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The obtainment of 30 new strains from native Trichoderma harzianum after UV light …

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- New strains obtained after UV treatment and protoplast fusion of native Trichoderma harzianum: their biocontrol activity on Pyrenochaeta lycopersici

Tomato crops (Lycopersicon esculentum Mill.) have turned to be in Chile and worldwide, the economically most important horticultural product. When grown under unheated greenhouse and in monoculture conditions, a severe disease pressure occurs. Among these, the corky root caused by P. lycopersici Schneider & Gerlach is observed. This fungus is one of the most important pathogens of tomato plants and other vegetable crops both in protected and field crops all over the world (Fiume and Fiume, 2003). The disease results in rotting of tomato fine roots, browning and suberization of main roots and, in extreme conditions, canker at the crown level of the plant.

Tomato diseases are normally controlled by means of selected fungicides and soil fumigation with methyl bromide (MeBr). Although the use of MeBr-chloropicrin eradicates many of the fungi involved in root rot complex, it has been shown that reinvasion by fast growing fungi may lessen the effectiveness of fumigation in the later growing season (Pinkerton et al. 2002). The control of P. lycopersici in Chile is accomplished through fumigation with this chemical, which contaminates the environment, affects the ozone layer, destroys the soil microflora, and must be applied every season because of its null residual activity and the rapid re-colonization of soils by the phytopathogens (Ristaino and Thomas, 1997; Pinkerton et al. 2002). Several disease management practices have been proposed to replace MeBr (Ristaino and Thomas, 1997; Fiume and Fiume, 2003).

The natural control of several phytopathogens is based on the presence of suppressive soils where different biocontrol microorganisms are detected, such as those belonging to Trichoderma, Gliocladium, Pseudomonas and Bacillus genera, among others (Weller et al. 2002; Montealegre et al. 2003; Guo et al. 2004; Huang et al. 2005; Montealegre et al. 2005). Considering the difficulty to generate suppressive soils similar to the natural ones, the use of selected biocontrol microorganisms could provide an alternative to the use of chemical fungicides.

Trichoderma spp. has proved to be useful in the control of phytopathogens affecting different crops (Benítez et al. 2004). Specifically, the use of Trichoderma harzianum Rifai native strains as biocontrol agents (BCAs) in tomato crops could be a useful alternative in the control of soilborne diseases, to be applied under monoculture conditions. Nevertheless, it seems necessary to improve the already positive biocontrol effect of T. harzianum (Pérez et al. 2002; Besoain et al. 2003), especially for its use on tomato crops developed under conditions where P. lycopersici easily produces the corky root disease. In addition, the high alkaline pH found in soils where this crop is cultivated in the central zone of Chile along with low temperatures, should be considered as challenging conditions in the obtainment of improved T. harzianum strains. Taking into account that previous results obtained after testing more than 35 native isolates from different Trichoderma species (T. harzianum, T. koningii, T. piluliferum and T. polysporum) on the development of P. lycopersici in conditions that favored the phytopathogens growth, allowed us to select isolates Th11, Th12, Th291 and ThV (Besoain et al. 2003). Therefore, we decided to improve the biocontrol activity of these selected Trichoderma strains using already described procedures for the obtainment of mutants. Thus, this paper describes the use of UV-A and UV-C radiation and protoplast fusion of native T. harzianum strains that have proved to behave as good BCAs, (Pérez et al. 2002; Besoain et al. 2003), for the obtainment of improved strains; their selection under low temperature and alkaline pH conditions, and the biocontrol effect of the selected new strains on P. lycopersici, under laboratory and glasshouse conditions.

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