Potatoes, cacao and biodiversity: The impacts of Phytophthora in the African continent.
Phytophthora is a genus of oomycete (Straminipilous fungi), which occurs globally and has been responsible for severe diseases of a variety of crops (Erwin and Ribeiro 1996), commercial forestry (Linde et al. 1994) and to native habitats (Garbelotto et al. 2003; Shearer et al. 2004). Phytophthora has been the subject for a vast amount of studies but little research from the African continent is known. In the last decade the number of newly described Phytophthora spp. has more than doubled the number of known Phytophthora spp. (Brasier 2008), but of these only two species descriptions originated from Africa (Maseko et al. 2007).
Recently, it was proposed that the number of extant Phytophthora species might be several times more than the number of known and described species (Brasier 2008). This was deduced from comparing the number of described species at the end of the twentieth century with that predicted by fungal diversity estimations (Hawksworth and Rossman 1997; Crous et al. 2006), i.e. that only 10% of the world's total fungi are described. These fungal diversity estimations are linked to the biodiversity of vascular plants and given the host dependant nature of Phytophthora species and Africa's extensive plant biodiversity and suitable habitat; it is safe to assume that Africa harbours a large undiscovered Phytophthora species diversity.
Despite the large probability for numerous undescribed Phytophthora species to exist in Africa, very few species has been identified or described in Africa. This lack of species discovery in Africa can be attributed an absence of studies investigating species diversity, a focus on a single or limited number of economically important species and to lack of funding and trained researchers. There is a great need for comprehensive studies focussing on Phytophthora species diversity, especially in native habitats.
1.1 African species occurrence
Twenty species of Phytophthora have been reported from Africa, of which 18 species has been reported in South Africa (Table 1) and only six species are known from the rest of Africa. P. megakarya and P. palmivora are not known to occur in South Africa but in many other African countries these two species causes a serious disease of Cacao (Theobroma cacao). The species occurring in South Africa as well as the rest of the continent includes P. cactorum (Mes 1934; Boughalleb et al. 2006), P. cinnamomi (Wager 1941; Huguenin et al. 1975), P. infestans (McLeod et al. 2001; Olanya et al. 2001) and P. nicotianae (Lamprecht 1973; Allagui and Tello-Marquina 1996).The most of the African reports of Phytophthora species thus originates from South Africa. Very little is known about Phytophthora species occurrences and diversity from other African countries.
In South Africa a distinct bias is seen towards the detection of Phytophthora species associated with the agricultural (14 species) industry compared to detection of Phytophthora species associated with the forestry (7 species) and ornamental plant industry (5 species) and native ecosystems (5 species). Although a large amount of species has been reported from South Africa, little further research has been done on the source, origin and additional hosts of these species. Of all the Phytophthora species detected from South Africa only P. cinnamomi has received extensive attention and most of the research done on native ecosystems only focused on P. cinnamomi. Those few studies that was not focussed on a single specific species did in fact retrieve several Phytophthora species (Von Broembsen 1989b; Bezuidenhout et al. 2009), indicating that there are potentially many more Phytophthora spp., be it known or unknown species, present in South Africa's native ecosystems.
2. Phytophthora in the African Cacao Industry
The Cacao plant (Theobroma cacao) is commercially cultivated for its seeds, known as cocoa beans, which is used for the production of cocoa, chocolate and cocoa butter. Originally from South America, it is now grown in many countries across the globe. Africa is the world's largest cacao producer as it produces on average 70% of the world's cacao, of which 38% is produced by Côte d'Ivoire and 19% by Ghana (ICCO 2007; ICCO 2009). When this is contrasted to the production of the Americas (12%) and Asia and Oceania (17%) it becomes clear that Cacao production is one of Africa's greatest industries.
The dominant cultural practise applied to Cacao in Africa is intercropping. Cacao cultivation involves the partial clearance of an area to retain shade trees and other economical plants and the subsequent planting of the Cacao plants. The Cacao plants are then intercropped with other economically important or food crops. Intercropping allows for the maximum land usage with minimal soil disturbance and this system is a superior approach to that of slash and burn practises (Duguma et al. 2001). The diversity of the Cacao intercropping system results in complex agroforests which resembles natural forests.
