Disease Control

NOTE: For the disease control products listed below, one product trade name and formulation is provided for each active ingredient (common name) as an example of rates, days to harvest (dh), REI, and special instructions. In many cases, there are other products available with the same active ingredient. Please see Table 28 and Fungicides and Bactericides Alphabetical Listing by Trade Name for more information on products with the same active ingredients.

The symbol OG   indicates a product is listed by the Organic Materials Review Institute (OMRI) as approved for use in organic production. See Organic Certification section for more detail.

Botrytis Leaf Blight

Avoid close planting and orient rows in the direction of prevailing winds. Do not plant on poorly-drained areas. Remove cull piles and practice crop rotation. Avoid excessive nitrogen fertilization as this increases canopies and susceptibility. Plant less susceptible cultivars. Irrigate early in the day to reduce leaf wetness periods. Incorporate crop residues after harvest. Apply fungicides based on a disease forecast system (i.e., BOTCAST). Disease development, based on weather conditions near your farm, can be monitored on-line (www.newa.cornell.edu).

For dry bulb onions:

azoxystrobin (Quadris): 9.0 to 15.5 fl oz/A. (0 dh, REI 4h, Group 11).

azoxystrobin plus chlorothalonil (Quadris Opti): 1.6 to 3.2 pt/A. (7 dh, REI 12h, Groups 11 & M5).

azoxystrobin plus difenoconazole (Quadris Top): 12.0 to 14.0 fl oz/A. (1 dh, REI 12h, Groups 11 & 3).

azoxystrobin plus propiconazole (Quilt): 14 to 27.5 fl oz/A. (14 dh, REI 12h, Groups 11 & 3).

boscalid (Endura): 6.8 oz/A. (7 dh, REI 12h, Group 7). 

chlorothalonil (Bravo Weather Stik): 1 to 3 pt/A. (7 dh, REI 12h, Group M5).  To suppress Botrytis neck rot in storage, a minimum of 3 weekly applications prior to lifting using 1.25 to 1.8 lb/A is recommended.

cyprodinil (Vanguard): 10 oz/A. (7 dh, REI 12h, Group 9).

cyprodinil plus fludioxonil (Switch 62.5 WG): 11 to 14 oz/A. (7 dh, REI 12h, Groups 9 & 12).

difenoconazole plus cyprodinil (Inspire Super): 16 to 20 fl oz/A. (7 dh, REI 12h, Group 3 & 9). Apply in sufficient volume to achieve thorough coverage.

fluazinam (Omega 500F): 1 pt/A. (7 dh, REI 48h, Group 29).

fluxapyroxad + pyraclostrobin (Merivon): 8-11 fl oz/A. (7 dh, REI 12, Groups 7 & 11).  For best results, begin applications before disease onset.

iprodione (Rovral 4F): 1.5 pt/A. (7 dh, REI 12-24h, Group 2). If tank-mixing with another chemical registered for Botrytis blight, reduce iprodione rate to 1 pt.

mancozeb (Dithane F45): 2.4 qt/A. (7 dh, REI 24h, Group M3).

penthiopyrad (Fontelis): 16 to 24 oz/A. (3dh, REI 12h, Group NC).

polyoxin D (OSO 5%SC): 3.75 to 13.0 fl oz/A. (0 dh, REI 4h, Group 19).

potassium phosphite + chlorothalonil (Catamaran): 4-7 pt/A. (7 dh, REI 12, Groups 33 & M5).

propiconazole (Tilt): 4.0 to 8.0 oz/A. (14dh, REI 12, Group 3).

pyraclostrobin plus boscalid (Pristine): 14.5 to 18.5 oz/A. (7 dh, REI 12h, Groups 11 & 7). Do not make more than 2 sequential applications of Pristine before alternating with a fungicide with a different mode of action.

pyrimethanil (Scala SC Fungicide): 18 fl oz/A. (7 dh, REI 12h, Group 9). 

