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.

Allium Leafminer (Phytomyza gymnostoma)

Allium leafminer (ALM) is a relatively new invasive pest species associated with many allium hosts. ALM is a true fly species that was first detected in Lancaster County, PA in 2015. The northward expansion of the fly's range has been rapid, but it has not yet been confirmed in New England north of Massachusetts. 

In the Northeast, ALM populations exhibit 2 flights per season. These flights are separated by a summer aestivation (“summer hibernation”) period that often precludes the fly from causing significant damage in crops grown and harvested during the summer months (e.g., garlic and bulb onions). The first flight (of the overwintering population) begins in mid- to late April, ending in May. The second flight does not begin until September and typically extends into early October. Female flies will make multiple punctures on leaves with their ovipositor (apparatus for laying eggs) that leaves a distinct line of easily visible white dots. These oviposition “scars” are the primary diagnostic indicator for the presence of the pest. Eggs are laid singly inside of leaves where the oviposition marks are made. When ALM eggs hatch, larvae enter the leaves and actively “mine” the plant tissue. After several days, larvae move towards the center and base of the plant. After several weeks of active feeding, larvae typically pupate near the base of the plant within the foliage or may exit the plant and pupate in the soil. 

The most vulnerable allium host plants tend to be those harvested during the early spring and fall (primarily leeks and scallions). The removal of infected host plants and other allium residues from earlier harvested alliums is an important practice for reducing potential outbreaks in fall allium crops. Insect exclusion netting or other types of row covers can effectively exclude ALM flies if securely applied before the second flight begins and as long as covers are not applied over soil infested with ALM pupae. Foliar chemical applications have also been shown to be effective for reducing ALM damage. 

IRAC Group 3:

• pyrethrin (PyGanic EC5.0^OG): 4.5 to 15.61 oz/A, or 0.25 to 0.50 oz/gal and 3 gal/1000 sq ft in greenhouse for backpack sprayers; PHI 0d, REI 12h, Bee: M.

IRAC Group 4:

• dinotefuran (Scorpion 35SL): 5.25 to 7 oz/A foliar, 8.75 to 10.5 oz/A soil.; PHI 1d, REI 12h, Bee: H, Group 4A. Do not apply to vegetables grown for seed.

IRAC Group 5:

• spinetoram (Radiant SC): 6 to 10 oz/A; PHI 1d, REI 4h, Bee: M. Efficacy may be improved with the addition of adjuvant.
• spinosad (Entrust SC^OG): 3 to 6 oz/A; PHI 1d, REI 4h, Bee: M.

IRAC Group 17:

• cyromazine (Trigard): 2.66 oz/A; PHI 7d, REI 12h, Bee: H. Apply when leafminers first appear. 

IRAC Group UN:

• azadirachtin (Azatin O^OG): 6 to 16 oz/A foliar or drench, 6 to 16 oz/100 gal in greenhouses; PHI 0d, REI 4h, Bee: L. When using lower rates, combine with adjuvant for improved spray coverage and translaminar uptake.

Leek Moth (Acroleopiosis assectella)

Though leek moths feed upon most cultivated alliums, leeks are the most preferred and susceptible host species. Because garlic and onions are generally harvested for their bulbs, leek moth feeding damage tends to be less of a concern in these crops, as even moderate damage to the above-ground foliage typically does not lead to significant reductions bulb size or yield.

There are 3 flight periods of leek moth per season. The first flight (of the overwintering generation) begins in mid‐ to late April, ending in mid‐May. The second flight period (the first generation) begins in mid‐June, ending in early to mid‐July. The third flight period begins in late July, ending in mid‐ to late August. Although leek moth activity slows down after the last flight period, the damage can still develop on remaining allium vegetables, especially leeks, in the field. These larvae are considered to be the third generation and become the overwintering adults or pupae.

Eggs are laid singly on lower leaf surfaces whenever night temperatures are above 50°F–54°F. Females lay up to 100 eggs over a 3- to 4-week period. After hatching, larvae begin to burrow into the stem and move towards the center of the plant where young leaves are formed. In onions, leek moth larvae enter the hollow leaves and continue to feed on the inner cuticle of the leaf leading to the characteristic “window-pane” damage. After several weeks of active feeding, larvae exit the foliage and initiate pupation on the outside of leaves. Pupation lasts about 12 days, depending on weather conditions. 

