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Potential for Blackleg Leaf Spot in Canola

By Mike Stamm
 
Blackleg leaf spot has been observed in canola plots near Manhattan, Kansas (Figure 1). This is an intentional canola-on-canola study looking at control of different fungicide treatments. Nonetheless, conditions have been favorable for blackleg leaf spot development this year. For Kansas producers, blackleg would be of greatest concern in fields where canola has been grown before, where blackleg has been previously observed, and where tight crop rotations are being used (e.g. one year of wheat/one year of canola).
 
Figure 1. Canola plants showing some leaf loss from blackleg in a fungicide trial near Manhattan, Kansas on October 31, 2018
 
Once the blackleg fungus is present in a field, fruiting bodies called pseudothecia form on infected residue in the fall and spring. Pseudothecia release a type of spore known as ascospores. These spores can become airborne, starting the infection cycle in a developing canola crop. Because of the ascospore release, blackleg will spread to fields where it may not have been present. Weather conditions play a large part in how significant the ascospore release may be. Generally, cooler temperatures and wet weather, like we have seen in fall 2018, are conducive to widespread ascospore release. Blackleg may also be spread through infected seed, however use of a seed treatment at planting is one way to slow its spread.
 
Identification of blackleg
 
Beige-colored, round or irregularly shaped lesions will appear on older leaves (Photo 2). Upon closer inspection of the lesions, tiny black dots can be seen within the borders. These dots are pycnidia. From within these pycnidia, tiny spores known as conidia are released. Conidia are splashed by wind and rain onto the stems and leaves of nearby plants where new lesions will form. The areas of heaviest fall infection will likely show up as disease hot spots later in the growing season.
 
Figure 2. Beige, irregularly-shaped blackleg lesions showing picnidia on older canola leaves 
 
Once the lesions form on leaf and stem tissue, the fungus grows through the plant’s vascular tissue to the crown where decay begins, causing the stem canker phase of the disease. Stem cankers are usually first observed in the spring as inconspicuous bluish lesions at a petiole scar near the soil line. The lesions develop into elongated, light brown sunken areas with purplish or black margins. As the lesions lengthen, stems become girdled and blackened.
 
Control options
 
Applying foliar fungicides when blackleg lesions are present in the fall is a common practice for canola producers. Some fungicides labeled for blackleg control are provided in Table 1. Generally, applications made in areas where disease pressure is greatest are more economical than applications made in areas where disease pressure is low. There are no thresholds for foliar applications in the fall, but if lesions are appearing on newly emerged canola leaves, then a foliar application may be justified.
 
Table 1. Foliar fungicides labelled for control of blackleg in canola.

Active Ingredient (MOA Group)

Common Name

Rate

Comments

Azoxystrobin (11)

Quadris

6.0 to 15.5 fl. oz.

Apply at the 2- to 4-leaf stage when leaf spot first appears.

Prothioconazole (3)

Proline 480SC

4.3 to 5.7 fl. oz.

Apply at the 2- to 4-leaf stage when leaf spot first appears.

Pyraclostrobin (11)

Headline

6 to 12 fl. oz.

Apply at the 2- to 4-leaf stage when leaf spot first appears.

Pyraclostrobin (11) + fluxapyroxad (7)

Priaxor

4 to 8 fl. oz.

Apply at the 2- to 4-leaf stage when leaf spot first appears.

*Adapted from 2017 OSU Extension Agents’ Handbook of Insect, Plant Disease, and Weed Control. E-832. Oklahoma State University.

**Consult product label for directions. Brand names used are for product identification purposes only.