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Forests are the sinks and reservoirs of permanently absorbed carbon as their biomass constitutes 289 giga tons of carbon (FAO, 2010). Tropical forests store 40% of all terrestrial carbon, out of which 58% is stored in the vegetation itself. According to Global Forest Resource Assessment (FRA), the global forest area was estimated four billion hectares in the year 2015 that equals to 30. 6% of total land area of the world (FAO, 2015).
The estimated forest cover by FRA in the year 1990 was 4. 13 billion hectares (31. 6% of world’s land area). Deforestation is a crucial issue to the world forest; however, the rate of annual forest loss was slowed down by more than 50% during 2010-2015 (3. 3 million hectares per year) compared to 1990-2000 (7. 3 million hectares). This was due reduction in forest loss in some countries and afforestation in others (FRA, 2015). Deforestation contributes highly to global carbon emissions and it is a major concern in the tropics. Land conversions and agricultural encroachments into the forest are the main reasons of deforestation and the loss of biodiversity especially in the tropics (Doumenge et al. , 2012). It is important to protect the forest resources as they mitigate the climate change, serve as a potential source of bioenergy and provides protection to the biodiversity (Zomer et al. , 2008).
To meet the global subsistence and to fulfill commercial demand for wooden products, governments and the landowners have relied on forest plantation. The most recent survey of food and agriculture organization (FAO) estimated that plantations cover 7. 0% of global forest area (FAO, 2010). In Paraguay, the investment on plantation forestry is surpassed over last two decades, hence the plantation coverage have doubled since 1990 (FAO, 2010). In South Africa, the forest industry relies exclusively on monocultures specifically Pine and Eucalyptus species (Wingfield, 1997). Since the natural forest has become either scarce or inaccessible, plantations are considered as an alternative. The global coverage of plantation forestry has increased 39 times since 1965 to produce industrial round wood, fuel wood, paperboards, wood based panels and other non-timber forest products (Evans and Turnbull, FAO, 2010).
Eucalyptus are the second major tree species, after pines, that is cultivated globally as plantation forests with a prime position in the southern hemisphere (Old et al. , 2003). Eucalyptus species are a diverse genus of flowering trees and shrubs that belongs to the family Myrtaceae. One of the species Eucalyptus regnans is considered as the tallest flowering plant known on earth (Commonwealth of Australia, 2017). Over 500 species of Eucalyptus exist worldwide and most are native to Australia (Damel, 2000). They are exceptionally unique in Australia as they have evolved in the harsh constraints of the Australian environment and had selection pressure from their major pest and diseases (Old et al. , 2003). Their ability to grow fast, easy propagation and adaptability to grow on difficult sites has led to largescale eucalyptus plantation all over the world (Grossman, 2012).
Eucalyptus plantations have expanded dramatically in the tropics particularly in the Southern Hemisphere during the course of the last hundred years. The nature of these plantations has been changed considerably as a result of hybridization, selection, vegetative propagation, breeding and introduction of other advanced techniques to improve the planting stock (Wingfield et al. , 2008). There is a wide range of pathogens that attack shoots, stems, and leaves of Eucalyptus even in their native environment. However, their natural existence in heterogeneous forest communities and broad genetic base of species provide the protection against epidemics of diseases (Old et al. , 2003). Identical clones prepared through tissue culture are cultivated on large scale targeting to uniform growth and better quality. There is a high risk of disease epidemics in such monocultures that can destroy plantations on a wider scale. The risk has been increased internationally by the movement of germ plasm across the countries, as the planting stock or seeds may be infested by pathogens (CIFOR, 2003).