Pre-harvest strategies: For smaller plots, insect exclusion netting can be an effective strategy for reducing leek moth damage by directly reducing exposure to egg-laying female moths. Chemical applications are typically well-suited for larger growing areas and are best applied 1–2 weeks following peak moth flights, which can be monitored using traps baited with pheromone lures. In addition, research has shown that the timely release of the parasitoid wasp, Trichogramma brassiceae, can significantly reduce leek moth damage in leeks and other alliums. 

Post-harvest strategies: The primary concern for garlic and onion growers is the potential damage that may occur during post-harvest curing and/or storage. Damage during curing and storage is generally the result of larvae being brought into the storage area following harvest. A simple low-risk strategy for reducing the prevalence of leek moth larvae in these areas is to remove as much of the foliage as possible prior to curing. This “topping” strategy leaves larvae in the field where they no longer can access the bulbs. Research has also shown that topping prior to curing or storage does not affect bulb quality or shelf life.

IRAC Group 5:

• spinetoram (Radiant SC): 6 to 10 oz/A; PHI 1d, REI 4h, Bee: M. Efficacy may be improved with the addition of adjuvant.
• spinosad (Entrust SC^OG): 3 to 6 oz/A; PHI 1d, REI 4h, Bee: M. 

IRAC Group 11:

• Bacillus thuringiensis kurstaki (Dipel DF^OG): 0.5 to 2 lb/A; PHI 0d, REI 4h, Bee: L. Must be ingested. Bt may be unlikely to provide adequate control by itself due to the mining behavior of leek moth larvae.

Onion Maggot (Delia antiqua)

See the Onion, Scallion, and Shallot insect control section for information on onion maggot.

IRAC Group 1:

• diazinon (Diazinon* AG500): 2 to 4 qt/A; REI 3d, Bee: H, Group 1B. Broadcast and incorporate just before planting. Will not control organophosphate-resistant onion maggots. Do not make more than 1 application per year.

Onion Thrips (Thrips tabaci)

See the Onion, Scallion, and Shallot insect control section for information on onion thrips.

IRAC Group 1:

•  malathion (Malathion 57 EC): 1.5 to 2 pt/A; PHI 3d, REI 24h, Bee: H, Group 1B.

IRAC Group 3:

• deltamethrin (Delta Gold*): 1.5 to 2.4 oz/A; PHI 1d, REI 12h, Bee: H.
• pyrethrin (PyGanic EC5.0^OG): 4.5 to 15.61 oz/A, or 0.25 to 0.50 oz/gal and 3 gal/1000 sq ft in greenhouse for backpack sprayers; PHI 0d, REI 12h, Bee: M.
• zeta-cypermethrin (Mustang* Maxx): 2.88 to 4 oz/A; PHI 7d, REI 12h, Bee: H.

IRAC Group 4:

• acetamiprid (Assail 30SG): 5 to 8 oz/A; PHI 7d, REI 12h, Bee: M, Group 4A.
• dinotefuran (Venom): 3 to 4 oz/A foliar, 5 to 6 oz/A soil at planting; PHI 1d foliar, REI 4h, Bee: H, Group 4A. Do not apply to vegetables grown for seed. 
• imidacloprid (Admire Pro): 14 oz/A; PHI 21d, REI 12h, Bee: H, Group 4A. Soil applications only.

IRAC Group 5:

• spinetoram (Radiant SC): 6 to 10 oz/A; PHI 1d, REI 4h, Bee: M. Thorough coverage is essential. Efficacy may be improved with the addition of adjuvant.

IRAC Group 23:

• spirotetramat (Movento): 5 oz/A; PHI 7d, REI 24h, Bee: M. Must be tank-mixed with a spray adjuvant with spreading and penetrating properties to maximize leaf uptake and systemicity. Don't use sticker adjuvants. Controls immature stages. Not for use in greenhouses or high tunnels.

IRAC Group 30:

• isocycloseram (Vertento): 3.1 to 4.1 oz/A; PHI 7d, REI 12h, Bee: H. Begin applications when thrips populations are low. Do not treat plants grown for transplanting. Not for use in greenhouses. 

