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, preharvest interval (PHI), restricted entry interval (REI), and special instructions. In many cases, there are other products available with the same active ingredient. Please see Table 25 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.

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

Damping-off and seed rot (Pythium and Rhizoctonia)

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

mefenoxam (Apron XL): 0.085 to 0.64 fl oz/100 lbs seed; REI 48h, Group 4. Seed treatment.

fludioxonil (Maxim 4FS): 0.08-0.16 fl oz/100 lb seed; REI 12h, Group 12. Does not control Pythium and Phytophthora

Downy Mildew (Peronospora destructor)

During extended periods of cool, humid weather, this disease can be highly destructive causing losses in both yield and bulb quality. Practice a 3- to 4-year rotation. Plant on well-drained soil and orient rows in the same direction as prevailing winds. Avoid overhead irrigation. A regular program of fungicide spraying based on climatic conditions and disease forecasts should be followed.

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

azoxystrobin (Quadris): 9.0 to 15.5 fl oz/A; PHI 0d, REI 4h, Group 11.  Do not make more than one application of Quadris before alternating with fungicides that have a different mode of action.

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

copper hydroxide (Kocide 3000): 0.75 to 1.5 lb/A; PHI 0d, REI 48h, Group M1. 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; PHI 0d, REI 12h, Group 40. Forum must be applied as a tank mix with another fungicide with a different mode of action.

famoxadone & cymoxanil (Tanos):  8 oz/A; PHI 3d, REI 12h, Groups 11 plus 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, Group 11. Do not alternate with other Group 11 fungicides.

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

pyraclostrobin (Cabrio): 12 oz/A; PHI 7d, REI 12h, Group 11. Do not make more than one application of Cabrio before alternating to a fungicide with a different mode of action. Do not rotate with Quadris or Cabrio.

Purple Blotch (Alternaria porri)

Practice long rotations with unrelated crops and practices that reduce hours of leaf wetness, i.e., plant spacing and good air drainage. Use of a single fungicide throughout the growing season is not recommended.

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

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

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

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

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

chlorothalonil (Bravo Weather Stik): 1.5 to 3 pt/A; PHI 7d, REI 12h, Group M5. When disease is present, use the higher rate.

copper hydroxide (Kocide 3000): 0.75 to 1.5 lb/A; PHI 0d, 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): 10.0 oz/A; PHI 7d, REI 12h, Group 9.

cyprodinil plus fludioxonil (Switch 62.5 WDG): 11.0 to 14.0 oz/A; PHI 7d, REI 12h, Groups 9 & 12.

difenoconazole plus benzovindiflupyr (Aprovia Top): 10.5 fl oz/A; PHI 7d, REI 12h, Groups 3 & 7. No more than 2 sequential applications before alternating to a non-Group 7 fungicide.

difenoconazole plus cyprodinil (Inspire Super): 16.0 to 20.0 fl oz/A; PHI 7d (dry bulb); PHI 14d (green), REI 12h, Group 3 & 9. Apply in sufficient volume to achieve thorough coverage. 

famoxadone plus cymoxanil (Tanos):  8 oz/A; PHI 3d, REI 12h, Groups 11 plus 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 SC): 5.5 fl oz/A; PHI 7d, REI 12h, Group 11. Do not alternate with other Group 11 fungicides.

fluxapyroxad plus pyraclostrobin (Merivon): 5.5 to 11 fl oz/A; PHI 7d, REI 12, Groups 7 & 11.  For best results, begin applications before disease onset.

penthiopyrad (Fontelis): 16.0 to 24.0 oz/A; PHI 3d, REI 12h, Group NC.

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

potassium phosphite plus chlorothalonil (Catamaran): 4.0 to 7.0 pt/A; PHI 14d, REI 12, Groups 33 & M5.

propiconazole (Tilt): 4 to 8 fl oz/A; PHI 0d, REI 12h, Group 3. 

pyraclostrobin (Cabrio EG): 8 to 12 oz/A; PHI 7d, REI 12h, Group 11. Do not exceed six applications or 72 oz/A. Do not rotate with other Group 11 fungicides.

pyraclostrobin plus boscalid (Pristine):  10.5 to 18.5 oz/A; PHI 7d, REI 12h, Groups 11 & 7.

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

tebuconazole (Orius 3.6F): 4.0 to 6.0 fl oz/A (for dry onion); PHI 7d, REI 12h, Group 7.

White Rot (Sclerotium cepivorum)

Avoid planting in infested fields. Destroy infected plants and spot treat the soil around the plants with fumigants. Warm season production of host crops may greatly reduce sclerotial inoculum. Practice crop rotation. Soil fumigation may provide partial control. In general, neither fungicides nor fumigation have provided consistent, reliable, full-season control of white rot. Hot water treatment of seed can reduce the potential to spread white rot sclerotia. Winter flooding and soil solarization can reduce the number of sclerotia. White rot symptoms are less severe on leeks.