2.1 Phytophthora pod rot Epidemiology
In Africa two Phytophthora species are responsible for causing disease under Cacao trees. Initially P. palmivora was implicated as the cause for Black Pod disease (Phytophthora pod rot) of Cacao, but later it was shown that P. palmivora from Cacao comprised another species, P. megakarya (Brasier and Griffin 1979). P. palmivora occurs globally whereas P. megakarya is restricted to the equatorial regions of Africa. Both species cause similar symptoms when infecting Cacao trees.
The most economically important disease caused by P. palmivora and P. megakarya is Black Pod disease. This entails the infection of Cacao pods by sporangia or zoospores and the subsequent dark discoloration of the pods, from there the name of the disease. This disease directly results in yield losses as cocoa pods shrivel up and do not produce usable cocoa beans. It has been noted that P. megakarya is much more virulent than P. palmivora on Cacao and subsequently results in much higher yield losses (Appiah et al. 2004; Ndoumbe-Nkeng et al. 2004). P. megakarya infection further differs from that of P. palmivora in that pod lesions caused by P. megakarya have serrated edges, where as P. palmivora lesions have smooth edges (Erwin and Ribeiro 1996).
Phytophthora can also infect the stems of Cacao trees to cause stem cankering. Rain splash dispersal of sporangia as well as physical contact between diseased pods and stems can result in the infection of Cacao stems (Erwin and Ribeiro 1996). These infections progress to form reddish-brown lesions which exudes a similar coloured fluid. It was found that P. palmivora infected areas had higher incidences of stem cankers compared to that of P. megakarya infected regions (Appiah et al. 2004). The same study also found that most cankers are caused through infection of the flowering cushions and that P. megakarya had a significantly higher incidence of soil-borne related cankers than P. palmivora. Although the incidence and impact of stem cankers are much lower than that of Black Pod, it remains an important source of secondary innoculum.
P. megakarya has been shown to occur in alternative hosts. Shade trees in Cacao plantations have been found to be infected with P. megakarya. In Ghana P. megakarya was found to occur in four tree species (Funtumia elastica (Apocynaceae), Sterculia tragacantha (Sterculiaceae), Dracaena mannii (Agavaceae) and Ricinodendron heudelotii (Euphorbiaceae)) (Opoku et al. 2002) and in Cameroon it was reported from an Irvingia sp. (Holmes et al. 2003). These alternative hosts might act as a reservoir and source of innoculum for P. megakarya but this has not been confirmed. The role that these alternative hosts will have on the epidemiology and control of P. megakarya is still uncertain.
2.2 History of P. palmivora in Africa
P. palmivora was first implicated as the cause for Black Pod disease of Cacao in Africa in the early late 1920's (Ashby 1929). This study included Ghanaian P. palmivora isolates from Cacao and a Mimusops sp. and characterized these in terms of morphology, sexual compatibility and pathogenicity to Cacao. Based on these characters two strains of P. palmivora were designated: the “Cacao Group” and the “Rubber” group. Later it was reported that in Ghana (the erstwhile Gold Coast) P. palmivora had been found to occur on Cacao, Mimusops sp. and Hevea brasiliensis (Rubber tree) (Dade 1940).
In Nigeria and the island of Bioko (formerly Fernando Po) Cacao was also found to be affected by Black Pod disease. The incidence of Black Pod was correlated to rainfall and humidity by using the incidence of epiphytes as an indication of humidly and rainfall (Thorold 1952; Thorold 1955b). The disease severity in terms of number of black pods per tree was thus positively correlated to the humidity and to the total number of pods per tree. The Nigerian symptoms of Black Pod disease was found to differ from those from Ghana: Nigerian Cacao pods were mostly infected from their distal ends and sides, whereas Ghanaian Cacao pods were mostly infected from the bases (Thorold 1955a). This study also found that suitable climatic conditions (temperature and humidity) are crucial for sporangial formation but that epidemics will only occur when a sufficient number of pods are available.