IRAC Group UN:

• Beauveria bassiana (Mycotrol ESO): 0.25 to 3 qt/A; PHI 0d, REI 4h, Bee: L. Use high rate for high pest pressure and/or dense foliage.
• Burkholderia spp. strain A396 (Venerate XC^OG): 2 to 4 qt/A; PHI 0d, REI 4h, Bee: M. Suppression only.
• Chromobacterium subtsugae strain PRAA4-1 (Grandevo^OG): 2 to 3 lb/A; PHI 0d, REI 4h, Bee: M. Suppression only. 
• kaolin (Surround WP^OG): 25 to 50 lb/A, 0.25 to 0.5 lb/gal; PHI 0d, REI 4h, Bee: L. Suppression only. Good coverage down into plant crown is essential for efficacy. Supplemental pest control methods my be needed for complete control.

See section on Soil Fumigation Outdoors. Proper use of Vapam can provide control of most weed species.

Stale Seedbed

The following herbicides are nonselective and are used to control weeds that are present in a field prior to planting or before the crop emerges, which is especially useful for the Stale Seedbed Technique (see the Weed Management section for more information).

• glyphosate (Roundup Power Max): PHI 14d, REI 4h, Group 9.
• pelargonic acid (Scythe): PHI 1d, REI 12h, Group 0.

Preemergent Herbicides (before weeds germinate)

HRAC Group 3:

• pendimethalin (Prowl H2O): Up to 3 pt/A (PHI 60d) or 3.1 to 8.2 pt/A (200d); REI 24h. Do not exceed 8.2 pts/A per year. Apply to the soil surface at least 1 day before transplanting.

HRAC Group 14:

• flumioxazin (Chateau SW): 4 fl oz/A. REI 12h; 1 application per year. Apply no later than 2 days before transplanting.

Pre- and Postemergent Herbicides

HRAC Group 14:

• oxyfluorfen (Goal 2 XL): 4 to 6 pt/A; PHI 5d, REI 24h. Do not exceed 6 pt/A per year. Apply a minimum of 60 days after transplanting. Contact with crop will cause damage.

Postemergent Herbicides (after weeds germinate)

HRAC Group 0:

• pelargonic acid (Scythe): 3% to 10% solution; PHI 1d, REI 12h. Apply before planting, or as a directed and shielded spray during crop growth. Contact with crop will cause damage.

HRAC Group 1:

• clethodim (Select Max): 9 to 16 fl oz/A; PHI 5d, REI 24h. Do not exceed 64 fl oz/A per year. Apply to actively growing grasses.

• sethoxydim (Poast): 0.5 to 2.5 pt/A; PHI 7d, REI 12h. Do not exceed 5 pt/A per year. Apply to actively growing grasses. BASF has stopped production and will discontinue the registration at the end of 2027.

HRAC Group 2:

• halosulfuron (Sandea): 1 to 2 oz/A; PHI 5d, REI 12h. Do not exceed 2 oz/A per year. Avoid contact with plastic mulch. Contact with crop will cause damage.

HRAC Group 9:

• glyphosate (Roundup Power Max): 10 fl oz to 3.1 qt/A; PHI 5d (spot treatment), 14d (all other uses), REI 4h. Do not exceed 5 qt/A per year. Apply during fallow intervals, prior to planting, at planting, before crop has emerged, or to row middles with a hooded or shielded sprayer or wiper applicator. Spray contact can cause crop death.

HRAC Group 14:

• carfentrazone-ethyl (Aim EC): 0.5 to 2 fl oz/A; PHI 0d, REI 12h. Do not exceed 6.1 fl oz/A per year. Apply as a preplant burndown no later than 1 day before transplanting or as a hooded application between beds.

HRAC Group 22:

• paraquat (Gramoxone SL 2.0*): 1.7 to 2.7 pt/A; PHI 1d, REI 48h. Do not exceed 5.3 pt/A per year. Apply between rows as a directed shielded spray after crop establishment. May be fatal if swallowed or inhaled. *Restricted use: Applicators must complete an EPA-approved paraquat training every 3 years: https://www.epa.gov/pesticide-worker-safety/paraquat-dichloride-training-certified-applicators.