Relation between Suppressiveness to Tomato Fusarium Wilt and Microbial Populations in Different Growth Media

Tomato Fusarium wilt suppressiveness of three growth media, with respect to peat and vermiculite, and the relationship between microbial populations of growth media in the severity of this pathogen were examined. The growth media evaluated were olive oil husk + cotton gin trash composted and mixed with rice husk (OC+R), spent mushroom composted and mixed with peat (SM+P) and coir fibre. To determine the role of the microflora in the suppression, the two composts were also heated to 60oC for 6 days. Three bioassays were carried out with these infested growth media and controls not infested. Disease severity was recorded during 25 days after tomato seedling transplantation. In these seven growth media the density of culturable groups of microorganisms was determined by dilution plating on semi-selective media before the bioassays. The two composts showed suppressiveness to Fusarium wilt with respect to vermiculite and peat while coir fibre was conductive. Furthermore, in OC+R its suppressiveness is due more to the growth media’s non heat-labile properties than in SM+P. Significant negative correlations were found between severity and Bacillus spp. , cellulolytic actinomycetes and bacteria, oligotrophic actinomycetes and bacteria, copiotrophic bacteria, total actinomicetes (cellulolytic + oligotrophic + copiotrophic) and total bacteria (cellulolytic + oligotrophic + copiotrophic) populations in these growth media. Bacillus spp. are known antagonists, so they may account for the suppressiveness in our composts. Cellulolytic and oligotrophic actinomycete populations were associated with Fusarium wilt suppressiveness for another growth media. High microbial activity has been reported as a Fusarium wilt suppressiveness factor. Copiotrophic, celullolitic and oligotrophic bacteria contribute to this microbial activity. INTRODUCTION Tomato production with intensive systems in Spain is increasingly using potting culture with organic substrates. One of the tomato crop problems is Fusarium wilt which cause important yield losses, and for these problems there are no effective chemical control measures. However, biological control is becoming an efficient alternative for controlling these diseases. Some growth media formulated with composts are able to suppress Fusarium wilt with respect to peat (Chef et al., 1983; Orlikowski, 1983; Trillas-Gay et al., 1986; Garibaldi, 1988; Pera and Calvet, 1989; Hoitink et al., 1991; Serra-Wittling et al., 1996; Szczech, 1999; Cotxarrera et al., 2002; Reuveni et al., 2002; Trillas et al., 2002). The nature of soils and composts with suppressiveness to Fusarium wilt is the result of complex interactions between the abiotic characteristics of the media and microbial populations (Hoitink et al., 1993; Alabouvette et al., 1996; Weller et al., 2002; Borrero et al., 2004b). Several biocontrol agents have been identified in these soils and composts (Hoitink et al., 1993; Weller et al., 2002). The microbiological nature of Fusarium wilt suppressiveness for many compost-amended growth media has been demonstrated previously (Chef et al., 1983; Orlikowski, 1983; Trillas-Gay et al., 1986; Garibaldi, 1988; Hoitink et al., 1991; SerraWittling et al., 1996; Szczech, 1999; Cotxarrera et al., 2002; Reuveni et al., 2002; Trillas et al., 2002). The suppressiveness to soilborne plant pathogenic fungi mechanisms involve general suppression, which is related to high microbial activity, and specific suppression, which is related to an increase in the population of specific microorganisms or groups of microorganisms that act as antagonists to the pathogen (Cook and Baker, 1983). On the other Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilan Acta Hort. 697 ISHS 2005 426 hand, general disease suppression can be dependent on communities of microorganisms. These communities may be associated with a substrate at a particular stage of decomposition under certain environmental and management conditions (Boehm et al., 1993, 1997). The composition of functional groups (rather than individual species) may determine the character of the community while individual species within a functional group may be interchangeable (van Bruggen and Semenov, 2000). The aims of this research were to evaluate three growth media Fusarium wilt suppressiveness with respect to peat and vermiculite and to study the relationship between microbial populations of growth media on the severity of Fusarium wilt in order to predict disease suppressiveness of growth media. MATERIAL AND METHODS Plant Growth Media Two composted residues from agricultural and industrial wastes and fertilized coir fibre (Cocopeat, Projar, Valencia, Spain) were evaluated for Fusarium wilt suppression. The growth media made with composts were olive oil husk + cotton gin trash, 1:1 v/v, composted and mixed with rice husk (1:1 v/v) (OC+R), and spent mushroom composted (Recomsa, Quintanar del Rey, Spain) mixed with light peat (1:1 v/v) (SM+P). The OC+R was composted as described elsewhere (Trillas et al., 2002). Fertilized light peat (Klasmann, Valinex, Palleter, Spain) and vermiculite (Vermiculita y derivadas, Gijon, Spain) was used as a reference. Peat was neutralized with 4 g L CaCO3. Fertilized peat and coir fibre had 0.33 g L of K2O (50% high solubility granulated; Compo Agricultura S.L., Barcelona, Spain) and 4.15 g L of P2O5 (18% granulated; Fertiberia, Madrid, Spain). Plant growth media made with composts were not fertilized. To standardize initial conditions, the growth media were incubated at a water tension of 1000 Pa (adjusted for weight) for 14 days at 25oC. To determine the role of microflora in suppression, the two composts were also heated to 60oC for 6 days (OC+R60 and SM+P60). Assessment of Disease Severity Disease suppressive properties of growth media were measured by a Fusarium wilt bioassay described elsewhere (Cotxarrera et al., 2002). The bioassays were developed with tomato `Roma and a monosporic isolate of Fusarium oxysporum f. sp. lycopersici race 2 (FOL-2). This isolate was obtained from tomato plants and stored in silica gel. Liquid culture, 500 ml 1% malt extract (Sigma Chemical Company, St Louis, MO), of FOL-2 was prepared and grown with continuous agitation (130 rpm) for 10 days at 25oC. Conidia were recovered after filtration and centrifugation at 5000 rpm, 15 min, (Eppendorf 5810 R, Hamburg, Germany) and rinsed twice with sterile distilled water. The concentration of conidia was determined with a hemocytometer. The seven growth media were infested with FOL-2 (10 microconidia ml growth media), mixed vigorously and poured into 9 cm diameter plastic pots (330 ml volume). Non infested growth media were used as controls. Four tomato seedlings (1 to 2 true leaf stage) grown in vermiculite were transplanted into each pot. Plants were irrigated as needed and fertilized with Peters foliar feed 27-15-12 (Scotts Heerlen, The Netherlands), 0.5 g L enriched with 0.6 g L of CaCl2, 0.696 g L MgSO4.7H2O and 0.3 g L of urea (46%). They were grown in a growth chamber (27oC, 280 μE/m2/s PAR intensity, 16:8 h light:dark photoperiod and 60% RH). Disease severity was monitored for 25 days after planting and was scored every two days based on a symptom severity scale where: 0 = asymptomatic plants; 1 = weakly infected plants (≤ 50% of leaves chlorotic or wilted); 2 = highly infected plants (> 50% of leaves wilted but plants not dead) and 3 = dead plants (Cotxarrera et al., 2002). The area-under-the-disease-progress-curve (AUDPC) was calculated, and AUDPCs was also calculated by dividing AUDPC by the total time duration (days) of the epidemic of each bioassay for comparison between epidemics in different bioassays (Campbell and Madden, 1990). These bioassays were repeated at least three times with five pots per treatment. At each rating time, the mean of the disease severity and the percentage of symptomatic plants per pot were calculated. This mean was considered as one value. Treatments were arranged in a randomized block design. Microbial Populations