A survey of all the Cacao producing countries of Africa found that isolates of P. palmivora can be divided into three definite strains. It was observed that much variation occurred within isolates of P. palmivora. This variation included differences in growth rate, sporangial dimensions and lesion development (Turner 1960b). Three strains of P. palmivora was designated based on these variation (Turner 1960a). The Ghanaian (G) strain was found to occur in Ghana, Côte d'Ivoire, Democratic Republic of Congo (then Belgian Congo) and Sierra Leone. The Nigerian (N) strain occurred in Nigeria, Cameroon, Bioko and Gabon. The Angolan (A) strain was less common and was only found from Angola and Sierra Leone. The G strain was characterized as causing fast growing lesions with a regular margin, producing little aerial mycelium and sporangia on unripe Cacao pods, which is contrasting to the N strain which caused slower growing lesions with irregular margins and producing almost no aerial mycelia but large amounts of sporangia. The A strain had sharply delimited edges and produced almost no aerial mycelium or sporangia. These strains also differed in regards to their sporangial morphology as the G and A strains had elongated sporangia whereas the N strain had round sporangia. Lastly this study found that the G and N strain were complementary to one another in terms of mating type. It was subsequently shown that the N strain is in fact a new and separate species from P. palmivora, namely P. megakarya (Brasier and Griffin 1979).
2.3 Control of Cacao Black Pod
Most of the research done on Cacao Black Pod in Africa focussed on control methods for this disease. The most widely used control strategy is the use of chemical fungicides, but lately there has been growing interest in using alternative approaches. These alternatives include phytosanitary practices, biological control agents and breeding of resistant Cacao cultivars. The above mentioned practices can be combined to complement one another and thus achieve lower levels of disease incidence compared to using individual approaches.
Chemical fungicides have been used widely against Cacao Black Pod. A wide variety of Copper-based contact fungicides (Opoku et al. 2007a) has been used as well as semi-systemic (metalaxyl and copper-1-oxide (Rodomil 72 plus)) (Opoku et al. 2004; Opoku et al. 2007b) and systemic (potassium phosphonate) fungicides (Opoku et al. 2007a) are applied to protect against Black Pod. The disadvantage of using chemical pesticides is that it is costly, requires multiple applications throughout the season and application thereof is labour intensive (Ndoumbe-Nkeng et al. 2004).
Phytosanitary cultural practices are aimed at reducing innocula production by creating unsuitable conditions for Phytophthora and by removing infected material. Recommended cultural practices include weeding, removing mistletoes, eliminating excess shade and removing infected and mummified pods (Akrofi et al. 2003). It has been shown that Black Pod incidence can be lowered by removing infected or mummified pods and that this practice also increases the amount of new pods formed (Ndoumbe-Nkeng et al. 2004). Sanitary practices alone are not sufficient to control Black Pod but when combined with fungicide applications it considerably reduces disease incidence(Opoku et al. 2007a). This can also reduce the amount of fungicide applications needed per year, although the reduction in disease incidence is not as good as with the normal fungicide regime (Opoku et al. 2007b). Phytosanitary cultural practices are effective supplemental practices to increase the protection offered by fungicides.
Recently the biological control of Black Pod by mycoparasites has been researched. Isolates of endophytic Geniculosporium spp. were tested to determine whether they reduce the severity or spread of P. megakarya (Tondje et al. 2006): one isolate, BC13, had a notable reduction in the growth and lesion formation of P. megakarya and another isolate, BC177, reduced the amount of sporangia formed. Despite this reduction in virulence and sporulation of P. megakarya by these two isolates, no complete inhibition of infection or spread was seen. A similar study investigated the ability of Trichoderma asperellum in reducing incidence of Black Pod (Tondje et al. 2007). This demonstrated that four isolates of T. aperellum reduced the incidence of Black Pod but that this did not compare to the reduction of disease incidence seen when using chemical fungicides. Further field trials using one of the above T. asperellum isolates were done and confirmed the reduction of disease incidence as well as outperformance of this biological control strategy by chemical fungicides (Deberdt et al. 2008). Current biological control agents of Black Pod disease cannot offer complete protection and this it should be integrated with chemical fungicides and cultural practises.
