Botrytis cinerea, commonly known as grey mold or Botrytis blight, is a fungal disease that affects over 200 plant species, including many vegetables, fruits, herbs, and ornamentals. The fungus causes a grey, fuzzy mold to grow on the infected plant tissues, leading to blighting of flowers, fruits, leaves, and stems. Proper identification, prevention, and management of Botrytis is crucial for avoiding significant economic losses in agriculture and horticulture.

What Causes Botrytis?

Botrytis cinerea is an opportunistic pathogen that infects plant tissues under favorable conditions. The main factors leading to Botrytis infection include:

High Humidity and Free Moisture

The fungal spores require high humidity or free moisture on plant surfaces to germinate and cause infection. Excessive rain, heavy dews, fog, and overhead irrigation wet plant surfaces and promote fungal growth.

Cool Temperatures

The optimum temperature range for Botrytis growth is between 50-75°F. Cool, damp weather encourages disease development.

Injuries and Mechanical Damage

Wounds from pruning, hail, wind, insect feeding, and other mechanical damage provide entry points for the fungus. Damaged or senescing tissues are more susceptible.

Dense Foliage and Poor Air Circulation

Thick, dense foliage prevents air circulation and keeps the microclimate humid and favorable for fungal growth.

Nutrient Deficiencies

Plants weakened by improper fertility are more prone to infection. Deficiencies in nitrogen, potassium, phosphorus, or calcium compromise plant health and increase susceptibility.

Identifying Botrytis Infections

Learning to properly identify Botrytis is the first step in effective management. Characteristic signs to look for include:

Grey Fungal Growth

The most diagnostic sign is the grey, powdery mold that develops on infected tissues. The fungus produces masses of greyish-brown spores that give it a dusty appearance.

Fluffy grey fungal growth on a strawberry infected with Botrytis. Source: Gardening Know How

Wilting or Blighting

Infected flowers, leaves, and shoots will rapidly wilt, wither, and turn brown. Flower petals brown and “blast” rather than opening normally.

Soft Watery Spots

Lesions first appear as soft, water-soaked spots that expand and cover the infected tissue. Petals and ripening fruit rot.

White Mycelium

In humid conditions, a dense white moldy growth of mycelium and spores develops on infected tissues and nearby surfaces.

Conditions Favoring Botrytis Blight

While Botrytis cinerea can infect plants under various conditions, certain environments encourage more severe disease outbreaks:

Cool, Wet Weather

The fungus thrives in cool, wet conditions between 60-70°F. Prolonged high humidity, frequent rainfall, heavy dews, and fog favor disease development.

Greenhouses and High Tunnels

The warm, humid, enclosed environment of greenhouses and high tunnels is ideal for rapid Botrytis growth. Condensation on surfaces promotes spore germination. Proper ventilation is critical.

Dense Plantings

Overcrowded, dense plantings prevent air circulation and keep foliage wet, creating optimal conditions for fungal growth. Wider spacing improves airflow.

Excess Nitrogen

Heavy nitrogen fertilization results in lush, tender growth that is highly susceptible to infection. Maintain proper plant nutrition to avoid excessive succulence.

Entry Wounds

Any wounds from mechanical damage, pruning cuts, wind injury, hail, or insect feeding provide prime entry points for Botrytis. Avoid unnecessary injury.

Senescing Tissues

Older, senescing flowers, fruits, and leaves are more prone to infection than vigorous tissues. Petal fall and overripe produce are vulnerable.

High Relative Humidity

Relative humidity consistently above 90% allows fungal spores to easily germinate and penetrate plant tissues. Proper ventilation to reduce RH is key.

