NC State Extension Publications

Introduction

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Gray mold is a common problem for many food crops produced in North Carolina. Because it has a wide host range, crop rotation strategies may not be effective. This disease is most commonly found in greenhouse production, and when plants are in cool, wet conditions. This disease can affect all parts of the plant, causing damages to leaves, stems, flowers, and roots.

Pathogen

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Gray mold is caused by Botrytis cinerea (teliomorph Botryotinia fuckeliana), which is a necrotrophic fungus that can cause severe damages to industrial hemp. B. cinerea produces several cell wall-degrading enzymes and toxins that degrade tissues and can also infect plant wounds caused by insects or pruning.

The fungus produces gray-white masses of mycelium (fungal mats) and spores on the surface of affected tissues (Figure 1). As tissues become decayed, the pathogen forms sclerotia (fungal overwintering structures) that are black, irregularly-shaped structures and mimic rodent droppings.

photo of gray mold on hemp leaves

Figure 1. Gray mold fungus on the surface of industrial hemp leaves.

L. Thiessen

Host Range

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B. cinerea affects over 200 plant species, including vegetables, peanut, strawberry, grape, and ornamentals. Economic losses may be found in both field and greenhouse production, and crops can suffer losses at almost any growth stage, including postharvest.

Symptoms

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Gray mold can cause brown, water-soaked lesions on all portions of the plant, including stems, leaves, and flowers. Leaves and flower lesions eventually become necrotic and produce a mass of gray spores (Figure 1). When stems/branches are infected, lesions may girdle the stem and cause limb or stalk breakage. Similarly to leaf and flower infections, necrotic tissues may be covered with a mat of gray spores. Within stem tissues, black sclerotia may develop.

Botrytis cinerea may also cause damping off of small seedlings or cuttings in the greenhouse when conditions are cool and wet. plants may girdle at the soil line and roots become necrotic and water-soaked.

Diseases With Similar Symptoms

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Fusarium Flower Mold (Fusarium graminearm complex, Figure 2)

This pathogen can have similar appearance on leaves and flowers. The two pathogens can be distinguished under conducive conditions by the absence of the gray spore mats produced by B. cinerea.

Fusarium Stem Canker (Fusarium graminearum complex, Figure 3)

Stem canker caused by the Fusarium graminearum complex can also cause necrotic stem lesions and girdling. This fungus does not produce gray spores on the surface of lesions and does not produce sclerotia.

Hemp Canker (Sclerotinia sclerotiorum)

This stem canker is characterized by shredding and breaking of stems at the lesion and the presence of white fungal mats with black, irregularly-shaped sclerotia inside of the lesion. This pathogen does not produce gray spores.

Photo of Fusarium flower mold of hemp

Figure 2. Fusarium flower mold on industrial hemp flower.

L. Thiessen

Photo of Fusarium stem canker

Figure 3. Fusarium stem canker causing stem girdling and breakage.

L. Thiessen

Disease Cycle

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This fungus overwinters as sclerotia in plant debris or in soil. Under conducive conditions, sclerotia germinate to form hyphal mats and the production of conidia (spores) on conidiophores. Conidia are dispersed in wind or water splash and land on a new host. Spores germinate and form new infections of any part of the plant, including leaves, stems, and flowers. As the tissues are depleted of resources, the fungus forms sclerotia to overwinter until favorable conditions are present.

Favorable Conditions for Disease

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Temperatures between 65-75°F [18-24°C], coupled with moist or humid conditions, are most conducive to the growth and dispersal of B. cinerea. As a result, most crops will suffer from infections in the late winter and early spring. These conditions may also be found in greenhouse conditions year-round, and altering the greenhouse environment to limit suitable conditions may be necessary to prevent disease.

General Management

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Gray mold can be difficult to control because it has a wide host range and can survive for extended periods of time in the absence of a host as sclerotia. A single mode of prevention alone is unlikely to be successful in preventing gray mold.

Sanitation is an important component to preventing spread of gray mold. Clean all equipment used on/near plants. Removing dead and infected plants and plant parts is essential to preventing, slowing, and stopping the spread of infection. Avoid removing affected plant parts when the field is wet or humid to prevent the spread of spores in conditions that favor further infection. Proper spacing will promote air flow between and around plants, also reducing the risk of infection. Because B. cinerea thrives in cool, moist environments, maintaining a properly watered environment with as little excess moisture as possible should reduce epidemics. Reduce humidity (below 50% RH) in greenhouses. Avoid over-fertilization to limit excess growth of the plant.

There are currently no effective, legal chemistries available to control gray mold in industrial hemp.

Useful Resources

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Acknowledgements

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This factsheet was prepared by the NCSU Field Crops and Tobacco Pathology Lab in 2019.

Authors

Undergraduate Research Assistant
Entomology & Plant Pathology
Assistant Professor & Extension Specialist
Entomology & Plant Pathology

Publication date: Sept. 9, 2019

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