Studies on a further race (W. 4) of Bremia lactucae Regel

A further race (W. 4) of Bremia lactucae was identified in a sporangial collection from an infected field crop of the cultivar Avondefiance by tests on detached seedlings and leaf disks of various diagnostic lettuce cultivars. Race W. 4 appeared to be unstable, with a similar host range to race W. 3 , but differed from it in its inability to infect the crisp-lettuce cultivars Grand Rapids, Calmar, Caravan, Francisco and Great Lakes.
Races W. 1, W. 2 and W. 3 were also found in tests starting with the sporangial collection from Avondefiance. Although W. 3 could have been present in small quantities in this original collection, it was considered that W. 1 and W. 2 arose spontaneously from W. 4 during subculturing.
A means of eliminating W. 2 from mixtures of W. 1 and W. 2 is described.
A laboratory experiment indicated that when competing in a crop of Avon-defiance or Cobham Green, race W. 2 might predominate over race W. 3.