Black mold is caused by the fungus Aspergillus niger, and can be a problem wherever onion is grown. The fungus can grow saprophytically on dead plant and animal tissues, and is a common soil inhabitant. The fungus can also grow within all onion tissues as an endophyte, but only causes symptoms on plants when environmental conditions are favorable. Infection occurs when spores land on and colonize leaves through wounds or natural openings, through wounds or cut necks at harvest, or endophytically by contaminated seed. The pathogen can survive between onion crops as a saprophyte on decaying organic matter, in onion seed, and on cull onions.
Blue mold is caused by several species of the fungus Penicillium, and is principally a pathogen only on stored bulbs. Like A. niger, the fungus can grow saprophytically on dead plant and animal tissues, and is a common soil inhabitant. The pathogen can survive between onion crops as a saprophyte on decaying organic matter, in or on onion seed, and on cull onions.
Black mold symptoms generally begin at the top of the bulb near cut necks and progress downward. Infected tissues initially become water-soaked, but later a black mold is apparent between scales; small black sclerotia later form. This tissue will dry and shrivel, but can become soft and watery if invaded by bacteria. Bulbs can appear healthy on the outside, but have at least one diseased inner scale when cut in cross-section. Losses can be significant if high temperature curing is not monitored closely.
Blue mold symptoms include a soft, watery rot with a blue-green mold apparent on the surface.
Biological control strategies have not been developed for black and blue mold.
Plant high quality onion seed free from the storage mold pathogens. Practice crop rotations of at least three years between onion crops. Thoroughly cure bulbs in the field and storage shed with ambient air, but avoid bruising and wounding during harvest. High temperature curing can favor black mold, so bulbs must be closely monitored to prevent infection of bulbs during forced air curing.
Seed treatments and timely applications of fungicides, especially those with some systemic activity, may reduce storage mold losses.
Product List for Storage Molds (Black Mold and Blue Mold):
Pesticide |
Rate per 100 lb seed |
Application Frequency (days) |
Remarks |
|
Azoxystrobin |
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|
Quadris |
0.4-0.8 fl oz/1000 ft |
Planting time treatment |
Suppression of Rhizoctonia spp. |
|
Captan |
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|
Captan 75 |
6-9 oz |
Seed treatment |
Broad spectrum, but weak against Pythium spp. |
|
Captan 30-DD |
1.33-2.33 fl oz |
Seed treatment |
Broad spectrum, but weak against Pythium spp. |
|
Captan 400 |
1.5-3.0 fl oz |
Seed treatment |
Broad spectrum, but weak against Pythium spp. |
|
Captan 400-C |
1.5-3.0 fl oz |
Seed treatment |
Broad spectrum, but weak against Pythium spp. |
|
Fludioxinil |
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|
Maxim 4FS |
0.08-0.16 fl oz |
Seed treatment |
Suppression of Fusarium and Rhizoctonia spp. |
|
Thiram |
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|
42-S Thiram |
6 fl oz |
Seed treatment |
Broad spectrum |
|
Thiram 50WP |
6 oz |
Seed treatment |
Broad spectrum |
|
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: Onion, Disease, Storage Molds, Black Mold, Blue Mold
Date: 04/01/2007