Preventing hemp disease By C. M. Ocamb

Recognizing and understanding gray mold and powdery mildew of hemp can help growers manage these diseases.

The fungus, Botrytis cinerea, is a common pathogen of plants and infects many different crops worldwide. It causes gray mold, also known as Botrytis bud blight and stem canker, on hemp (Cannabis sativa L.). This fungus initially infects weak, damaged, or senescing leaf, bud, or stem tissues, especially when environmental conditions are favorable. Temperatures between 50 and 68° F and a high relative humidity (> 95%) or free water on plant surfaces favor infection of plants and subsequent sporulation by Botrytis. Once infection has occurred, the fungus can continue colonizing plants when the temperature is 32 to 80° F.

Hemp grown inside greenhouses is at higher risk for gray mold throughout plant development due to the higher relative humidity that occurs inside. Rainy periods, especially during flowering and bud maturation, can result in significant disease in outdoor plantings or inside greenhouses with poor ventilation and heating.

Figure 1. Hemp bud in early stage of gray mold development on a cola, denoted by the brown-colored tissues (A), a hemp plant with bud blight due to gray mold with multiple affected colas (B), a hemp bud infected with Botrytis that is exhibiting gray-brown sporulation (C), and a portion of a hemp stem with a Botrytis canker and profuse gray-colored sporulation (D) characteristic of this disease. Photos: C.M. Ocamb, Oregon State University.

Botrytis produces a gray-brown mass of asexual spores known as conidia on infected tissues; the spores are dispersed by wind currents as the relative humidity drops. Conidia can be produced throughout the growing season, depending on environmental conditions. Small, black hyphal structures known as sclerotia can also develop on or inside affected plant tissues, although they occur infrequently with Botrytis. These durable sclerotia are released from decomposing plant tissues, fall onto or are mixed into the soil, where they survive for months to years. There may be a widespread presence of B. cinerea in the local landscape (ornamental plants, herbs, fruit crops, vegetables, etc.) throughout the year, depending on the local climatic conditions, so sclerotia are not necessary for Botrytis outbreaks in hemp.

The economic impact of Botrytis on hemp grown for cannabinols, buds or seed is primarily through the infection of flowers and bud tissues. Tissues will initially exhibit a water-soaked appearance where infection has occurred. Infected buds eventually turn brown as the affected tissues die, and the fungus often produces spores on dead tissues (Fig. 1A-C). Botrytis can also infect stems with pale to light-brown colored areas developing as well as gray-colored sporulation (Fig. 1D) under conducive conditions. Stem cankers can result in splitting or breaking of branches as well as result in death of the growing tip on affected branches. Infected hemp tissues can be killed rapidly, potentially within one week. Young hemp seedlings can also be killed by Botrytis.

Management of gray mold in hemp

• Minimize conductive conditions for disease by increasing plant and row spacing to promote air movement within and around plants. In greenhouses, maintain warm temperatures and high light intensities while keeping the relative humidity below 50%.
• Avoid practices that keep plants wet for 12 hours or longer. Limit irrigation during bloom. Use bottom-watering methods rather than overhead irrigation. Prevent standing water or pooled water inside greenhouses.
• Avoid over-fertilization with nitrogen as this creates a more dense plant canopy that impedes air movement and promotes higher humidity conditions.
• Avoid mechanical injuries to plants, including those caused by insect feeding on buds, since Botrytis more easily infects wounded tissues.
• Apply a protective fungicide program during bud development that is efficacious on gray mold. Contact your state department of agriculture or Extension faculty at a land-grant university in your state for recommended fungicides.

Powdery mildews in hemp

The fungi that can cause powdery mildew in hemp include Golovinomyces ambrosiae and Podosphaera macularis. Golovinomyces ambrosiae is the species that most commonly infects hemp and has been reported on cannabis in Oregon, New Jersey, and Kentucky as well as Canada. Golovinomyces ambrosiae may also infect members of the Asteraceae family; it has been reported on sunflower, dahlia, daisy, Jerusalem artichoke, zinnia and ragweed. Podosphaera macularis more commonly occurs on hop, but it has been reported on hemp in the Pacific Northwestern U.S., Canada and Switzerland.

