera helix, Podocarpus salignas.

<i>Phytophthora plurivora</i> Species of single-celled organism

Phytophthora plurivora is a very aggressive soil-borne plant pathogen, with worldwide distribution and a wide variety of hosts.

Phytophthora quercina is a papillate homothallic soil-borne plant pathogen causing root rot of oak tree species in Europe. It is associated with necrotic fine roots.

Black pod disease is a fungal disease of Cocoa trees. This pathogen if left untreated can destroy all yields; annually the pathogen can cause a yield loss of up to 1/3 and up to 10% of total trees can be lost completely.

Buckeye rot of tomato is caused by three species of pathogens in the genus Phytophthora: P. nicotianae var. parasitica, P. capsici, and P. drechsleri. It is an oomycete that thrives in warm, wet conditions and lives in the soil. It is characterized by a bull’s eye pattern of dark brown rotting on the tomato fruit, and affects fruit that is close to, or lying on the soil. The easiest management is to keep the plant out of contact with the soil, although other chemical methods can be very effective. This disease commonly occurs in the southeast and south central areas of the United States. The disease has affected a large portion of crop yield in the United States as well as India. The relatively small genome size of Phytophthora parasitica compared to Phytophthora infestans gives researchers the unique ability to further examine its ability to cause disease.

Black rot on orchids is caused by Pythium and Phytophthora species. Black rot targets a variety of orchids but Cattleya orchids are especially susceptible. Pythium ultimum and Phytophthora cactorum are known to cause black rot in orchids.

Cranberry Root Rot (CRR) is a disease in cranberries that can cause a decline in yield.

References

  1. 1 2 3 4 5 6 7 8 9 Opoku, IY; Appiah, AA; Akrofi, AY; Owusu, GK (2000). "Phytophthora megakarya: A potential threat to the cocoa industry in Ghana". Ghana Journal of Agricultural Science. 33 (2). doi:10.4314/gjas.v33i2.1876.
  2. ↑ Ploetz, Randy C. (December 2007). "Cacao diseases: important threats to chocolate production worldwide". Phytopathology. 97 (12): 1634–1639. doi: 10.1094/PHYTO-97-12-1634 . ISSN   0031-949X. PMID   18943724.
  3. 1 2 Evans, Harry C. (December 2007). "Cacao diseases-the trilogy revisited". Phytopathology. 97 (12): 1640–1643. doi:10.1094/PHYTO-97-12-1640. ISSN   0031-949X. PMID   18943725.
  4. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Akrofi, A. Y. (2015). "Phytophthora megakarya: A Review on its Status as a Pathogen on Cacao in West Africa". African Crop Science Journal.
  5. ↑ Phytophthora megakarya (Black Pod of Cocoa). “Invasive Species Compendium”. CABI
  6. 1 2 3 M. A. Barreto et al. (2015). “Detection of genetic resistance to cocoa black pod disease caused by three Phytophthora species”. Springer Science+Business Media Dordrecht
  7. 1 2 3 4 Helliwell, Emily E.; Vega-Arreguín, Julio; Shi, Zi; Bailey, Bryan; Xiao, Shunyuan; Maximova, Siela N.; Tyler, Brett M.; Guiltinan, Mark J. (March 2016). "Enhanced resistance in Theobroma cacao against oomycete and fungal pathogens by secretion of phosphatidylinositol-3-phosphate-binding proteins". Plant Biotechnology Journal. 14 (3): 875–886. doi:10.1111/pbi.12436. hdl: 10919/97896 . ISSN   1467-7652. PMID   26214158. S2CID   25355847.
  8. 1 2 Latijnhouwers, Maita, Pierre J.G.M. De Wit, and Francine Govers. "Oomycetes and Fungi: Similar Weaponry to Attack Plants." Cell Press 11.10 (2003): 462-69.
  9. 1 2 Prabha, K Peter and R. Chandramohanan (2014). Integrated management of black pod disease caused by Phytophthora palmivora. Research article, International Journal of Plant Protection, Volume 7 issue 1.
  10. 1 2 3 4 5 Agriculture and Consumer Protection. Integrating crops and wildlife in West Africa. Chapter 1. http://www.fao.org/docrep/004/x6543e/x6543e01.htm
  11. 1 2 3 Akrofi, A.Y; Appiah, A.A; Opoku, I.Y (1 April 2003). "Management of Phytophthora pod rot disease on cocoa farms in Ghana". Crop Protection. 22 (3): 469–477. doi:10.1016/S0261-2194(02)00193-X. ISSN   0261-2194.
  12. 1 2 3 4 Prabha, K Peter and R. Chandramohanan (2014). Integrated management of black pod disease caused by Phytophthora palmivora. Research article, International Journal of Plant Protection, Volume 7 issue 1.
  13. ↑ Opoku, Isaac Y.; Akrofi, Andrews Y.; Appiah, Alex A. (1 February 2007). "Assessment of sanitation and fungicide application directed at cocoa tree trunks for the control of Phytophthora black pod infections in pods growing in the canopy". European Journal of Plant Pathology. 117 (2): 167. doi:10.1007/s10658-006-9082-8. ISSN   0929-1873. S2CID   6620069.
  14. 1 2 3 4 Mbarga, Joseph; Begoude, Didier; Ambang, Zachee; Meboma, M; Kuate, J; Schiffers, Bruno; Ewbank, William; Dedieu, Laurence; Ten Hoopen, Gerben (1 October 2014). "A new oil-based formulation of Trichoderma asperellum for the biological control of cacao black pod disease caused by Phytophthora megakarya". Biological Control. 77: 15–22. doi:10.1016/j.biocontrol.2014.06.004.
  15. 1 2 David Guest, (2006). “Black Pod: Diverse Pathogens with a Global Impact on Cocoa Yield” Cacao Diseases: Important Threats to Chocolate Production Worldwide
  16. ↑ "Top 10 Cocoa Producing Countries." WorldAtlas. 22 April 2015. Web. 22 October 2015.
  17. ↑ Djiekpor EK, Goka K, Lucas P, Partiot M, (1981). Cocoa black pod rot caused by Phytophthora sp. in Togo: assessment and control strategies. Cafe Cacao The, 25(4):263-268
  18. ↑ Ford, Tamasin, Jonathan Vit, Rupert Neate, Tania Branigan, and Emine Saner. "The Cocoa Crisis: Why the World's Stash of Chocolate Is Melting Away." The Guardian. 21 November 2014. Web. 21 October 2015.
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