Update on Potato Late Blight

     Laboratory tests at NDSU this past summer indicated that 96%
of the North Dakota samples positive for late blight were of the
A2 mating type, race US 8; only 4% were of the A1 mating type,
race US 1.  In previous years, the only mating type present was
A1, race US 1.  Race US 8 is extremely agressive, and in
greenhouse trials produced symptoms in 3 days, with severe dying
of foliage by 5 days.  In contrast US1 was just beginning to
produce symptoms by 5 days.  Moreover, US 8 produces more spores
and also produces stem lesions that can rapidly destroy large
portions of a plant.  The fact that US 8 became dominant in a
single season indicates how agressive it is.  In areas where both
the A1 and the A2 mating types are present, the fungus can
undergo sexual reproduction; this may lead to greater genetic
diversity and to the development of new races of the late blight
pathogen.   
     We can expect that US 8 will be the dominant late blight
race in 1996, and that there will be problems if weather
conditions favor late blight.   Moreover, with the increase in
overhead irrigation, it is highly likely that we will see at
least some late blight in 1996, particularly near the centers of
pivots.  Since US 8 is so agressive and develops so quickly,
careful scouting will be critical to late blight control in 1996. 
Use of the Blight Hot Line will also be very important.  Timely
action will be essential for late blight control.  Other factors
that are important include the destruction of cull piles,
volunteers and seedlings.