Alternaria black spot is caused by the fungi Alternaria brassicae, A. brassicicola, and A. raphani. The pathogens infect their hosts by direct penetration or through wounds and natural openings. The disease is favored by warm, humid weather during mid- and late-season development. These pathogens are disseminated as conidia within and among fields by splashing water and wind. The pathogens survive between mustard and canola crops in infested crop debris and on seed.
Alternaria black spot symptoms appear as brown to black circular spots on leaves that enlarge under favorable conditions. Lesions become gray with concentric rings and develop a purple or black border. Lesions on stems and pods are black or black with gray centers. Under dry conditions, lesions remain small and black, and often develop a yellow halo. Severe defoliation can occur during disease-favorable weather. Pods with infected pedicels fail to develop and drop off the plant. Severely spotted pods dry, shrink, and may split open prematurely, allowing shrunken seeds to drop to the ground. The Alternaria black spot pathogens can also cause a seed rot or postemergence damping-off.
No biological control strategies have been developed for Alternaria black spot.
Plant high quality seed free from the Alternaria black spot pathogens. Practice a three-year or longer rotation to non-hosts such as small grains. Promptly incorporate crop residues after harvest to hasten their breakdown. Eliminate volunteer canola, mustard and cruciferous weeds.
Chemical controls are most effective when combined with sound cultural practices. Copper fungicides and neem oil are not highly effective.
Product List for Alternaria Black Spot:
Pesticide |
Rate per Acre |
Application Frequency |
Remarks |
Azoxystrobin |
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Quadris |
6.2-15.4 oz |
7-14 days |
Maximum of three applications (27 oz); Beginning applications at 10-25% bloom; Do not make application past 95% petal fall; |
Boscalid |
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Endura |
5-6 oz |
21 days |
Maximum of two applications per season; also provides white mold suppression |
Copper fungicides: not all formulations listed |
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Champ DP |
0.33-0.66 lb |
7 days |
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Cuprofix Disperss |
1.5-2.5 lb |
7-10 days |
Can cause phytotoxicity |
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Kocide 2000 |
0.75-1.5 lbs |
7-10 days |
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Nordox |
1.0-3.0 lb |
7-10 days |
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Nu Cop 50 WP |
1.5-2.0 lb |
7-10 days |
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Neem |
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Trilogy |
1% solution |
7-14 days |
Apply in 25 to 200 gallons |
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The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality. |
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Categories: Canola and Mustard, Disease, Alternaria Black Spot
Date: 12/30/2004