Occurrence of race 4 of Phytophthora nicotianae in tobacco

While conducting tests on a set of host differentials to identify races 0 and 1 of P. nicotianae, a new race, race 4, was identified based on the ability to overcome a resistance gene from Nicotiana longiflora but not the Ph gene from N. plumbaginifolia. Race 1 overcomes both genes and race 0 neither gene. Race 4 was recovered from field sites in NC and Kentucky where the N. longiflora gene was deployed. In addition, race 4 was recovered from multiple field sites in NC where varieties with only partial resistance from Fla. 301 were deployed. The development of race 4 on tobacco varieties with partial resistance was confirmed in greenhouse tests. Flats were planted with varieties with low, medium, or high levels of partial resistance and soil was infested with an isolate of race 0, 1, or 4. Isolates were recovered from soil after 140 days and inoculated onto host differentials. Races 0 and 4 were recovered from flats infested with race 4, and race 4 was recovered from all flats infested with races 0 and 1. Race 1 was not recovered from race 0 or race 4 flats. The mechanism for this previously unreported race shift is under investigation.