Survival of a tropical isolate of Xanthomonas oryzae was studied in different substrates in relation to temperature and humidity, using a streptomycin-resistant mutant and direct isolation on a streptomycin agar.  Survival was longer at low than at high relative humidities (RH) and temperatures; but higher temperatures with very low RH also permitted long survival.  X. oryzae survived less than one month in warm (30 C), flooded, or moist soil or in leaves buried in such soil.  In ooze droplets, or in diseased leaves in air at warm temperatures and 100% RH, the pathogen survived only 5-40 days.  Lowering RH to 0-20% lengthened such survival to almost one year.  The pathogen survived only briefly in warm rice-paddy water.  These data indicate that the field disease cycle should be breakable in the wet tropics by having hosts absent for two months.  In regions with dry or cold off-seasons, the disease cycle should continue regardless of periodic host absence.

Additional key words: bacterial ecology.