Trichoderma Biocontrol Taxa And Strains

The genus Trichoderma currently consists of more than 40 known taxa, which are usually cosmopolitan, (although some species display a geographic bias: Kubicek et al. 2002; Kullnig et al. 2000), and typically soilborne or wood decaying Teleomorphs of Trichoderma occur in the genera Hypocrea, Podostroma, and Sarawakus of the Hypocreaceae (Gams and Bissett 1998; Rossman et al. 1999). The latter two genera thereby most likely being synonyms of Hypocrea (GJ Samuels, personal communication). Rossman (1999) proposed that necrotrophy (on basidiomycetes) is the original habitat of these Hypocrea spp. and their lignicolous properties have developed later when the species were following their hosts into their habitat (wood and decaying wood in soil). Rossman et al. (1999) claim that the Hypocrea spp., which are found on decaying wood, actually are necrotrophic on the fungi in the wood. Several of the individual teleomorphic and anamorphic partners have been detected recently, and examples relevant to biocontrol are given in Table 1. The more than 100 species of Hypocrea with Trichoderma anamorphs which Doi and Doi (1986) described constitute unexplored source of potential biocontrol agents.

Most of the isolates of the genus Trichoderma, which have been found to act as biocontrol agents, have been classified as T. harzianum Rifai, leading to the fact that T. harzianum is generally synonymized as a "biocontrol agent." However, most of the Trichoderma strains used for biocontrol were identified at the species level exclusively on the basis of morphological and phenotypical characters, showing high convergence in many cases (Kullnig-Gradinger et al. 2003). Therefore, reports of a pronounced genetic variability of T. harzianum isolates by analyzing carbon

Table 1 Teleomorphs known for Trichoderma taxa used in biocontrol

Anamorph

Teleomorph

T. harzianum

H. lixii (former H. nigricans)

T. atroviride

H. atroviride

T. virens

H. virens

T. asperellum

Not known

T. parceramosum

Not known

T. longibrachiatum

Not known

source utilization patterns (Manczinger and Polner 1985), secondary metabolite production (Okuda et al. 1982), isoenzyme polymorphism (Grondona et al. 1997; Stasz et al. 1989), RAPD profiles (Fujimori and Okuda 1994; Gomez et al. 1997; Muthumeenakshi et al. (1994); Turoczi et al. 1996; Zimand et al. 1994), RFLP patterns (Bowen et al. 1996; Muthumeenakshi et al. 1994), rDNA sequence (Grondona et al. 1997; Muthumeenakshi et al. 1994) and karyotype (Gomez et al. 1997) must be treated with caution. On the basis of a rigorous comparison of a pool of seventeen bonafide "T. harzianum" biocontrol strains with the neo-ex type strain of T. harzianum, Hermosa et al. (2000) showed that they actually comprised of four different species i.e., T. harzianum, T. atroviride, T. longibrachiatum and T. asperellum. Consistent results were also reported by Kullnig (2001), who by sequence analysis of the internally transcribed spacer regions of the rDNA (ITS1 and ITS2), the small subunit of the mitochondrial DNA (mtSSUrDNA), and part of the coding region of the 42-kDa endochitinase encoding gene ech42- reassessed the species identity of eight T. harzianum isolates, which are being used by several laboratories for key investigations on the genetics, biochemistry, and physiology of biocontrol. Thereby the strains T. harzianum CECT 2413, T-95, T-22, and T-11 were confirmed as T. harzianum, "T. harzianum" ATCC 74058, IMI 206040, ATCC 36042 identified as T. atroviride, and "T. harzianum" T-203 assessed as T. asperellum. As outlined above, there may be other species capable of biocontrol as well, T. virens being the most prominent example. In addition, molecular proof for identity of other species as biocontrol agents has been presented for T. ghanense (previously T. parceramosum; Arisan-Atac et al. 2002) and T. stromaticum (Samuels et al. 2000).

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