Purple blotch is caused by the fungus Alternaria porri. It can infect all above-ground parts of the plant in addition to the bulb. Initial symptoms appear on older leaves, usually late in the summer as spores are blown from infested debris. Older leaves and plants are more susceptible to infection. The disease is most severe when frequent rain or persistent dews occur during moderate (77 to 81°F) temperatures. Almost no infection occurs below 55°F. The fungus is disseminated within and among fields by splashing water and wind, and overwinters in and on infested crop debris. The pathogen may also be seed-borne.
Lesions are elongate, small, sunken and whitish with a purple center. Concentric light and dark zones later appear over part or all of the purple area. These blotches may enlarge (up to four inches long) and become covered with black fruiting bodies (spores). Leaves wilt and die. Bulbs can be infected at harvest if the pathogen enters neck wounds. Storage symptoms appear as a dark yellow to wine-red spongy rot of outer or inner scales of bulbs. The disease can reduce bulb yield and grade 20% or more.
Biological control strategies have not been developed for purple blotch.
Plant high quality seed and transplants free from the purple blotch pathogen. Practice a three-year or longer rotations to non-hosts such as small grains and corn. Reduce over- wintering and survival of the pathogens by eliminating culls, onion debris, and volunteers. Avoid dense plantings of late maturing varieties. Apply adequate but not excessive nitrogen fertilizer before bulb initiation. Undercut onion roots and harvest at full maturity when intact necks top over naturally, ideally during dry weather. Thoroughly cure bulbs in the field and before storage with ambient to warm (90 to 95°F) air. Maintain storage temperatures near 32°F with a 70% or less relative humidity.
Chemical controls are most effective when combined with sound cultural practices. Copper fungicides are registered for control of purple blotch, but are not highly effective.
Product List for Purple Blotch:
Pesticide |
Product per Acre |
Application Frequency
|
Remarks |
|
Boscalid |
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Endura |
6.8 oz |
7-14 days |
Max of 6 applications (41oz) per season; rotate with a different mode of action; 7 day PHI |
|
Chlorothalonil and Chlorothalonil Mixtures—not all formulations listed |
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|
Bravo S |
3-4 pt |
7-10 days |
|
|
Bravo Ultrex |
0.9 –2.7 lb |
7 days |
Maximum of 18 pounds per season; 7 day PHI |
|
Bravo Weather Stik |
1-3 pt |
7 |
Maximum of 20 pints per season; 7 day PHI |
|
Chlorothalonil 4L |
1 -2 1/8 pt |
7-10 days |
|
|
Equus 720 |
1-2 pt |
5-10 days |
Maximum of 20 pints per acre; May be applied through sprinkler irrigation at no more than 10-day interval; other formulations of Equus are available |
|
Quadris Opti |
1.6-3.2 pt |
5-14 days |
Maximum of three applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
|
Ridomil Gold Bravo |
2 lb |
14 days |
Maximum of 4 applications, 7 day PHI |
|
Ridomil Gold Bravo Liquid |
1 pack per 10 acres |
14 days |
Maximum of 4 applications, 7 day PHI |
|
Terranil 6L |
1-2 pts |
7-10 days |
7 day PHI |
|
Copper-based Fungicides—not all formulation listed |
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|
Champ DP |
1.33 lb |
7-10 days |
|
|
Copper-Count-N |
2 qt |
7-10 days |
|
|
Cuprofix |
2.5-6 lb |
7-10 days |
Can be phytotoxic |
|
Cuprofix MZ |
5-7.25 lb |
3-7 days |
Maximum of 78 pounds per season; 7 day PHI |
|
Kocide 2000 |
1.5 lb |
7-10 days |
Can be phytotoxic to leaves |
|
Kocide 3000 |
0.75 lb |
7-10 days |
Can be phytotoxic to leaves |
|
Nordox |
2-4 lb |
7-10 days |
|
|
NuCop 50WP |
2.0 lb |
7-10 days |
|
|
Top Cop with S |
2-3 qt |
7-10 days |
|
|
Cyprodinil/Fludioxinil |
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|
Switch 62.5WG |
11-14 oz |
7-10 days |
Maximum of 56 ounces per season; rotate with a different mode of action; 7 day PHI |
|
EBDC and EBDC Mixtures—not all formulations listed |
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|
Cuprofix MZ Disperss |
5-7.25 lb |
3-7 days |
Maximum of 78 pounds per season; 7 day PHI |
|
Dithane DF |
3 lbs |
7 days |
Maximum of 32 pounds per season; Amended with a non-ionic surfactant to improve performance; 7 day PHI |
|
ManKocide |
2.5 |
3-7 days |
Maximum of 160 pounds per season; 7 day PHI |
|
Manex |
1.2-2.4 qt |
7 days |
Maximum of 24 quarts per acre; 7 day PHI |
|
Penncozeb 75DF |
2-3 lb |
7 days |
Maximum of 24 pounds per season; 7 day PHI |
|
Ridomil Gold MZ |
2.5 lb |
14 days |
Maximum of 4 applications; 7 day PHI |
|
Iprodione—not all formulations listed |
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|
Rovral |
1.5 lb |
7-14 days |
Maximum of 5 applications per season; 7 day PHI |
|
Rovral 4 Flowable |
1-2 pt |
7-14 days |
Maximum of 5 applications per season; 7 day PHI |
|
Pyrimethanil |
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|
Scala SC |
18 oz |
7-14 days |
Maximum of 54 fl oz; 7 day PHI |
|
Strobilurin and Strobilurin Mixtures |
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|
Cabrio |
8-12 fl. oz |
14 days
|
Maximum of 6 applications and 72 oz per season; Rotate with fungicides with a different mode of action; 7 day PHI |
|
Pristine |
10.5-18.5 oz |
14 days |
Maximum of 6 applications and 111 oz per season; alternate with different mode of action; 7 day PHI |
|
Quadris FL |
6.2 - 15.4 fl oz |
5- 14 days |
Maximum of four applications per season; Rotate with fungicides with a different mode of action; 0 day PHI |
|
Quadris Opti |
1.6-3.2 pt |
5-14 days |
Maximum of three applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
|
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, Purple Blotch
Date: 04/01/2007