Components of aggressiveness in during adaptation to multiple sources of partial resistance in tobacco.

Black shank is a devastating disease of tobacco caused by . Host resistance has been an integral part of black shank management, but after the loss of single-gene resistance following its widespread deployment in the 1990s, growers have relied on varieties with varying levels of partial resistance. Partial resistance is effective in suppressing disease, but continued exposure can result in an increase in pathogen aggressiveness that threatens durability of the resistance to . Aggressiveness components in were characterized following adaptation on two sources of partial resistance, Fla 301 and the gene from . An aggressive isolate of the two major races of , race 0 and race 1, were adapted for either 'one/two' or 'five/six' generations on the two resistance sources, giving four sets of isolates based on race, number of generations of adaptation, and source of resistance. Across the four sets of isolates, adapted isolates infected higher proportions of root tips, produced more sporangia per infected root tip, and caused larger lesions than their respective non-adapted isolates of the same race and from the same resistance source. Adapted isolates also produced more aggressive zoospore progeny than the non-adapted isolates. Adaptation to partial resistance involves multiple aggressiveness components that results in the increased aggressiveness observed for . These results improve our knowledge on the nature of adaptation to partial resistance in tobacco and indicate that different resistance sources are likely to select for similar aggressiveness components in the pathogen.