Cacao resistance to Black Pod disease has also received attention. Resistance to Black Pod can potentially negate or reduce the need for costly chemical fungicides (Nyassé et al. 1995). Breeding of resistant Cacao plants is a lengthy process and so is the screening process whereby Cacao trees are assessed as they succumb to natural infection. A succession of Cameroonian and Ivorian studies was carried out to identify indicators, which would allow for faster screening of Cacao resistance to Phytophthora pod rot. Different parts of the Cacao plant were investigated for use as indicator for resistance and it was found that leaves are the best indicator (Nyassé et al. 1995; Tahi et al. 2000). Leaves effectiveness as early indicator of resistance were confirmed by subsequent breeding study (Nyassé et al. 2002; Tahi et al. 2006a). The influence of experimental factors on leaf based resistance screening was also determined and it was found that leaf ages and incubation time after inoculation played an important role when assessing a trees resistance to Phytophthora pod rot (Tahi et al. 2006b): 50- 60 day old leaves and an incubation time of 5-7 days where found to be most indicative of resistance level of Cacao trees. The above foliar resistance screening was used to confirm resistance of trees selected in Ivorian (Pokou et al. 2008) and Cameroonian (Efombagn et al. 2007) farms for low disease incidence.
Control of Phytophthora pod rot is best achieved by combining different control approaches. It has been widely suggested that optimal control can be achieved by combining chemical control, phytosanitary cultural control, biological control and host resistance (Ndoumbe-Nkeng et al. 2004; Opoku et al. 2007b; Deberdt et al. 2008). The protection against Black Pod is increased when chemical and phytosanitary cultural approaches are used together (Akrofi et al. 2003) and it has been shown that by supplementing chemical control with fertilizer treatment can sustain and significantly increase Cacao yields in severely infected areas (Opoku et al. 2004). The most efficient control of Phytophthora pod rot can thus be achieved by using a combination of approaches.
3. Late Blight in Africa
Potato (Solanum tuberosum) is globally cultivated for its carbohydrate rich tubers. Global potato production per year is around 300 million metric tons, whereas African potato production range from 12-18 million metric tons per year (http://faostat.fao.org). The ten largest potato producing countries in Africa is Egypt, Malawi, South Africa, Algeria, Morocco, Rwanda, Kenya, Nigeria, Uganda and Tanzania. These countries are responsible for 80% of Africa's potato production. Most of Africa's potato produce is consumed locally, although a small amount is exported. African countries, on average, imports slightly more (470970 tons) potatoes than it exports (386240), based on 2001-2007 data (http://faostat.fao.org). Algeria imports the highest quantity of potatoes annually and Egypt the second highest, although Egypt is Africa's largest exporter of potatoes.
Late blight of potato is a serious disease caused by P. infestans. This disease was responsible, in part, for the Great Irish Famine of the 1840's and is caused by P. infestans (De Bary 1876). Late blight causes significant yield losses in potato crops, which can be as high as 90% (Sengooba and Hakiza 1999), although considerable variation in disease severity has been reported between regions and potato cultivars (Mukalazi et al. 2001b). Weather patterns have a definite affect on the severity of late blight and it has been shown that a succession of days with high humidity and low light levels are conducive to late blight epidemics (El-Bakry 1972; El-Bakry et al. 1983; Fahim et al. 2003).
The first reported African incidence of late blight was first observed in 1913 from potato and in 1922 from tomato (Solanum lycopersicum) from various locations in South Africa (Doidge and Bottomley 1931; Wager 1941). The first outbreak of late blight in the rest of Africa only occurred during the 1940's in Kenya, where it infected potatoes and various solanaceous plants (Nattrass and Ryan 1951). By the mid 1950's late blight had spread into the rest of Central Africa, including Cameroon, Uganda and Tanzania (erstwhile Tanganyika territory) (Russell 1954) and in the 1960's it was found in Egypt (El-Bakry 1972).
P. infestans' centre of origin is believed to be Mexico (Goodwin et al. 1992; Goodwin et al. 1994a). The A2 mating type was first identified, outside Mexico, from Switzerland (Hohl and Iselin 1984). By 1990 the A2 mating type and additional genotypes had spread through Europe, large parts of Asia, Egypt and South America (Fry et al. 1993). The A2 mating type of P. infestans was first reported from Africa from infected potato tubers originating from Egypt (Shaw et al. 1985). A later study did not observe the A2 mating type from Egyptian P. infestans isolates (Baka 1997). Most African countries appeared to have P. infestans populations consisting only of the A1 mating type. The A2 mating type has recently been observed in Morocco (Sedegui et al. 2000; Hammi et al. 2001; Hammi et al. 2002).The presence of both mating types and more genetic diversity of P. infestans could allow it to adapt faster to fungicides than the ancestral (US-1) clonal populations could and also to survive outside of its hosts.
