Influence of soil saturation and temperature on Erwinia chrysanthemi soft rot of carrot

In 1998, soft rot caused by Erwinia chrysanthemi resulted in an estimated l oss of 1,800 tons of carrots in California. The disease appeared to be rela ted to unusually high temperatures and excessive irrigation. To determine t he optimum conditions for development of soft rot under controlled conditio ns, pots of carrots inoculated with E. chrysanthemi were saturated with wat er and incubated at 20, 25, 30, or 35 degrees C. Plants were harvested and examined for disease 12, 24, 36, 48, 72, and 96 h after inoculation. Neglig ible disease occurred after 12 h. Disease severity and incidence increased with increasing temperature and duration of saturation from 24 to 96 h. In a second experiment, carrot disks were inoculated with three isolates each of E. chrysanthemi and E. carotovora subsp. carotovora and incubated at 15, 20, 25, 30, and 35 degrees C. After 48 h, the disks were washed to remove rotted tissue and reweighed. Neither bacterium reduced carrot disk weight a t 15 degrees C. In general, moderate weight reduction occurred at 20 and 25 degrees C. The greatest degree of soft rot was caused by E. chrysanthemi a t 30 and 35 degrees C. E. carotovora subsp. carotovora isolates were relati vely less virulent than E. chrysanthemi at 30 degrees C and none of the E. carotovora subsp. carotovora isolates reduced carrot disk weight at 35 degr ees C. This is the first report of E. chrysanthemi causing soft rot of carr ot in California. Based on these results, growers should limit the length o f time carrot roots are exposed to saturated soil, especially at high soil temperatures.