Re-evaluation of seed transmission of Clavibacter michiganensis subsp. nebraskensis in Zea mays.
The spread of Goss's bacterial wilt and leaf blight of corn (Zea mays), caused by Clavibacter michiganensis subsp. nebraskensis, to a wider geographic range in the early 2000s compared with the late 1960s has generated concern about the possible role of seed transmission in long-distance spread. The objectives of this research were: (1) to determine the percentage of seed infection found in seed harvested from inoculated and noninoculated plants of hybrids that varied in resistance to Goss's wilt; and (2) to estimate the seed transmission rate from these infected seed lots. The greatest percent seed infection was detected in seed from inoculated plants of the most susceptible hybrid and the least in seed from the most resistant hybrid. Seed lots with seed infection that ranged from 3.6 to 37.0% were planted in three field and three greenhouse trials. A total of 12 seed transmission events (Goss's wilt symptomatic seedlings) were identified among 241,850 plants examined, for a seed transmission rate of 0.005%. When the seed transmission rate was recalculated to consider only the infected seed portion of each seed lot, the rate increased to 0.040% (12 events from 30,088 potentially infected plants). Based on the low seed transmission rate observed and previous research on disease spread from a point source, it seems unlikely that seed transmission could introduce enough inoculum to create a serious disease outbreak in a single growing season. However, risk of seed transmission is relevant for phytosanitary restrictions and preventing the introduction of the pathogen to new areas. To date, Goss's wilt has not been detected outside North America, and while the risk of seed transmission is very low, the risk is not zero. Fortunately, the presence of C. michiganensis subsp. nebraskensis in corn seed is readily detectable by established seed health testing methods.