BACTERIAL LEAF-SPOT OF CELERY IN CALIFORNIA - ETIOLOGY, EPIDEMIOLOGY,AND ROLE OF CONTAMINATED SEED

Pseudomonas syringae pv. apii, causal agent of bacterial leaf spot (BL S) of celery, was first identified in California in 1989. By 1991, BLS was apparent in all celery-growing areas of the state. Greenhouse-pro duced transplants were affected most severely, and disease incidence a pproached 100% in some greenhouses. In this study, sources of inoculum and factors contributing to disease development were investigated in three Salinas Valley greenhouse operations during the 1991, 1992, and 1993 celery transplant seasons (January to August). Epiphytic P. syrin gae pv. apii was not detected on celery transplants until April or May of each year. Increased epiphytic populations preceded BLS outbreaks, and high-pressure, overhead irrigation favored bacterial infiltration and disease development. In seed-wash assays, P. syringae pv. apii wa s recovered from 5 of 24 commercial celery seed lots. In field tests, epiphytic P. syringae pv. apii was found on umbels of inoculated celer y plants, and seeds from these plants were heavily contaminated with P . syringae pv. apii. Contaminated seed produced seedlings with large e piphytic P. syringae pv. apii populations. Hot-water treatment (50 deg rees C for 25 min) eliminated >99.9% of seed contamination. Based on t hese results, disease management techniques are proposed.