In the 1970's new genotypes of P. infestans have spread out of the centre of origin to other parts of the world (Fry et al. 1993). These genotypes are defined by mating type and two allozyme (Gpi and Pep) characteristics (Goodwin et al. 1994b). “Old” (historical) genotypes are of A1 mating type only and have Gpi and Pep profiles of 86/100;92/110, 86/100;100/100, 100/100;92/100 100/100;92/92 (Gpi;Pep). Genotypes newly spread during the 1970's are of either mating type and Gpi and Pep profiles of 90/100;83/100, 90/100;100/100, 100/100;83/100, 100/100;100/100 (Gpi;Pep) (Goodwin et al. 1994b). African reports of new genotypes have been very scarce. Most studies done on population characterization of P. infestans have found only the “old” genotype (Baka 1997; Erselius et al. 1997; Vega-Sanchez et al. 2000; McLeod et al. 2001). Only one case of “new” genotypes have been reported from Morocco, where A1 and A2 isolates of 100/100;100/100 Gpi and Pep profile were found (Sedegui et al. 2000). This study reports change in the Moroccan P. infestans population from dominated by A1 mating type and “old” genotypes to mostly A2 mating type and “new” genotypes.
P. infestans can also infect various other plant species other than potato. The ability of P. infestans to infect alternative hosts complicates control measures against it, as the alternative hosts can act as a continuous source of innocula, especially where intercropping is applied. Apart from potato, P. infestans can also infect and cause late blight of tomato (Solanum lycopersicon) (Erselius et al. 1997) and Huckleberry (S. scabrum) (Fontem et al. 2003). It was also found that P. infestans can infect other solanaceous plants such as gboma eggplant (S. macrocarpon)(Fontem et al. 2004), S. indicum, S. incanum and S. panduriforme (Nattrass and Ryan 1951) as well as the asteraceous weeds Billy goatweed (Ageratum conyzoides), Dichrocephala (Dichrocephala integrifolia), haemorrhage plant (Aspilia africana) and worowo (Solanecio biafrae) (Fontem et al. 2004). The host specificity of P. infestans isolates have been studied and in some cases host specific strains of P. infestans were observed on potato and tomato (Erselius et al. 1997), whereas others did not find any evidence thereof (Fontem et al. 2005).
3.2 Control of Late blight in Africa
In Africa a great deal of research focus is placed on optimal usage of chemical control methods. Due to the high cost associated with control chemicals the aim is to maximize the economic benefit and not necessarily to maximize the crop production (Kassa and Beyene 2001). Commonly used chemicals for the control of P. infestans include the contact fungicide Dithane M-45 (mancozeb), the systemic fungicide Ridomil (metalaxyl) and a combination of metalaxyl and mancozed, Ridomil MZ (Olanya et al. 2001; Tumwine et al. 2002b; Kankwatsa et al. 2003). Mancozeb has the advantages of being effective against a broad range of pathogens, but being a contact fungicide its protection is severely reduced after rainfall. Metalaxyl on the other hand is not influenced by rainfall but P. infestans has been known to develop resistance to metalaxyl. In Africa metalaxyl resistant P. infestans have been reported from several countries like Uganda (Erselius et al. 1997; Vega-Sanchez et al. 2000; Mukalazi et al. 2001a), Kenya (Erselius et al. 1997; Vega-Sanchez et al. 2000) , Morocco (Sedegui et al. 2000; Hammi et al. 2002), Cameroon (Fontem et al. 2005) and South Africa (McLeod et al. 2001).
Chemical control, although the most effective and widely applied control method, is not the only option applied in Africa. Host resistance is important as it can significantly decrease disease incidence and increase yield, but it is only effective when integrated with chemical control (Kankwatsa et al. 2002; Namanda et al. 2004). Regulating the planting time of potato crops in Africa, in order to avoid conditions favourable to late blight, is difficult due to lack of weather monitoring systems and the fact that it is widely grown as a subsistence crop (Kankwatsa et al. 2002). It has also been shown that integration of sanitation (the removal of infected material), sheltering and intercropping is an effective option to control late blight in tomatoes, although not as effective as chemical control (Tumwine et al. 2002a). Integration of various control strategies contains significant advantageous to improve crop yields and reduce associated expenses, although it has been indicated in a Kenyan survey that most farmers are ignorant of integrated control methods (Nyankanga et al. 2004). Thus there exists a need for education of farmers on how to best combat late blight and maximize their yields.
