Late blight is a disease that we deal with to some extent in the mid-Atlantic each year.  When reports come in you may see something mentioned about the lineage that was detected.  What exactly are we telling you when we mention this information?  Each lineage has unique genetic characteristics that may impact management practices and also level of concern in the region.  Some lineages prefer to cause disease on potatoes, some tomatoes, and some both.  In addition, some are sensitive, intermediate, or insensitive to mefanoxam fungicides.  As a quick reference, you can view the recent lineages reported in the US HERE.  I have simplified the information for you below.


Genotypea Host Mating type Sensitivity to mefenoxamc
US-8f Potato A2 I/R
US-11 Potato/Tomato A1 R
US-20 Tomato A2 I/R
US-21 Tomato A2 S/I/R
US-22g Potato/Tomato A2 S/I
US-23 Potato/Tomato A1 S/I
US-24 Potato A1 I


Source  Summary of multilocus genotypes of Phytophthora infestans collected in the US and Canada, 2002-2009 (from Hu et al., Plant Dis. 2012 and Fry et al., APSnet Features 2012)