For green onions, leeks or shallots:

azoxystrobin (Quadris): 9.0 to 15.5 fl oz/A. (0 dh, REI 4h, Group 11).

azoxystrobin plus chlorothalonil (Quadris Opti): 1.6 to 3.2 pt/A. (14 dh, REI 12h, Groups 11 & M5). See label for tank mix precautions.

azoxystrobin plus difenoconazole (Quadris Top): 12.0 to 14.0 fl oz/A. (1 dh, REI 12h, Groups 11 & 3).

azoxystrobin plus propiconazole (Quilt): 14.0 to 27.5 fl oz/A. (0 dh, REI 12h, Groups 11 & 3).

boscalid (Endura): 6.8 oz/A. (7 dh, REI 12h, Group 7).  Use higher rate and shorter interval when disease pressure is high. Do not make more than 1 application of Endura before alternating with a fungicide with a different mode of action (FRAC group).

chlorothalonil (Bravo Weather Stik): 1.5 to 3.0 pt/A. (14 dh, REI 12h, Group M5). When disease is present, use the higher rate.

cyprodinil (Vanguard): 10 oz/A. (7 dh, REI 12h, Group 9).

cyprodinil plus fludioxonil (Switch 62.5 WDG): 11 to 14 oz/A. (7 dh, REI 12h, Groups 9 & 12).

difenoconazole plus cyprodinil (Inspire Super): 16 to 20 fl oz/A. (14 dh, REI 12h, Group 3 & 9).

fluxapyroxad + pyraclostrobin (Merivon): 8-11 fl oz/A. (7 dh, REI 12, Groups 7 & 11).  For best results, begin applications before disease onset.

penthiopyrad (Fontelis): 16 to 24 oz/A. (3dh, REI 12h, Group NC).

polyoxin D (OSO 5%SC): 3.75 to 13.0 fl oz/A. (0 dh, REI 4h, Group 19).

potassium phosphite + chlorothalonil (Catamaran): 4-7 pt/A. (14 dh, REI 12, Groups 33 & M5).

propiconazole (Tilt): 4.0 to 8.0 oz/A. (0 dh, REI 12, Group 3).

pyraclostrobin plus boscalid (Pristine): 14.5 to 18.5 oz/A. (7 dh, REI 12h, Groups 11 & 7). Do not make more than 2 sequential applications of Pristine before alternating with a fungicide with a different mode of action.

pyrimethanil (Scala SC Fungicide): 18 fl oz/A. (7 dh, REI 12h, Group 9).

Neck Rot (Botrytis alli)

Primarily a storage disease. Do not apply nitrogen late in the season and avoid high-density planting. Undercut bulbs to promote drying. Allow the tops to dry completely and have tight necks before topping.  Avoid condensation on bulbs during storage. Maintain storage environment at 32°F to 33°F and 70% to 75% relative humidity. Fungicides applied to control Botrytis blight may also decrease neck rot.

azoxystrobin plus chlorothalonil (Quadris Opti): 1.6 to 3.2 pt/A. (14 dh, REI 12h, Groups 11 & M5).  Suppression only. See label for tank mix precautions.

cyprodinil plus fludioxonil (Switch 6.25 WG): 11.0 to 14.0 oz/A. (7 dh, REI 12h, Groups 9 & 12). Suppression only. 

fluazinam (Omega 500F): 1 pt/A. (7 dh, REI 48h, Group 29).

fluxapyroxad + pyraclostrobin (Merivon): 8-11 fl oz/A. (7 dh, REI 12, Groups 7 & 11).  For best results, begin applications before disease onset.

iprodione (Rovral 4F): 1.5 pt/A. (7 dh, REI 24h, Group 2). If tank-mixing with another chemical registered for Botrytis blight, reduce iprodione rate to 1 pt.

mancozeb (Dithane M45): 2.4 qt/A. (7 dh, REI 24h, Group M3).

polyoxin D (OSO 5%SC): 3.75 to 13.0 fl oz/A. (0 dh, REI 4h, Group 19).

pyraclostrobin plus boscalid (Pristine): 14.5 to 18.5 oz/A. (7 dh, REI 12h, Groups 11 & 7).

pyrimethanil (Scala SC Fungicide): 18 fl oz/A. (7 dh, REI 12h, Group 9).

