The results revealed the identity of the isolates from the diseased V. tortilis as P. rubefaciens, P. herbarum, T. longibrachiatum and T. harzianum. All these fungi have not been reported previously on this plant species in Namibia. Penicillium and other fungus such as Aspergillus and Talaromyces (Eurotiomycetes) are amongst the genera that are commonly found indoors (Amend et al. 2010; Houbraken et al. 2011).

Penicillium species have been reported (Frisvad et al. 2004; Frisvad and Samson 2004) to be linked to bio deterioration in foods especially fruits and producing various mycotoxins. Penicillium species have also been positively used in the food industry for the production of cheese (Giraud et al. 2010; Visagie et al., 2014), fermented sausages (Lopez-Díaz et al., 2001; Ludemann et al., 2010), and medical industry for the production of penicillin (Visagie et al., 2014). Penicillium, Eurotium and Cladosporium species have also been isolated from the cork (outer layer of the Quercus suber L. tree.) by Barreto et al. (2012) and they argued that these fungal species colonize the cork alongside different kinds of fungi inhabiting different parts of the tree and surrounding soil. Depending on the fungal species, some form a parasitic relationship while others form a saprobiotic one in which case the cork provides the relevant nutrients for the fungal growth (Barreto et al., 2012). Moreover, Penicillium occurs in diverse environments that include soil, vegetation, air, indoor environments and in food (Visagie et al., 2014). To our knowledge, this is the first report on the presence of P. rubefaciens in A. tortilis. The effect of P. rubefaciens on A. tortilis remains unknown and further investigations to assess if this species is the primary disease causing pathogen on A. tortilis are vital.

The Phoma genus is diverse and composed of species that inhabit a wide range of environments such as soil and plants. A substantial amount of the species of this genus are saprophytic with some being opportunistic (Irinyi et al., 2007). In addition, most coelomycetes are associated with plants and in some cases are primary pathogens (Sutton, 1980). According to Boerema et al. (2005), approximately 2000 species of Phoma have been isolated and characterized. Furthermore, Garas et al. (2012) labeled Cytosperma chrysoperma and Phoma species such as; P. glomerata, P. cava and P. eupyrena as pathogens responsible for dieback disease in V. mellifera and V. karoo.

P. herbarum species have been isolated from fruit trees in which they infect the leaves and twigs (Valiuskaite, 2002) and sapwood of healthy and declining scots pine trees (Giordano et al., 2009). Giordano et al. (2009) revealed that P. herbarum, Thanatephorus cocumeris and Mucor plumbeus species are associated with the existence of mistletoes on scots pine trees. A mistletoe is a parasitic plant that affects the stand structure by inducing crown losses and tree mortality but also has some ecological importance in the survival of some trees because it functions as a carbon sink (Cullings et al., 2005; Mueller and Gehring, 2006). Giordano et al. (2009) further argued that mistletoe on a tree influences the diversity of endophytic mycoflora inhabiting that particular tree and declining scots pine trees possess different endophytic mycobiota in their sapwood compared to that of healthy trees. However, P. herbarum has been designated as the primary pathogen leading to twig dieback in A. mearnsii after its consistent isolation from diseased A. mearnsii trees (Gibson, 1979; Olembo, 1972; Roux et al., 1995). Gibson (1979), Olembo (1972) and Roux et al. (1995) further argued that P. herbarum caused fresh infections through wounds. The present study did not observe the presence of mistletoe on the V. tortilis tree but can infer that P. herbarum is associated with disease in the V. tortilis.

Trichoderma species are widely found in the soil and root ecosystems and are used as biological control agents against a variety of plant pathogens especially fungi (Mulaw et al., 2013). These species are found in a diverse range of habitats ranging from soil and growing on wood, bark, and other fungi and numerous sub­strates (Druzhinina et al., 2011). This report is the first one on Trichoderma occurring in V. tortilis in Namibia. The commonly used Trichoderma species as biological controls are T. harzianum, T. atroviride, T. viride, T. virens and T. koningii. These species are antagonistic microorganisms capable of preventing the growth of pathogens in the soil and plant (Singh et al., 2014). However, the effect of Trichoderma species on non-target organisms is less understood although it is relevant for their effective use. Trichoderma species have found prominent use because they prevent increase of the pathogens, produce enzymes that degrade the cell wall of pathogens, produce antibiotics capable of destroying pathogens, induce the development of the plant, and induce plant defensive mechanisms (Singh et al., 2014). Druzhinina et al. (2011) explained that the antagonistic properties of parasitizing and killing other fungi are common in Trichoderma species and have thus earned them widespread use as biological control organisms. Studies have revealed that T. longibrachiatum and T. harzianum are used as biological agents against fungal infection in plants (B³aszczyk et al., 2014; Howell, 2003; Limon et al., 2004; Mohd Zainudin et al., 2008; Mulaw et al., 2013).

The isolation of T. longibrachiatum and T. harzianum in A. tortilis in the present study suggests the relationship between the A. tortilis, and T. longibrachiatum and T. harzianum species is either commensal or mutualistic because of their antagonistic properties. It was revealed that P. herbarum is associated to twig dieback disease in V. tortilis while the effect of P. rubefaciens on V. tortilis remains unknown. In addition, pathogens have the ability to form synergistic relationships which have been detected in various plant species (Lamichhane and Venturi, 2015). Synergistic relationships range from fungi-fungi interaction, bacteria-bacteria interaction, virus-virus interaction, and mixed interactions such as fungi-bacteria interaction. Synergism justifies the existence of plant diseases in which more than a single pathogen is involved in the infection process (Lamichhane and Venturi, 2015). Synergism and its effects go undetected in monocultures and it probably occurs frequently in plant diseases. Glen et al. (2009) revealed fungi-fungi synergism in A. mangium infected with root rot disease in which the pathogens responsible were Ganoderma philippii, Ganoderma mastoporum, Ganoderma austral, and Amauroder marugosum. Consequently, it is possible that some of the isolates in this study were opportunistic pathogens that benefit from synergism. Moreover, with the current data set, it is not possible to draw firm conclusions that P. herbarum is the primary cause of the twig dieback although it is associated with twig dieback as reported in previous studies (Gibson, 1975; Olembo, 1972; Roux et al., 1995) because of the possible existence of other fungal species that are not or rarely detected in culture based methods.