Epiphytic survival of Pantoea ananatis on Richardia scabra L. in Georgia

Abstract

Pantoea ananatis, the causal organism of center rot of onion (Allium cepa L.), can survive on different weeds but, in a previous survey, it was most commonly found on Florida pusley (Richardia scabra L.). The epiphytic survival of P. ananatis on R. scabra under different temperature and moisture regimes was investigated. Weed seedlings were spray inoculated with rifampicin-resistant strain PNA 97-1rif at either 103 or 108 CFU/ml and incubated in a growth chamber at 15.5 or 21.1°C at 65% relative humidity for 96 h postinoculation (hpi), which represented the mean environmental conditions during mid-March to mid-May in Vidalia, GA when onion production and R. scabra presence overlap. For plants inoculated with P. ananatis at 103 CFU/ml, the bacterium survived for 96 hpi when incubated at 21.1°C, with mean populations of 1.7 × 102 CFU/g of leaf tissue. In contrast, no viable bacteria were detected after 72 hpi at 15.5°C. For plants inoculated with P. ananatis at 108 CFU/ml, the bacterium survived for 96 hpi at 21.1°C (3.8 × 105 CFU/g) whereas, during the sample time period, viable bacterial populations were not detected at 15.5°C. Survival of P. ananatis on R. scabra was also monitored during alternating 12 h wet and 12 h dry periods, or continuous wet or dry periods for 96 hpi at 15.5 or 21.1°C. Compared with initial or continuous dry periods, P. ananatis survived significantly better with a 12 h wet/12 h dry cycle or a continuous 96 hpi wet period at both 15.5 and 21.1°C. Unlike at 15.5°C, P. ananatis populations (7.4 × 102 CFU/g) survived for 96 hpi at 21.1°C under a cycle of 12 h dry and 12 h wet. These results demonstrate that P. ananatis can survive on R. scabra leaves under conditions of 21.1°C and prolonged leaf wetness and may potentially serve as a source of inoculum to onion.