Downy Mildew (Peronospora destructor)

The fungus overwinters in infected bulbs or sets and on other host plants, in cull piles, and in fall-seeded onion. Only a few spores are needed to initiate an epidemic. Eliminate cull piles, volunteer onion plants, and wild Allium species. Practice a 3 to 4-year rotation. Plant only on well-drained land. Apply fungicides when mildew is present in the area and weather favors disease increase based upon a disease forecasting system (i.e., DOWNCAST).  Disease development, based on weather conditions near your farm, can be monitored on-line (www.newa.cornell.edu).

For dry bulb onions:

acibenzolar-s-methyl (Actigard 50 WG): 0.75 to 1.0 oz/A (7dh, REI 12h, Group 21). Actigard acts as a plant activator and should be applied preventatively.

ametoctradin plus dimethomorph (Zampro): 14.0 fl oz/A. (0 dh, REI 12h, Groups 45 and 40).

azoxystrobin (Quadris): 9.0 to 15.5 fl oz/A. (0 dh, REI 4h, Group 11).

azoxystrobin plus chlorothalonil (Quadris Opti): 2.4 to 3.7 pt/A. (7 dh, REI 12h, Groups 11 & M5). See label for tank mix precautions.

copper hydroxide (Kocide 3000): 0.75 to 1.5  lb/A. (0 dh, REI 48h, Group M1). Can cause phytotoxicity to leaves. Do not apply in a spray solution having a pH less than 6.5 or tank mix with Aliette.

dimethomorph (Forum): 6 fl oz/A.  (0 dh, REI 12h, Group 40). Forum must be applied as a tank mix with another fungicide with a different mode of action.

famoxadone plus cymoxanil (Tanos):  8 oz/A. (3 dh, REI 12h, Groups 11 & 27). Must be tank mixed with an appropriate contact fungicide with a different mode of action. Do not alternate or tank mix with other Group 11 fungicides. 

fenamidone (Reason 500 SC): 5.5 fl oz/A. (7 dh, REI 12h, Group 11). Do not make more than 1 application of Reason before alternating to a non-Group 11 fungicide.

fluazinam (Omega 500F): 1 pt/A. (7 dh, REI 48h, Group 29).

fosetyl-Al (Aliette): 2 to 3 lb/A. (7 dh, REI 12h, Group 33). Do not tank mix with copper.

mancozeb (Dithane M45): 2.4 qt/A. (7 dh, REI 24h, Group M3).

mancozeb plus copper hydroxide (ManKocide): 2.5 lb/A. (7 dh, REI 48h, Groups M3 & M1).

mandipropamid (Revus): 8.0 fl oz/A. (7 dh, REI 12h, Group 40).

mefenoxam plus chlorothalonil (Ridomil Gold Bravo SC): 2.5 pt/A. (7 dh, REI 48h, Groups 4 & M5).

mefenoxam plus mancozeb (Ridomil Gold MZ): 2.5 lb/A. (7 dh, REI 48h, Groups 4 & M3).

phosphorous acid (Fosphite): 1.0 to 3.0 qt/100 gal. (0 dh, REI 4h, Group 33). Do not apply to heat or moisture stressed plants or to plants recently treated with copper.

potassium phosphite + chlorothalonil (Catamaran): 4-7 pt/A. (7 dh, REI 12, Groups 33 & M5).

pyraclostrobin (Cabrio EG): 12 oz/A. (7 dh, REI 12h, Group 11).

pyraclostrobin plus boscalid (Pristine): 18.5 oz/A. (7 dh, REI 12h, Groups 7 & 11). For suppression only. Rotate with a downy mildew fungicide with a different mode of action.

