Role of the shrub Tamarix nilotica in dissemination of Fusarium oxysporum f. sp radicis-lycopersici

The saltcedar shrub Tamarix nilotica grows as a weed in the Arava region of Israel. This weed is commonly found in cultivated fields naturally infeste d with Fusarium oxysporum f. sp. radicis-lycopersici, the causal agent of t omato crown and root rot. Young bushes, 20 to 40 cm tall, were randomly upr ooted from different fields. The roots were cut into segments which were pl aced on Fusarium-selective medium. Although the plants did not show any sym ptoms of disease, the roots of the shrub were colonized by the pathogen. Th e incidence of infected saltcedar plants and level of root colonization by F: oxysporum f. sp. radicis-lycopersici decreased with increasing distance of the sampling location from a tomato field infected with crown and root r ot. F. oxysporum f. sp. radicis-lycopersici was also isolated from chaff of inflorescence samples taken from mature T. nilotica shrubs. Identity of th e pathogen isolates obtained from T. nilotica roots and chaff samples was v erified by pathogenicity and vegetative compatibility tests. Roots of T. ni lotica plants sown under greenhouse conditions in soil naturally infested w ith F. oxysporum f. sp. radicis-lycopersici became colonized by the pathoge n. Uprooting and removing saltcedar plants throughout the season from field s not cultivated with tomatoes lowered the inoculum density of F. oxysporum f. sp. radicis-lycopersici in the soil from 611 to 6 and from 176 to 10 CF U/g of soil in the 1998-99 and 1999-2000 growing seasons, respectively. The se results demonstrate that T. nilotica may contribute to the buildup of th e pathogen populations in the absence of a susceptible host. Colonization o f saltcedar by F: oxysporum f. sp. radicis-lycopersici is an additional mec hanism for survival of this pathogen in the fields and for dissemination th rough the spread of infested seed or chaff of T. nilotica.