Y REKAH¹, D SHTIENBERG and J KATAN

¹Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agricultural, Food and Environmental Quality Sciences, Rehovot 76100, Israel; Department of Plant Pathology, ARO, The Volcani Center, Bet Dagan 50250, Israel

Background and objectives Tomato crown and root-rot, caused by Fusarium oxysporum f. ;sp. radicis-lycopersici is a potentially destructive disease. It is generally accepted that most soil-borne pathogens are monocyclic in nature, whereas foliar pathogens are polycyclic, although some exceptions exist. Such characterization of pathogens has important consequences for disease management. The purpose of this study was to determine whether F. ;o. f. ;sp. radicis-lycopersici is a polycyclic pathogen, namely, if it has the ability to complete several disease cycles per growing season.

Materials and methods
The spatial and temporal development of tomato crown and root-rot, was followed in naturally infested fields in the years 1996 and 1997. Disease incidence assessments were initiated shortly after the onset of the disease and continued at 14-day intervals. Geostatistic analysis was used to quantify the temporal and spatial distribution of the disease [1].

Semivariogram function was calculated separately for each experimental plot for each of the disease assessments. The spread of the pathogen from diseased to neighbouring plants was also studied in fumigated plots using tagged inoculum as nit mutants of the pathogen [2]. On planting day, bags with inoculum were placed near the plant root and pathogen's spread was determined by plating roots of diseased adjacent plants on chlorate amended selective medium.

Results and conclusions
Geostatistic analyses and semivariogram calculations showed that during the season the disease spreads in the field to a distance of a few metres. In the experiments where the chlorate-resistant nitrate non-utilizing (nit) mutant of F. ;o. f. ;sp. radicis-lycopersici was used, it was found that the pathogen was able to spread from one plant to another and infect adjacent plants during the growing season up to four plants (2.0 ;m) from an 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, thus further indicating a dissemination via root to root contact.

These findings suggest a polycylic nature of F. ;o. f. ;sp. radicis-lycopersici, which differs from the monocyclic nature of many soil-borne pathogens.

References
1. Journel AG, Huijbregts CJ, 1978. Mining Geostatistics. New York: Academic Press.
2. Hadar E, Katan J, Katan T, 1989. Plant Disease 73, 800-3.