Golovinomyces and Podosphaera produce copious amounts of asexual spores (conidia) in chains (Fig. 2). The white powdery growth that is characteristic of powdery mildew is due to these conidia. Colonies become visible approximately seven to 14 days after infection. Conidia can be moved via wind currents, endangering neighboring hemp plants or fields in the vicinity. A high relative humidity promotes infection while wet plant surfaces will inhibit infection by conidia. Warm conditions are conducive for the production of conidia and infection; around 77° F is ideal for conidia to be produced by P. macularis on hops, but conidia production and infection are inhibited when temperatures exceed 86° F. It is anticipated that temperature effects would be similar for P. macularis on hemp. No information is currently reported regarding conditions conducive for sporulation and infection by G. ambrosiae in hemp.

These two powdery mildew fungi can produce overwintering fruiting bodies known as chasmothecia that yield sexual spores (ascospores), but evidence of ascospores in association with cannabis is lacking. Podosphaera macularis has not been found to produce chasmothecia on hop grown in the Pacific Northwest, but chasmothecia are produced on hops in other regions. Ascospores are forcibly ejected from chasmothecia beginning in the spring and then move with wind currents. Ascospores require a high relative humidity or free moisture in order to infect susceptible host plants. Podosphaera macularis is also known to overwinter as mycelium on infected crown buds of hop plants in the Pacific Northwest. These two powdery mildew fungi may also survive/over-winter on infected host plants in greenhouses.

Infection of hemp plants by powdery mildew fungi can occur at any stage of plant growth, but in regions with dry summers, onset of powdery mildew in field production is more common during the latter part of crop development, especially during flowering. Infections appear as whitish, powdery spots typically on the upper leaf surface. Entire leaf surfaces can be covered with powdery mildew, and petioles as well as flower bracts may be infected. As foliar infections of Golovinomyces and Podosphaera age, if chasmothecia are produced, they appear as tiny, roundish specks ranging from yellow to dark-brown to black coloration within powdery mildew colonies.

Figure 2. Hemp plants exhibiting powdery mildew. A plant in the field (A) and a close-up of a leaf (B) that are infected by Golovinomyces ambrosiae. A hemp branch with buds that is infected with Podospaera macularis (C). Photos A and B: C.M. Ocamb. Photo C: T.A. Bates, Oregon State University.

Management of powdery mildew in hemp

• Removal of infected leaves early in disease development can delay disease buildup but requires routine disease scouting.
• Maintain adequate nitrogen levels but do not over-apply because more succulent tissue is more susceptible to infection.
• Applications of silicon (300 to 600 kg/ha) applied to hemp roots suppressed powdery mildew in studies conducted by the University of Kentucky. Silicon applications may be especially helpful for greenhouse grown plants produced normally under low levels of soluble silicon in the growing medium.
• A protective spray program may be warranted but thorough spray coverage before powdery mildew is present is essential. Contact your state department of agriculture or land-grant university personnel for recommended fungicides that are legal to apply in your region.
• Bicarbonates such as Kaligreen, MilStop SP can be very effective when used on regular intervals prior to onset of disease. Some formulations are OMRI listed. Studies by the University of Tennessee showed good efficacy when bicarbonate was applied at weekly intervals on greenhouse grown hemp.
• Oils including Bonide BonNeem II, Mildew Cure, Monterey Horticultural Oil, Omni Supreme Spray, etc., can help to control powdery mildew. Do not spray if the temperature is below 50 or above 90° F or when plants are wet or under heat/moisture stress. Some formulations are OMRI listed.
• Regalia can offer reasonable control of powdery mildew as shown by field and greenhouse studies conducted by the University of Tennessee. OMRI listed.
• Sulfur formulations can be very effective at controlling powdery mildew; using it above 85° F is not recommended. A greenhouse study by the University of
Tennessee showed good efficacy with sulfur applied on seven-day intervals.

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C. M. Ocamb

C.M. Ocamb is a plant pathology Extension specialist at Oregon State University. She can be reached at

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