Studies on seed-transmission of Pantoea (Erwinia) stewartii
Studies have been carried out in United States on the seed transmission (plant-to-seed and seed-to-seedling) of Pantoea (Erwinia) stewartii (EPPO A2 quarantine pest), considering also the influence of resistant and susceptible seed parents. The authors noted that differences in rates of transmission of P. stewartii reported in the early literature and in recent studies, where only very low transmission is observed (e.g. 29 infected seedlings from 53,600 seeds from infected plants), may be partly due to improved levels of resistance in maize cultivars and hybrids. In the field, plants of sweet maize and maize were inoculated with a rifampicin-resistant strain or wild-type strains of P. stewartii, seeds produced were tested for the bacterium and the transmission from seed to seedling was evaluated in field and greenhouse plantings. Results showed that P. stewartii was detected in seeds produced on four hybrids which had presented systemic symptoms after leaf inoculation. But the bacterium was not detected in cultivars and hybrids which did not show systemic symptoms after leaf inoculation. When plants were inoculated in ear shoots, ear shanks or silk channels, P. stewartii could then be isolated from cob and shank tissues and from seeds. The authors pointed out that in their studies, there was no evidence of seed-to-seedling transmission of P. stewartii when over 75,000 seeds from infected plants were planted in field or glasshouse trials. They felt that this could be due to the limited extent to which seed-parent plants were infected, as systemic infection was only observed in a few cases. They concluded that the risk of transmitting P. stewartii to seedlings is nearly nil for seeds obtained from plants with less than 50 % leaf area diseased, and that quarantine restrictions imposed on maize seeds will need to be revised. This statement will certainly stimulate further discussions on this difficult subject of seed transmission.
Khan, A.; Ries, S.M.; Pataky, J.K. (1996) Transmission of Erwinia stewartii through seeds of resistant and susceptible field and sweet corn.
Plant Disease, 80(4), 398-403.