Teratosphaeria zuluensis was previously described as Coniothyrium zuluensis by M. J. Wingfield. , Crous and T. A. Cout (Cortinas et al. , 2006). It is one of the most significant pathogens that cause the stem canker of Eucalyptus species (Jimu et al. , 2014). Eucalyptus stem canker is also known as Teratosphaeria canker or Coniothyrium canker; it has emerged as a most damaging disease in Eucalyptus plantations especially in non-native regions (Jimu et al. , 2014). It was believed to be caused by the single pathogen species for several years. However, multigene phylogenetic studies led to the identification of other pathogen species that cause same disease described as Colletogloeopsis gauchensis by M. N. Cortinas, M. J. Wingfield and Crous in 2006 (Cortinas et al. , 2006). Both fungal pathogens have gone through several name changes due to the application of novel technologies and currently accepted names are T. zuluensis and T. gauchensis (Jimu et al. , 2014).
The canker causing pathogen of Eucalyptus, T. zuluensis, was firstly discovered in 1988 in South Africa (Wingfield et al. , 1996). Thereafter, the pathogen was reported from several countries of different continents including Mexico (Roux et al. , 2002), Vietnam (Gezahgne et al. , 2003), Malawi (Roux et al, 2005), China (Cortinas et al. , 2006), Zambia (Chungu et al. , 2010) and Uganda (Jimu et al. , 2014). South Africa might be the origin of T. zuluensis as suggested by Wingfield et al. (1996), due to the fact that the pathogen is present in the countries where Eucalyptus is distributed from South Africa (Wingfield et al. , 1996). Nonetheless, the origin of the pathogen is still unknown (Chen et al. , 2011).
Teratosphaeria canker is a serious disease of Eucalyptus that causes damage on stems and branches of the tree (Wingfield et al. , 1996). The disease is primarily caused by two different pathogen species T. zuluensis and T. gauchensis (M. N. Cortinas, M. J. Wingfield and Crous, 2006). T. zuluensis is characterized by the development of depressed isolated lesions that often combine to form bigger necrotic cankers on susceptible eucalyptus trees (Wingfield et al. , 1997). Sometimes, the lesions form into the parallel cracks on the stem that make the cat’s eye shaped appearance and pycnidia may appear on the dead bark. The gum called kino exudes from the cracks that stain the stem and branches of a susceptible tree (Wingfield et al. , 1997).
The disease may result into deformed branches and brush like crowns while severe infection inhibit the growth and might lead to death of the tree (Wingfield, 1996; Cortinas, 2006). The wood from infected trees is also unsuitable for sawn timber. However it can be used for pulp production (Jimu et al. , 2014). Moreover, the sunken lesions also restrict the peeling of eucalyptus bark prior to pulping (Roux et al. , 2002). The economic losses due to this pathogen for forest industry come from decreasing the value of saw timber to pulp material.
The first symptoms of disease are small necrotic spots characterized by sunken lesions on the stem and branches of eucalyptus. The brown colored spots on green stem can be clearly seen which later coalesce to form larger cankers (Wingfield et al. , 1997; Van Zyl et al, 2002; Gezahgne et al. , 2003; Old et al. , 2003). It often creates cracks on stems that ultimately girdle the stem and branches that may lead to the death of tree parts (Wingfield et al. , 1997). The infected tree forms the kino pockets which contain pycnidia in the xylem vessels (Gezahgne et al. , 2003). Teratosphaeria canker is often referred to measles disease due to the fact that initial infections occur on the young, green stem tissue and give rise to small discrete dark spots on the bark ((Jimu et al. , 2014). The lesions merge to form larger black spots on highly susceptible trees and form cracks that exudes profuse amount of kino (Fabinet, 2002). Furthermore, stunted growth, deformed stems and brush like crowns are clear symptoms of severe infection of T. zuluensis.
The pathogen T. zuluensis is noted to be slow growing on artificial media in the laboratory (Wingfield et al. , 1996). Additionally forester’s reports in Paraguay indicated that T. zuluensis canker behaves differently according to its exposure to sunlight (Silva, unpublished data), this led us to compare nutrient utilization and how light would affect the growth of the pathogen. The growth of this pathogen was tested in different volumes of light inside the incubation chamber in different types of media. We studied different substrates for the growth preferences of the pathogen on media through Phenotypic Microarrays.
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