Disease Control

The most common controls for leek diseases include the use of disease-resistant cultivars and pathogen-free seed, and cultural practices such as crop rotation and crop residue management that restrict pathogens.

Damping-off and Seed Rot (Pythium and Rhizoctonia)

Avoid excessive soil moisture by improving soil drainage or by utilizing raised beds. Seed and soil treatments with fungicides are a recommended practice.

FRAC Group 4:

• mefenoxam (Ridomil Gold SL): 0.5 to 1 pt/A; REI 48h. Labeled for Pythium damping-off as a pre-plant or at-planting application.
• metalaxyl-m [mefenoxam] (Apron XL): 0.085 to 0.64 fl oz/100 lb seed; REI 48h. Seed treatment for Pythium damping-off. 

FRAC Group 11:

• azoxystrobin (Quadris F): 0.4 to 0.8 fl oz/1000 row ft; PHI 0d, REI 4h.Labeled for Rhizoctonia damping-off.

FRAC Group 12:

• fludioxonil (Maxim 4FS): 0.08 to 0.16 fl oz/100 lb seed; REI 12h. Does not control Pythium or Phytophthora. 

FRAC Group BM01:

• thyme oil (Guarda): 1 gal/29–159 gal water (0.625%–3.3% v/v); PHI 0d, REI 0h.

FRAC Group BM02:

• Gliocladium virens strain GL-21 (SoilGard^OG): 1 to 1.5 lb/ cu yd soil (dry) or 0.5 to 2 lb/100 gal or 2 to 10 lb/A (drench); REI 4h. Incorporate into soil or soilless media as dry granules or in solution as a soil drench. 
• Pseudomonas chlororaphis strain AFS009 (Howler EVO^OG): 2.5 to 7.5 lb/A; PHI 0d, REI 4h. Use preventatively. 

Downy Mildew (Peronospora destructor)

See the Onion, Scallion, and Shallot disease control section for information on allium downy mildew.

FRAC Group 11:

• azoxystrobin (Quadris F): 9 to 15.5 fl oz/A; PHI 0d, REI 4h. Do not make more than 1 application before alternating with a non-Group 11 fungicide.
• azoxystrobin + chlorothalonil (Quadris Opti): 2.4 to 3.7 pt/A; PHI 14d, REI 12h, Groups 11 & M05. See label for tank-mix precautions. 
• famoxadone + cymoxanil (Tanos): 8 oz/A; PHI 3d, REI 12h, Groups 11 & 27. Must be tank-mixed with an appropriate contact fungicide from a different FRAC Group. Do not alternate or tank-mix with other Group 11 fungicides. 
• fenamidone (Reason 500 SC): 5.5 fl oz/A; PHI 7d, REI 12h. Do not alternate with other Group 11 fungicides.
• fenamidone (Reason 500 SC): 5.5 fl oz/A; PHI 7d, REI 12h. Do not alternate with other Group 11 fungicides.
• pyraclostrobin (Cabrio EG): 12 oz/A; PHI 7d, REI 12h. Do not make more than 1 application before alternating to a non-Group 11 fungicide.

FRAC Group 27:

• famoxadone + cymoxanil (Tanos): 8 oz/A; PHI 3d, REI 12h, Groups 11 & 27. Must be tank-mixed with an appropriate contact fungicide from a different FRAC Group. Do not alternate or tank-mix with other Group 11 fungicides. 

FRAC Group 40:

• ametoctradin + dimethomorph (Zampro): 14 fl oz/A; PHI 0d, REI 12h, Groups 45 & 40. Do not make more than 3 applications per year. 
• dimethomorph (Forum): 6 fl oz/A; PHI 0d, REI 12h. Must be tank-mixed with another fungicide from a different FRAC group.

FRAC Group 45: 

• ametoctradin + dimethomorph (Zampro): 14 fl oz/A; PHI 0d, REI 12h, Groups 45 & 40. Do not make more than 3 applications per year. 

FRAC Group M01:

• copper hydroxide (Kocide 3000): 0.75 to 1.5 lb/A; PHI 0d, REI 48h. Do not apply in a spray solution having a pH less than 6.5 or tank mix with Aliette. An OMRI-listed formulation is available (Kocide 3000-O^OG).

FRAC Group M05:

• azoxystrobin + chlorothalonil (Quadris Opti): 2.4 to 3.7 pt/A; PHI 14d, REI 12h, Groups 11 & M05. See label for tank-mix precautions. 
• mefenoxam + chlorothalonil (Ridomil Gold Bravo SC): 2.5 pt/A; PHI 14d, REI 48h, Groups 4 & M05. 

Purple Blotch (Alternaria porri)

Alternaria porri is most likely to become problematic when temperatures are warm (77°F–85°F) with high humidity. Small lesions will form on stems or leaves, eventually enlarging and becoming tan or purple. The pathogen overwinters in allium crop residue. Disease development is favored by warm, moist conditions.

Practice long rotations with non-allium crops and use practices that reduce hours of leaf wetness, e.g., increased plant spacing. Promote good airflow and good drainage. Drip irrigation is preferable to overhead. Remove or plow under crop residues promptly after harvest. Apply fungicides as canopy becomes denser and leaf-wetness periods increase. Manage weeds. Rotate between fungicides in different FRAC groups. Purple blotch disease development, based on weather conditions near your farm, can be monitored online (www.newa.cornell.edu). 

