Evaluation of Trichoderma harzianum Strains to Control Crown and Root Rot of Greenhouse Fresh Market Tomatoes

Greenhouse tomato growers in the United States have few products available for chemical control of plant pathogens. Biological control of soilborne plant pathogens by antagonistic microorganisms is a potential alternative to the use of chemical pesticides during greenhouse production. Biological control experiments were conducted to test the effects of commercial and noncommercial strains of Trichoderma harzianum against Fusarium oxysporum f. sp. radicis-lycopersici on tomato plants grown in two different hydroponic media, coir and rockwool. Trichoderma harzianum is a fungus that attacks a range of economically important phytopathogenic fungi. Tomato (Lycopersicon esculentum Mill., cultivar ‘Caruso’) plants were inoculated with T. harzianum strains (PlantShield, T22 and T95) prior to challenge with the pathogen. They were applied into growing media prior to sowing and to roots at transplanting at two inocula densities, 10 or 10 conidia/ml. The results of this study demonstrated that T. harzianum strains, especially applied at transplanting, decreased disease incidence 79% for coir and 73% for rockwool, decreased disease severity 45% for coir and 48% for rockwool, and increased fruit yield 37% for coir and 25% for rockwool on tomato for Fusarium control. INTRODUCTION Fusarium crown and root rot (FCRR) caused by Fusarium oxysporum f. sp. radicis-lycopersici Jarvis & Shoemaker (FORL) is one of the most prevalent soil borne diseases of tomato. This disease occurs in both greenhouse and field worldwide and causes significant losses in tomato production. In closed systems, with recirculation of nutrient solution and rock wool as a growing medium, crown and root rot of tomato is a serious problem (Hartman and Fletcher, 1991; Rattink, 1992). The use of Fusarium-resistant tomato cultivars can provide some degree of control of FCRR, but the occurrence and development of new pathogenic races is a continuing problem. Currently there are no commercially acceptable cultivars with adequate resistance to FORL (Jarvis, 1988; Jones et al., 1991; McGovern et al., 1993). FCRR is generally controlled in tomato by pre-plant soil fumigation with methyl bromide (MBr). Tomatoes represent the largest single-crop use of MBr in the United States accounting for 25% of the total MBr use for soil fumigation (Anonymous, 1994). However, fumigation with MBr is expensive and not always an effective measure due to rapid colonization of growing media by FORL (Rowe et al., 1978; Gabor and Wiebe, 1997). In addition to other potential health, safety and environmental risks, concerns over the ozone-depleting properties of MBr have led to the phasing out of its use. Therefore, alternative control measures are necessary and need to be made available as soon as possible. Biological control is an alternative to the use of chemical pesticides. Biological fungicides may act to suppress the population of the pathogenic organism through competition with pathogenic organisms, stimulate plant growth, which may allow plants to quickly outgrow any pathogen effects, or damage the pathogen by means of toxins produced (Cook, 2000; Gilreath, 2002). A variety of soil microorganisms have Proc. XXVI IHC – Managing Soil-Borne Pathogens