Common Host Plants for Botrytis

Botrytis cinerea has an extremely wide host range. Some of the most commonly affected plants include:

Fruits

Strawberries, raspberries, blackberries, grapes, cherries, peaches, nectarines, citrus, tomato, cucurbits, apple, pear, kiwi

Vegetables

Lettuce, peas, beans, crucifers, onion, garlic, asparagus, eggplant, pepper, potato

Herbs

Basil, mint, parsley, cilantro, dill, rosemary, sage, oregano, marjoram

Flowers

Roses, geranium, petunias, zinnias, pansies, snapdragons, stock, peonies, chrysanthemums, dahlias

Ornamentals

Poinsettia, lilac, hydrangea, iris, tulip, lily, bleeding heart, columbine, phlox

Shrubs and Trees

Azalea, rhododendron, camellia, magnolia, crabapple, flowering cherry, dogwood, euonymus

Preventing Botrytis Blight

An integrated approach utilizing cultural, biological, and chemical controls is most effective for preventing Botrytis outbreaks:

Provide Good Air Circulation

Use proper spacing, prune out dense inner growth, and avoid overcrowding to allow good airflow that reduces humidity in the plant canopy.

Avoid Wetting Foliage

Water at the base of plants, use drip irrigation, water early in the day, and space/stake plants to promote rapid drying and prevent prolonged wetness.

Reduce Humidity in Greenhouses

Ventilate greenhouses, use horizontal airflow fans, heat and dehumidify when needed, and avoid condensation on plant surfaces.

Apply Protective Fungicides

Use registered fungicides like chlorothalonil, fluoxastrobin, fludioxonil, boscalid, and mancozeb as protective applications before disease onset.

Practice Sanitation

Remove and destroy infected plant debris. Disinfect gardening tools, equipment, and greenhouse surfaces regularly.

Grow Resistant Varieties

Select varieties with genetic resistance when available. Consult seed catalogs or local Extension lists.

Use Biological Controls

Introduce beneficial microbes like Bacillus subtilis and Gliocladium catenulatum to suppress Botrytis.

Avoid Excess Nitrogen

Moderate nitrogen inputs to prevent succulent, tender growth vulnerable to infection.

Minimize Injury

Handle plants carefully to avoid wounds. Prune, trellis, and stake plants to prevent mechanical damage. Control insects.

Managing Active Botrytis Infections

If Botrytis is already present, these tactics can help reduce disease impact:

Improve Air Circulation

Open up dense foliage by pruning out affected shoots, thinning, or increasing plant spacing. Use fans in enclosed spaces.

Remove Affected Tissues

Prune out diseased stems, leaves, and flower parts well below visible symptoms to eliminate sources of new spore production.

Apply Targeted Fungicides

Use fungicides that specifically target Botrytis, such as fenhexamid, cyprodinil, pyraclostrobin, or boscalid, following label directions carefully.

Reduce humidity and moisture

Water plants at the base only. Improve drainage. Ventilate greenhouses to lower relative humidity below 85% during the day.

Remove nearby weeds

Eliminate alternate hosts like wild mustards, mints, and nightshades that can harbor Botrytis and re-infect crops.

Disinfect surfaces

Clean and sanitize greenhouse structures, tools, and equipment with products like Lysol, bleach, or quaternary ammonium to destroy fungal spores.

Use postharvest treatments

Apply registered fungicides like Scholar or Merivon as postharvest dips or sprays to reduce storage rots in produce. Refrigerate immediately after treatment.

Apply biofungicides

Use microbial products with Streptomyces or Bacillus species to suppress Botrytis on fruit, vegetable, and ornamental crops during growth and postharvest.

Frequently Asked Questions About Botrytis Blight

What is the best time to spray fungicides for Botrytis?

Fungicide applications for Botrytis prevention are most effective before initial symptoms develop. Begin applications when conditions favor disease, such as during bloom, petal fall, leaf emergence, or fruit set. Reapply every 7-14 days according to label directions to maintain protective coverage on expanding tissues.

How do I keep Botrytis out of my greenhouse?

To exclude Botrytis: disinfect the greenhouse structure, benches, tools, etc before each growing season, avoid introducing infected plant material, provide adequate spacing and air circulation using exhaust fans and horizontal airflow fans, scout regularly for issues, and apply preventative fungicides if necessary during ideal conditions for disease development.

What temperature kills Botrytis?

Botrytis does not thrive in hot weather. Temperatures consistently above 86 F slow fungal growth and can reduce spore germination. However, the fungus can persist as mycelium or sclerotia during hot periods and re-emerge when conditions cool. Maintaining optimum temperatures and reducing humidity are needed for long term control.

Is Botrytis harmful to humans?

