Plant pathologists from across the Prairies are surveying canola for disease levels. For each field surveyed, they select 20 random plants at 5 random locations for a total of 100 plants.
For blackleg, they pull up each plant and clip the stem just below ground level. Each stem is then rated for what percentage of the stem cross section is discoloured by blackleg infection. A rating of “0” means no blackleg stem discolouration. A rating of “5” means the stem is completely discoloured and blackleg has killed the plant. The graphic and embedded video show more detail on the other ratings.
Varieties with a resistance or R rating for blackleg should have very low infection levels — no more than a “2” rating. If R-rated varieties are showing damage levels above “2”, that means the blackleg population in that field has shifted and the variety is no longer resistant to the blackleg races in that field.