Biocontrol agents are non-pathogenic plant-associated microbes that suppress disease or enhance plant growth. Trichoderma harzianum is a fungus, naturally found in soils or associated to other fungi in aerial portions of plants, which has proved to be an efficient biocontrol agent of fungal plant pathogens. The biocontrol activity of Trichoderma on phytopathogens is accomplished by different mechanisms: a) competition for space and nutrients in order to exclude the presence of the phytopathogen from a certain portion of soil or tissue; and/or b) mycoparasitism in order to use the metabolic machinery of the phytopathogen for its benefit; and/or c) production of antibiotics that interfere with cellular functions of the phytopathogen; and/or d) production of hydrolytic enzymes that degrade the cell wall of the phytopathogen. Therefore, using one or more of these biocontrol mechanisms, Trichoderma mayinterfere with the development of the phytopathogen thus preventing or suppressing disease. On the other hand, Paenibacillus lentimorbus is a naturally occurring bacteria, which has also proved to be an efficient biocontrol agent, being the secretion of antibiotics its main mechanism of action. Based on the mechanisms used to interfere in the development of phytopathogens, both T. harzianum and P. lentimorbuscould replace at least in part the use of chemicals for their control, thus preventing or decreasing the environmental pollution and toxic effects of chemical pesticides, on animal and human beings. In addition, while T. harzianum is applied directly into the soil, P. lentimorbus is applied on roots of seedlings previous to transplanting into the soil, protecting the surface of roots in contact with the soil from the attack of phytopathogens and the surrounding soil from the development of them.

Solarization constitutes an additional alternative to control soil-borne phytopathogens, mostly as a pre-planting soil treatment. It is a process that employs solar radiation to heat soil producing temperatures that are detrimental to several of these type of pathogens. It is achieved by covering (mulching, tarping) tilled and irrigated soil with continuous transparent polyethylene during the hot season, thereby heating it and killing pests and pathogens. Solarization starts usually at the end of Spring for 3-6 weeks, for seed-bed nurseries and productive crops, respectively. The increase of temperature by solarization ranges between 8 and 15ºC, at depth of 20 cm, in heavy clay and sandy soil, respectively. Under these conditions soilborne pests and pathogens could be killed due to physical and biological processes, being a unique treatment which has a long term effect, 4-6 successive crops, if the fields are well kept from recontamination.

Tomato plants are infected by several soil-borne pathogens such as Rhizoctonia solani, Pyrenochaeta lycopersiand Fusarium oxysporum f. sp. lycopersici.Their control is accomplished through fumigation of soils with Methyl Bromide (MeBr). This chemical 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-colonisation of soils by the phytopathogens. Moreover, its use will be forbidden worldly in few years. Therefore, it is important to find alternatives to prevent tomato diseases caused by the pathogens already mentioned, which may include the use of biocontrol microorganisms alone or combined with the use of solarization. The present work describes the effect of treatments with T. harzianum (soil) with  P. lentimorbus (tomato seedlings) and solarization (soil) in different combinations, using treatment with MeBr as control for every assay.

Trichoderma harzianum isolate 650 (Th650) and Paenebacillus lentimorbus isolate 629 (Pl629) selected earlier for their ability to control Rhizoctonia solani, Fusarium solani and F. oxysporum in vitro, were specifically applied to a soil with a high inoculum level, for the control of tomato root rot caused by the complex F. oxysporum f. sp. lycopersici - Pyrenochaeta lycopersici - Rhizoctonia solani. They were applied alone or combined with solarization (summer assay) and/or with methyl bromide (MeBr) (summer and winter assays). Evaluations considered damage level caused by P. lycopersici, root damage caused by the complex, total production, first quality fruits and % of first quality fruits in total plants of each assay. Evaluations were also performed independently for root damage caused by R. solani in the summer assay.

The chemical fungicide used as control (MeBr) decreased tomato root damage caused by the complex from 88.7% to 21.2% in the summer assay and from 78.4% to 35.7% in the winter assay. None of the bio-controllers could replace MeBr for the control of the complex in the winter assay; nevertheless Th650 and Pl629 reduced the root damage caused by the complex in the summer assay. Treatments with the biocontrol microorganisms were improved when they were combined with solarization in this latter season, suggesting that they could be used as a partial alternative to control the complex F. oxysporum f. sp. lycopersici - Pyrenochaeta lycopersici - Rhizoctonia solani in the summer season.

Independent evaluations showed that both Th650 and Pl629 prevented root damage caused by R. solani when applied alone or after solarization. The expression of enzymes such as chitinases, β-1,3-glucanases and proteases (biocontrol mechanisms) by Th650 in terms of number of isoenzymes and level of activity is clearly better towards R. solani, than F. oxysporum f. sp. lycopersici or Pyrenochaeta lycopersici, a fact that could account for the ability of Th650 to control root damage produced by this pathogen and not for the complex. Taking into account these results it appears that the positive control of Th650 towards R. solani and the lack of effect on P. lycopersici correlates well with the endochitinase pattern expressed by Th650 in response to these phytopathogens. On the other hand, root damage caused by R. solani can be controlled at a similar level as it does MeBr in summer assays, thus representing an alternative to the use of this chemical fungicide for the control of this phytopathogen.

Evaluation of fruit yield and quality showed that MeBr alone was not better than the other treatments when total yield or first fruit quality were evaluated.

Taken together the results of assays performed in winter and in summer, it may be concluded that Th650 and Pl629 do not express bio-control mechanisms enough to prevent P. lycopersici development, but appear to be good alternatives to the use of MeBr for the control of R. solani. Solarization prevents root damage due to the presence of P. lycopersici or R. solani or of the complex; nevertheless, this prevention did not result in improvement of fruit yield or quality but was be improved by the addition of Th650 or Pl629 for the control of R. solani. The combination of solarization and biocontrolers for the control of R. solani in the summer season appears to be a good alternative to the use of MeBr.