4. Phytophthora cinnamomi
Phytophthora cinnamomi is one of the most serious and widespread plant pathogens in the world. Its success as a pathogen can be attributed to its huge host range (Erwin and Ribeiro 1996; Shearer et al. 2004). P. cinnamomi can persist in soil for extended periods of time in the form of chlamydospores (Weste and Vithanage 1978; Weste and Vithanage 1979; Kenerley and Bruck 1983). Due to its association with soil, P. cinnamomi is easily spread by human activities such as through vehicles, construction of roads or any other activity where chlamydospores-containing soil is transferred from one location to another (Weste 1975; Anonymous 2000) and have as a result been spread all across the world through human activities (O'Gara et al. 2005). P. cinnamomi has a near global distribution, as it has been found on every continent except Antarctica (Zentmyer 1985). The centre of origin of P. cinnamomi is currently not certain, but it has been noted that it might originate from Southeast Asia, Australia or South Africa (Zentmyer 1985; Zentmyer 1988).
P. cinnamomi is known to cause serious losses in a number of agriculturally important crops. Some of the most important affected crops includes, pineapple (Rohrbach and Schenck 1985), avocado (Coffey 1987) and macadamia (Zentmyer and Storey 1961). P. cinnamomi is also a pathogen of several tree species used by the commercial forestry industry, such as Pinus spp. (Chavarriaga et al. 2007; Reglinski et al. 2009) and Eucalyptus spp. (Tippett et al. 1985). P. cinnamomi has also had severe effects on the native Australian ecosystem; where due to its large host range it could infect many of the native plant species (Shearer et al. 2004) and thus cause a change in the species composition of the ecosystem (Hardham 2005).
4.1 P. cinnamomi in Africa
It has been known for some time that P. cinnamomi is present in a number of African countries, such as Cameroon, Congo, Guinea, Ivory Coast, Kenya, Morocco, South Africa, Uganda, Zaire, Zambia and Zimbabwe (Zentmyer et al. 1976; Zentmyer 1988). Despite the fact that P. cinnamomi occurs in above mentioned countries not much research has been done on it. It has been reported to cause root rot in avocado's in Cameroon (Huguenin et al. 1975) and Canary Islands (Llobet 1992). P. cinnamomi has also been found to cause stem cankers and root rot in Macadamia spp. in Kenya (Mbaka et al. 2009). No studies have been done to assess the incidence and impact of P. cinnamomi in native habitats of Africa, except in South Africa.
In South Africa P. cinnamomi is the most destructive disease found on avocado's (Kremer-Köhne and Mukhumo 2003). It was first detected in South Africa from Avocado trees showing dieback symptoms but was identified as P. cambivora but its pathogenicity could not be established (Doidge and Bottomley 1931; Wager 1931). Later upon re-examination of the above mentioned P. cambivora isolates Wager stated that they more closely resembles P. cinnamomi and subsequently regarded it as such (Wager 1941). P. cinnamomi was later found to be the cause of crown and trunk canker of avocado trees (Lonsdale et al. 1988). Apart from avocado, P. cinnamomi were also found to be the cause of crown and root rot of grapevines in the Western Cape of South Africa (van der Merwe et al. 1972). Other Phytophthora spp. were also found to occur and cause disease on grapes (see table 1) but P. cinnamomi were found to be the most virulent species on grapevines and that it could survive for extended periods within infected root-debris (Marais 1979; Marais 1980)
P. cinnamomi has also been prominent as causing disease in the South African forestry sector. Eucalyptus spp. and Pinus spp. are extensively planted in South African plantations and both these genera has been affected by root rot disease caused by P. cinnamomi (Wingfield and Knox-Davies 1980; Linde et al. 1994). P. cinnamomi has also been associated with significant losses due to seedling death caused by P. cinnamomi in forestry nurseries (Donald and von Broembsen 1977).