Green or bunching:

ametoctradin plus dimethomorph (Zampro): 14.0 fl oz/A. (0 dh, REI 12h, Groups 45 and 40).

azoxystrobin (Quadris): 9.0 to 15.5 fl oz/A. (0 dh, REI 4h, Group 11).

azoxystrobin plus chlorothalonil (Quadris Opti): 2.4 to 3.7 pt/A. (14 dh, REI 12h, Groups 11 & M5). See label for tank mix precautions.

copper hydroxide (Kocide 3000): 0.75 to 1.5  lb/A. (0 dh, REI 48h, Group M1). Can cause phytotoxicity to leaves. Do not apply in a spray solution having a pH less than 6.5 or tank mix with Aliette. 

dimethomorph (Forum): 6 fl oz/A. (0 dh, REI 12h, Group 40). Forum must be applied as a tank mix with another fungicide with a different mode of action.

famoxadone plus cymoxanil (Tanos):  8 oz/A. (3 dh, REI 12h, Groups 11 & 27). Must be tank mixed with an appropriate contact fungicide with a different mode of action. Do not alternate or tank mix with other Group 11 fungicides. 

mandipropamid (Revus): 8.0 fl oz/A. (7 dh, REI 12h, Group 40).

mefenoxam plus chlorothalonil (Ridomil Gold Bravo SC): 2.5 pt/A. (14 dh, REI 48h, Groups 4 & M5).

potassium phosphite + chlorothalonil (Catamaran): 4-7 pt/A. (14 dh, REI 12, Groups 33 & M5).

pyraclostrobin (Cabrio EG): 12 oz/A. (7 dh, REI 12h, Group 11).

pyraclostrobin plus boscalid (Pristine): 18.5 oz/A. (7 dh, REI 12h, Groups 7 & 11). For suppression only. Rotate with a downy mildew fungicide with a different mode of action.

Purple Blotch (Alternaria porri, Stemphylium vesicarium)

Purple Blotch can be caused by either fungus alone or both fungi together. Avoid Sweet Spanish onion because they are extremely susceptible. Plow under crop residues after harvest. Rotate with non-hosts. Select sites and practice irrigation to enhance rapid drying of foliage. Apply fungicides as canopy becomes denser and leaf-wetness periods increase. Disease development, based on weather conditions near your farm, can be monitored on-line (www.newa.cornell.edu).

azoxystrobin (Quadris): 6.0 to 12.0 fl oz/A. (0 dh, REI 4h, Group 11). Do not rotate with other Group 11 fungicides.

azoxystrobin plus chlorothalonil (Quadris Opti): 1.6 to 3.2 pt/A. (7 (dry) 14 (green) dh, REI 12h, Groups 11 & M5). See label for tank mix precautions. 

azoxystrobin plus difenoconazole (Quadris Top): 12.0 to 14.0 fl oz/A. (7 dh, REI 12h, Groups 11 & 3). 

azoxystrobin plus propiconazole (Quilt): 14.0 to 27.5 fl oz/A. (14 (dry) 0 (green) dh, REI 12h, Groups 11 7 3).

Bacillus amyloliquefaciens strain D747 (DoubleNickelOG): 0.25 to 3.0 lb/A. (0 dh, REI 4 h, Group 44). Disease suppression only. For improved control; mix or rotate with a chemical fungicide.

boscalid (Endura): 6.8 oz/A. (7 dh, REI 12h, Group7).

chlorothalonil (Bravo Weather Stik): 1.0 to 3.0 pt/A. (7 (dry) 14 (green) dh, REI 12h, Group M5).

copper hydroxide (Kocide 3000): 0.75-1.5 lb/A. (0 dh, REI 48h, Group M1). Do not apply in a spray solution having a pH less than 6.5 or tank mix with Aliette.

cyprodinil (Vanguard): 10 oz/A. (7 dh, REI 12h, Group 9).

cyprodinil plus fludioxonil (Switch 62.5 WDG): 11 to 14 oz/A. (7 dh, REI 12h, Groups 9 & 12).

difenoconazole plus cyprodinil (Inspire Super): 16 to 20 fl oz/A. (7 dh (dry bulb); 14 dh (green), REI 12h, Group 3 & 9). Apply in sufficient volume to achieve thorough coverage. 