FRAC Group 3:

• azoxystrobin + propiconazole (Quilt Xcel): 14 to 21 fl oz/A; PHI 14d, REI 12h, Groups 11 & 3. See label for phytotoxicity warning.
• difenoconazole + benzovindiflupyr (Aprovia Top): 10.5 fl oz/A; PHI 7d, REI 12h, Groups 3 & 7. 
• difenoconazole + cyprodinil (Inspire Super): 16 to 20 fl oz/A; PHI 14d, REI 12h, Groups 3 & 9. Apply in sufficient volume to achieve thorough coverage. 
• azoxystrobin + propiconazole (Quilt Xcel): 14 to 21 fl oz/A; PHI 14d, REI 12h, Groups 11 & 3. See label for phytotoxicity warning.
• propiconazole (Tilt): 4 to 8 fl oz/A; PHI 0d, REI 12h. 

FRAC Group 7:

• boscalid (Endura): 6.8 oz/A; PHI 7d, REI 12h. 
• difenoconazole + benzovindiflupyr (Aprovia Top): 10.5 fl oz/A; PHI 7d, REI 12h, Groups 3 & 7.
• fluxapyroxad + pyraclostrobin (Merivon Xemium): 5.5 to 11 fl oz/A; PHI 7d, REI 12, Groups 7 & 11. For best results, begin applications before disease onset.
• penthiopyrad (Fontelis): 16 to 24 oz/A; PHI 3d, REI 12h.
• pyraclostrobin + boscalid (Pristine): 10.5 to 18.5 oz/A; PHI 7d, REI 12h, Groups 11 & 7.

FRAC Group 9: 

• difenoconazole + cyprodinil (Inspire Super): 16 to 20 fl oz/A; PHI 14d, REI 12h, Groups 3 & 9. Apply in sufficient volume to achieve thorough coverage. 
• cyprodinil (Vangard WG): 5.5 to 10 oz/A; PHI 7d, REI 12h.
• cyprodinil + fludioxonil (Switch 62.5 WDG): 11 to 14 oz/A; PHI 7d, REI 12h, Groups 9 & 12.
• pyrimethanil (Scala SC): 9 to 18 fl oz/A; PHI 7d, REI 12h. See label for rates in tank-mixes and use alone.

FRAC Group 11:

• azoxystrobin (Quadris F): 6 to 12 fl oz/A; PHI 0d, REI 4h. Do not rotate with other Group 11 fungicides.
• azoxystrobin + chlorothalonil (Quadris Opti): 1.6 to 3.2 pt/A; PHI 14d, REI 12h, Groups 11 & M05. See label for tank-mix precautions. 
• azoxystrobin + propiconazole (Quilt Xcel): 14 to 21 fl oz/A; PHI 14d, REI 12h, Groups 11 & 3. See label for phytotoxicity warning.
• famoxadone + cymoxanil (Tanos): 8 oz/A; PHI 3d, 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; PHI 7d, REI 12h. Do not alternate with other Group 11 fungicides.
• fluxapyroxad + pyraclostrobin (Merivon Xemium): 5.5 to 11 fl oz/A; PHI 7d, REI 12, Groups 7 & 11. For best results, begin applications before disease onset.
• pyraclostrobin (Cabrio EG): 8 to 12 oz/A; PHI 7d, REI 12h. Do not exceed 6 applications per season. Do not rotate with other Group 11 fungicides.
• pyraclostrobin + boscalid (Pristine): 10.5 to 18.5 oz/A; PHI 7d, REI 12h, Groups 11 & 7.

FRAC Group 12:

• cyprodinil + fludioxonil (Switch 62.5 WDG): 11 to 14 oz/A; PHI 7d, REI 12h, Groups 9 & 12.

FRAC Group 19: 

• polyoxin D zinc salt (Oso 5% SC^OG): 6.5 to 13 fl oz/A; PHI 0d, REI 4h.

FRAC Group 27: 

• famoxadone + cymoxanil (Tanos): 8 oz/A; PHI 3d, 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. 

FRAC Group M01:

• copper hydroxide (Kocide 3000): 0.75 to 1.5 lb/A; PHI 0d, REI 48h. Do not apply in a spray solution having a pH less than 6.5 or tank mix with Aliette. An OMRI-listed formulation is available (Kocide 3000-O^OG).

FRAC Group M05:

• azoxystrobin + chlorothalonil (Quadris Opti): 1.6 to 3.2 pt/A; PHI 14d, REI 12h, Groups 11 & M05. See label for tank-mix precautions. 
• chlorothalonil (Bravo Weather Stik): 1.5 to 3 pt/A; PHI 14d, REI 12h, Group M05. When disease is present, use the higher rate. Use sufficient water to obtain thorough coverage of tops. 
• potassium phosphite + chlorothalonil (Catamaran): 4 to 7 pt/A; PHI 14d, REI 12, Groups P07 & M05.

FRAC Group BM02:

• Bacillus amyloliquefaciens strain D747 (Double Nickel 55^OG): 0.25 to 3 lb/A (foliar); PHI 0d, REI 4h. 

FRAC Group P07:

• potassium phosphite + chlorothalonil (Catamaran): 4 to 7 pt/A; PHI 14d, REI 12, Groups P07 & M05.

White Rot (Sclerotium cepivorum, aka Stromatinia cepivora)

See the Onion, Scallion and Shallot disease control section for information on white rot. White rot symptoms are typically less severe on leeks compared with other allium crops.