P. cinnamomi was first associated with native plant species in the early 1970's , where it was found to be the cause of root rot in Leucodenderon argenteum (Silver tree, Proteaceae) (Van Wyk 1973). Thereafter it was found to be extremely prevalent in the Fynbos vegetation of the South-Western Cape of South Africa as it was recovered from more than 80 native species including Agothsma spp., Erica spp., Leucadendron spp., Leucospermum spp., Protea spp. and Widdringtonia spp. (Von Broembsen and Kruger 1985; Wingfield et al. 1988; Bezuidenhout et al. 2009). P. cinnamomi has also been associated with as much as 63 species of commercially cultivated Proteaceae (Von Broembsen and Brits 1985). A survey of river systems in the South Western Cape region also indicated that P. cinnamomi is widely present in most river systems in that area (Von Broembsen 1984a).
It has been proposed that the South Western Cape of South Africa might be a centre of origin of P. cinnamomi (Von Broembsen and Kruger 1985; Zentmyer 1988) due to its presence in undisturbed native Fynbos vegetation. This hypothesis seems unlikely given data on the biogeographical distribution of P. cinnamomi mating types in South Africa, which indicated that only the A1 mating type is found in fynbos, the A2 mating type is found in cultivated crops and that both mating types are present in natural forests (Von Broembsen 1989a). More recent biogeographical data indicated that both mating types are present in Fynbos and that the A1 mating type is restricted to the South Western Cape of South Africa (Linde et al. 1997). A population study of South African P. cinnamomi isolates revealed a low level of gene diversity, indicating that P. cinnamomi is an introduced organism and that although both mating types occur in the same locations that sexual reproduction rarely occurs in South African populations (Linde et al. 1997).
Africa is severely affected by Phytophthora spp. and their associated plant diseases, but research in Africa is lagging behind that of the international community. This is due to lack of funding and trained experts in the field. Most research has been done on the control and crop yield optimization associated with Late blight and Black pod diseases. There is a lack of focus on Phytophthora spp. in native ecosystems: Invasive species could cause serious harm to native ecosystems and novel Phytophthora spp. might also be present in native African ecosystems.
Table 1: Phytophthora species occurrence in South Africa
Citrus, Apple, Grape
(Wager 1941; van der Merwe and Matthee 1973; Marais 1979; Marais 1980)
Paprika, Pumpkin, Tomato
(Thompson et al. 1994; Labuschagne et al. 2000; Labuschagne et al. 2003)
(Wager 1941; van der Merwe et al. 1972; van der Merwe and Matthee 1973; Marais 1979; Marais 1980; Lonsdale et al. 1988)
(von Maltitz and von Broembsen 1985)
(Doidge and Bottomley 1931; Wager 1931; Wager 1941; Schutte and Botha 2008)
(Marais 1979; Marais 1980; Thompson et al. 1995)
(Thompson 1987; Thompson 1988; Thompson and Phillips 1988)
(Doidge and Bottomley 1931; Wager 1931; McLeod et al. 2001)
(Thompson 1987; Thompson 1988)
Citrus, tabacco, tree lucern
(Lamprecht 1973; Ferreira et al. 1991; Botha 1993; Thompson et al. 1995)
Banana, grape, rhubarb
(Wager 1931; Wager 1935; Marais 1979; Marais 1980; Thompson 1981)
(Von Maltitz and Von Broembsen 1984)
Eucalypts, Black Wattle
(Linde et al. 1994; Roux and Wingfield 1997)
Eucalypts, Pines and in nurseries
(Donald and von Broembsen 1977; Wingfield and Knox-Davies 1980; Linde et al. 1994)
(Roux and Wingfield 1997)
(Maseko et al. 2001)
Eucalypts, Black Wattle
(Zeiljemaker 1971; Linde et al. 1994; Roux and Wingfield 1997)
(Maseko et al. 2007)
(Maseko et al. 2007)
(Mes 1934; Wijers 1937)
(McLeod and Coertze 2006)
(Thompson and Naudé 1992)
Native plants and habitats
Proteacae, rivers, Widdringtonia cederbergensis, Agathosma spp.
(Van Wyk 1973; Von Broembsen 1984a; Von Broembsen 1984b; Von Broembsen and Brits 1985; Von Broembsen and Kruger 1985; Wingfield et al. 1988; Bezuidenhout et al. 2009)
Osteospermum sp. Agathosma spp, Proteacea and rivers
(Von Broembsen 1989b; McLeod and Coertze 2007; Bezuidenhout et al. 2009)
Agathosma spp. and rivers
(Von Broembsen 1989b; Bezuidenhout et al. 2009)
(Von Broembsen 1989b)
(Bezuidenhout et al. 2009)
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