famoxadone plus cymoxanil (Tanos):  8 oz/A. (3 dh, REI 12h, Groups 11 & 27). Must be tank mixed with an appropriate contact fungicide with a different mode of action. Do not alternate or tank mix with other Group 11 fungicides.

fenamidone (Reason 500 SC): 5.5 fl oz/A. (7 dh, REI 12h, Group 11). Do not rotate with other Group 11 fungicides. 

fluazinam (Omega 500F): 1 pt/A (dry bulb only). (7 dh, REI 12-24h, Group 29).

iprodione (Rovral 4F): 1.5 pt/A (dry bulb only). (7 dh, REI 24h, Group 2). Do not make more than 5 applications per season. If tank-mixing with another chemical registered for purple blotch, reduce iprodione rate to 1 pt/A.

mancozeb (Dithane F45): 2.4 qt/A (dry bulb only).   (7 dh, REI 24h, Group M3).

mancozeb plus copper hydroxide (ManKocide): 2.5 lb/A (dry bulb only). (7 dh, REI 48h, Groups M3 & M1).

penthiopyrad (Fontelis): 16 to 24 oz/A. (3dh, REI 12h, Group NC).

polyoxin D (OSO 5%SC): 3.75 to 13.0 fl oz/A. (0 dh, REI 4h, Group 19).

potassium phosphite + chlorothalonil (Catamaran): 4-7 pt/A. (14 dh, REI 12, Groups 33 & M5).

propiconazole (PropiMax EC): 4.0 to 8.0 fl oz/A. (14 dh bulb; 0 dh green, REI 12h, Group 3).

pyrimethanil (SCALA SC Fungicide): 18 fl oz/A. (7 dh, REI 12h, Group 9).

pyraclostrobin (Cabrio EG): 8 to 12 oz/A. (7 dh, REI 12h, Group 11). Do not exceed 6 applications per season or 72 oz/A. Do not rotate with other Group 11 fungicides.

pyraclostrobin plus boscalid (Pristine): 10.5 to 18.5 oz/A. (7 dh, REI 12h, Groups 7 & 11). Do not rotate with another Group 11 fungicide.

tebuconazole (Orius 3.6F): 4 to 6 fl oz/A (dry onion). (7 dh, REI 12, Group 7).

White Rot (Sclerotium cepivorum)

White rot is associated with soil-borne inoculum; repeated cropping of Allium species results in increased disease over time. The sclerotia can persist in the soil for up to 20 years in the absence of a host. Select fields with no history of this disease. Use only disease-free transplants. Soil fumigation may be useful. Stimulating the sclerotia to germinate with onion exudate mimics (diallyl disulphide) can reduce soil inoculum.

azoxystrobin plus propiconazole (Quilt Xcel): 17.5 to 26 fl oz/A. (14 dh, REI 12h, Group 11 & 3).

Bacillus amyloliquefaciens strain D747 (DoubleNickelOG): 0.12 to 1.0 lb/A as a soil drench. (0 dh, REI 4 h, Group 44).

boscalid (Endura): 6.8 oz/A. (7 dh, REI 12h, Group 7). Apply in-furrow.

cyprodinil plus fludioxonil (Switch 6.25 WG): 7.0 to 14.0 oz/A. (7 dh, REI 12h, Groups 9 & 12). Apply at planting as in-furrow spray.

fludioxonil (Cannonball): 7 oz/A (0.5 oz/1,000 ft row). (7 dh, REI 12, Group 12).

penthiopyrad (Fontelis): 16 to 24 oz/A. (3dh, REI 12h, Group NC).

tebuconazole (Orius 3.6F): 20.5 fl oz/A (dry onion). (7 dh, REI 12, Group 7). Apply in a 4-6 inch band into/over each furrow. May also be applied by chemigation.

thiophanate methyl (Topsin M 70WP): 2.0 lb/A. (REI 72h, Group 1). Apply in furrow.