Spatial distribution and temporal development of Fusarium crown and root rot of tomato and pathogen dissemination in field soil

The spatial distribution and temporal development of tomato crown and root rot, caused by Fusarium oxysporum f. sp. radicis-lycopersici, were studied in naturally infested fields in 1996 and 1997. Disease progression fit a lo gistic model better than a monomolecular one. Geostatistical analyses and s emivariogram calculations revealed that the disease spreads from infected p lants to a distance of 1.1 to 4.4 m during the growing season. By using a c hlorate-resistant nitrate nonutilizing (nit) mutant of F. oxysporum f, sp. radicis-lycopersici as a "tagged" inoculum, the pathogen was found to sprea d from one plant to the next via infection of the roots. The pathogen sprea d to up to four plants (2.0 m) on either side of the inoculated focus plant . Root colonization by the nit mutant showed a decreasing gradient from the site of inoculation to both sides of the inoculated plant. Simulation expe riments in the greenhouse further established that this soilborne pathogen can spread from root to root during the growing season. These findings sugg est a polycyclic nature of F. oxysporum f. sp. radicis-lycopersici, a devia tion from the monocyclic nature of many nonzoosporic soilborne pathogens.