The Blackleg fungus is capable of causing yield loss in canola, so an early season fungicide application might be considered for fields at high risk for the disease. High risk scenarios include a short crop rotation, planting the same hybrid multiple times (even if resistant), a history of disease in the field and frequent rains after emergence. Importantly, multiple PG’s (think races) of the pathogen exist, some of which can overcome the genetic resistance in hybrids. Consequently, monitoring even resistant hybrids for disease is important.
The fungus that causes blackleg can survive on canola residues for two to three years and is capable of releasing spores during that period. These spores can be transported by air from neighboring fields or by water splash within the same field. While canola plants are susceptible to blackleg at any stage of their development, usually infections that take place when canola plants have less than four leaves result in economic yield losses. Lesions on cotyledons of susceptible hybrids resemble scalding of tissues with gray to bleached areas (Figure 1A), and under favorable weather conditions, could have pycnidia in them. Pycnidia have the appearance of tiny black dots in the lesions (Figure 1B). Sometimes infected cotyledons may turn yellow faster than normal (Figure 1C), in part due to the action of toxins produced by the pathogen. Blackleg resistant hybrids on the other hand, could create a ring of darkened tissues around the point of infection (Figure 1D) which limits the development of the pathogen and prevents it from reaching the stem tissues.
The protection offered by most seed treatments usually wears off approximately two weeks after emergence of the seedlings. After that, seedlings will be infected if inoculum is available and weather conditions are favorable.
Growers should scout their fields for symptoms of infection starting on the second week after planting and continuing until the plants have passed the four-leaf stage. If blackleg symptoms are present in 20% or more of the seedlings, consider a fungicide application. Growers should keep in mind that most fungicides registered to manage blackleg are more effective when applied at the two to four leaf stage. Fungicide applications made later will not control the pathogen effectively.
A list of fungicides registered for use in canola against blackleg can be found in the 2017 North Dakota Field Crop Plant Disease Management Guide (aka Fungicide Guide or publication PP-622). Remember to read and follow the label if making a fungicide application.
Extension Plant Pathologist, Broad-leaf Crops
Luis del Rio Mendoza