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.
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).
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.