More In-Depth Information

Powdery Mildew

Golovinomyces ambrosiae
G. chicoracearum
Podosphaera macularis

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infected leaf Powdery mildew with denser spotting on greenhouse-grown hemp.

infected leaf with lighter spotting
Powdery mildew with lighter spotting on greenhouse-grown hemp.

powdery mild damage Powdery mildew damage on field-grown hemp.

Hosts

  • Hemp
  • Bindweed
  • Other crops

Additional information about hosts

  • G. chicoracearum is found on bindweed and some ornamentals. However, there has been a debate if there are different strains of G. chicoracearum that are host-specific. To date, we have not seen G. chicoracearum on field-grown hemp in Utah despite bindweed nearby being covered with powdery mildew. This species has been found on greenhouse hemp.
  • Another powdery mildew reported on hemp is P. macularis. It also infects strawberry and hops. Only
  • G. ambrosiae has been found on field-grown hemp which also infects many cut flowers.

Symptoms

White, powdery spots that may expand to cover leaves, stems, or flower buds.

Disease Cycle

Powdery mildew survives as mycelium on green plant tissue in the greenhouse or outside in warmer climates. In colder climates it survives by producing, fruiting structures that contain overwintering spores (chasmothecia). As temperatures warm in the spring, the spores are released and colonize nearby green tissue. After colonizing the tissue, the characteristic white mycelium covers the tissue and new spores are produced. These spores cause new infections and the cycle can continue every 7 to 10 days as long as environmental conditions are favorable.

Time for Concern

Throughout the growing season.

When and Where to Scout

  • Starting a few weeks after planting, inspect hemp leaves for new infections. Start with the lowest leaves of the plant, and check the undersides and the leaf surface for white, powdery areas. 

Threat Level

Medium.

Occurrence in Utah

Powdery mildew on field-grown hemp has been very rare in Utah, but common on greenhouse hemp

Management

  • Space out field or greenhouse plantings to allow for air circulation.
  • Drip irrigation will lessen the chance of higher humidity, and reduce infection potential.
  • Approved fungicides are available.

When to Consider Treatment

When scouting reveals that approximately 5 to 10% of plants are infested with powdery mildew. This is an estimate based on threshold levels for other crops, and has not been verified on hemp. 

Look-alikes

None, as this disease is characteristic (white spots on leaves).

Fungicides for Powdery Mildew

Utah-Registered Product Active Ingredient(s) Notes
BioSafe Disease Control RTU hydrogen peroxide  
Bonide All Seasons Horticultural & Dormant Spray Oil Concentrate; Ready to Spray mineral oil Greenhouse use only (commercial).
Bonide Insecticidal Soap MultiPurpose Insect Control Ready To Use potassium laurate Greenhouse use only
Bonide Rose Rx 3 in 1 Concentrate clarified hydrophobic extract of neem  Greenhouse use only.
Dr. Earth Final Stop OMRI Disease Control Fungicide (RTU); Concentrate rosemary oil  
Jet-Ag hydrogen peroxide Cleans up spores but PM can come back in 7 days
Kopa Insecticidal Soap potassiums salts of fatty acid  
Mammoth Cancontrol thyme oil  
Natural Guard Brand by Ferti-lome Insecticidal Soap Concentrate potassium salts of fatty acids  
Natural Guard Brand Neem  clarified hydrophobic extract of neem oil  
Omni Supreme Spray  mineral oil  
Organocide 3-in-1 Garden Spray Concentrate, Ready to Spray; Ready to Use sesame oil  
Organocide Bee Safe 3-in-1 Garden Spray Concentrate; RTU sesame oil  
OxiPhos phosphorous acid, mono- and dipotassium salts of Use allowed prior to final transplant.
PureCrop1 soybean oil  
Rango neem oil, cold pressed  
Regalia CG Biofungicide Reynoutria sachalinensis
(knotweed)		 Outdoor use only,
Reliant Systemic Fungicide phosphorous acid, mono- and dipotassium salts of Can only be used prior to final transplant.
ReLoad Fungicide mono- dibasic sodium, potassium, and ammonium phosphites Greenhouse use only.
Safer Brand Garden Fungicide for Flowers, Fruit, and Vegetables, Concentrate sulfur  
Safer Brand Insect Killing Soap Concentrate II potassium salts of fatty acids  
SaferGro Mildew Cure corn oil  
Serenade Garden Disease Control Concentrate Bacillus subtilis qst713 strain  
Serenade Garden Disease Control Ready to Spray Bacillus subtilis qst713 strain  
Serifel NG Biological Fungicide  Bacillus subtilis mbi 600  
Summit Year-Round Spray Oil  mineral oil  
Surround WP Crop Protectant kaolin  
Triact 70 clarified hydrophobic extract of neem oil Older versions cannot be used on cannabis.
Triathlon BA Bacillus amyloliquefaciens strain d747  
Trilogy sulfur  
TriTek mineral oil  
ZeroTol 2.0 hydrogen peroxide Greenhouse use only.

Photo Credit 

All photos: USU Extension IPM Program

Precautionary Statement: Utah State University and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks, and not all are registered for edible crops. “Restricted use” pesticides may only be applied by a licensed applicator. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed in this publication.