Sinaloa state accounts for 23% of Mexico's tomato production. One constraint on this important crop is the Fusarium crown and root rot, caused by Fusarium oxysporum f. sp. radicis-lycopersici, which has been reported to reduce crop yield by up to 50%. In this study, we set out to identify bacterial populations which could be used to control this disease through natural antagonism. Five tomato rhizospheric soil samples were collected, dried for 1-week, and homogenized. Sub-samples were used to prepare an aqueous solution used to isolate microorganisms in pure cultures. Organisms were purified and grown separately, and used to generate a collection of 705 bacterial isolates. Thirty-four percent from this bank (254 strains) was screened against Forl, finding 27 bacteria displaying in vitro Forl growth inhibition levels from 5% to 60%. These isolates belonged to the genus Bacillus and their 16Sr DNA sequences showed that they are closely related to seven species and they were putatively designated as: B. subtilis, B. cereus, B. amyloliquefaciens, B. licheniformis, B. thuringiensis, B. megaterium, and B. pumilus. One isolate belonged to the genus Acinetobacter. Two B. subtilis isolates (144 and 151) and one B. cereus isolate (171) showed the best antagonistic potential against FCRRT when evaluated on seedlings. Plate and activity assays indicate that these isolates include a diverse repertoire of functional antagonistic traits that might explain their ability to control FCRRT. Moreover, bacteria showed partial hemolytic activity, and future research will be directed at ensuring that their application will be not harmful for humans and effective against Forl in greenhouse or field conditions.

Resumen. La diversidad de los microorganismos asociados a la rizosfera de diferentes especies vegetales en los suelos, en México se ha estudiado poco y se ha abordado de manera convencional, con técnicas microbiológicas limitadas debido al elevado porcentaje de microorganismos no-cultivables (95-99%). En el presente trabajo se empleó el análisis por secuenciación del ADN ribosomal (ADNr) para evitar esa limitante y explorar mejor la diversidad de los microorganismos cultivables y no-cultivables asociados al jitomate (Solanum lycopersicum L.) en un agroecosistema en Sinaloa. Se empleó ADN genómico extraído del suelo rizosférico para amplificar una región hipervariable en el ADNr empleando oligonucleótidos universales para ADNr procariota y eucariota. El análisis de 194 y 384 secuencias de ADNr de origen procariota y eucariota, respectivamente, mostró que los phyla eucariotes más abundantes fueron Ascomycota (59%), Chlorophyta (21%) y Basidiomycota (12%), y los más abundantes de origen procariote fueron Firmicutes (45%), Proteobacteria (14.7%) y Gemmatimonadetes (13.1%). El presente trabajo representa a la fecha la caracterización más completa de la diversidad de microorganismos de la rizosfera del jitomate. Se discute el papel que especies identificadas en este trabajo, pertenecientes a géneros procariotas (Bacillus y Paenibacillus) y eucariotas (Alternaria), pudieran desempeñar en la rizosfera del jitomate y en el control biológico de fitopatogénos en esta especie.