Coconut fiber grow bags as a source of primary inoculum of Fusarium oxysporum f. sp. radicis-lycopersici in greenhouse tomato crops in Almeria (Spain)

Fusarium oxysporum f. sp. radicis-lycopersici (Forl) is a specific pathogen that affects tomato plants in soil and substrate crops. In southeastern Spain, use of coconut fiber substrate has increased, overtaking use of non-organic substrates. Coconut fiber (CF) grow bags were used in a newly constructed greenhouse, where tomatoes were never grown before. In the second year of CF cultivation, crown and root rot symptoms appeared, while tomato plants grown in new CF did not show any symptoms. It was verified that Forl was associated with the symptoms. Once other primary inoculum sources were dismissed, it was determined whether or not the fungus was initially present in the CF. Different CF samples were analyzed by using Fusarium selective medium, and some of the so-identified isolates of F. oxysporum were inoculated on 'San Pedro' tomato seedlings. In parallel, CF samples were analyzed by soil phytopathometry (bait plants), mixing sterile vermiculite with CF in 1-L pots in which tomato seedlings susceptible to Forl were grown. The pots were kept under controlled conditions. CF samples from cultivated tomato bags showed a high Forl percentage. Both methods allowed the detection of the forma specialis. Unused CF bags, from the same batch as the previous, also displayed Forl, although the population was considerably lower. Other analyzed CF substrates showed no Forl. These results suggest that Fusarium oxysporum f. sp. radicis-lycopersici was present in unused CF grow bags, playing a role as a source of primary inoculum for the disease.