The Botrytis fungi that cause blights in plants are not considered human pathogens and do not infect or cause illness in people if accidentally ingested. However, handling or consuming moldy produce is not recommended. The fungi can cause allergic reactions in sensitive individuals. Use caution when handling blighted plants or produce and wash hands thoroughly after working with affected materials.

Can Botrytis spread from dead leaves on the ground?

Yes, leaf litter and plant debris on the soil surface can allow Botrytis spores to overwinter and serve as an early source of inoculum for new infections. Remove and destroy infected plant tissues promptly, including fallen leaves. Rake up and dispose of debris around susceptible plants. Turning under plant litter in the fall can help reduce carryover.

Conclusion

Botrytis blight is a challenging disease in greenhouse and field production of flowers, fruits, vegetables, and ornamentals. Proper identification of the characteristic grey mold growth is the first step. Preventing conditions favorable to fungal infection using an integrated approach is key. Once established, aggressive sanitation tactics are needed to reduce spore production and limit spread. With vigilance and a combination of cultural, biological, and chemical controls, the damaging impacts of Botrytis can be minimized.

What is Botrytis?

Botrytis cinerea, also known as grey mold or Botrytis blight, is a fungal pathogen that causes disease in over 200 plant species. The fungus produces airborne spores that infect plant tissues, causing them to rot and collapse.

Characteristics of Botrytis

• Botrytis is a fungus in the family Sclerotiniaceae. There are actually over 20 different species of Botrytis fungi, but B. cinerea is the most common and widespread.
• It can infect leaves, shoots, flowers, fruits, vegetables, and ornamental plants. On fleshy fruits like strawberries and grapes, it produces soft rot that quickly spreads across infected produce.
• The name Botrytis comes from the Greek word “botrys” meaning grapes or clusters of grapes. This refers to the grape-like clusters of fungal spores produced on infected tissues.
• The fungus produces greyish brown spores in abundance, giving plant tissues a powdery, moldy appearance. This is why it is commonly called grey mold.
• Under very humid conditions, Botrytis can also produce fuzzy white strands of mycelium on infected plants and nearby surfaces.
• The fungus overwinters as sclerotia (survival structures) in plant debris or as mycelium in infected living tissues. It produces spores prolifically in spring.
• Cool, wet weather promotes spore release, germination, and new infections by Botrytis. It thrives in temperatures between 60-70°F.
• In addition to grapes and strawberries, it causes major crop losses in flowers, lettuce, onions, stone fruits, and many other plants.

Life Cycle of Botrytis

Botrytis cinerea goes through the following lifecycle:

• Inoculum – Overwintering sclerotia and mycelium produce masses of greyish spores that serve as primary inoculum.
• Spore dispersal – Spores spread rapidly by wind, rain splash, and physical contact to nearby plants.
• Germination – Spores landing on susceptible wet plant tissues germinate by producing hyphal tubes that penetrate the epidermis.
• Infection – The fungus colonizes plant cells and grows into surrounding tissues.
• Sporulation – The classic grey mold develops as infected tissues are covered with layers of grey fungal spores. These spores are the secondary inoculum for spread within and between plants.
• Overwintering – As infected tissues die, thick-walled sclerotia form and allow Botrytis to survive cold temperatures and resurface the following season.

Effects of Botrytis Infection

Botrytis causes substantial damage across many types of plants. Some common effects of infection include:

Blossom Blight

• Flowers and flower buds wither, brown, and die without opening. Petals stick together in a moist, mushy decay.

Fruit Rot

• Infected ripening fruits like strawberries, grapes, and stone fruits develop spreading brown lesions. Produce quickly rots into mush.

Shoot Blight

• Young succulent shoots and stems turn brown and collapse. Canes of berries wither and die back.

Leaf Blight

• Foliar infections lead to expanding necrotic spots, browned areas, and leaf death.

Seedling Damping-Off

• Botrytis can attack germinating seeds and seedlings, causing them to wilt, collapse, and die soon after emergence.

Postharvest Decay

• Produce in storage, transit, and market settings is highly susceptible to infection, leading to rapid spoilage and unmarketable fruits and vegetables.

Reduction in Yield and Quality

• Destruction of flowers, fruits, leaves, and shoots results in greatly diminished yields. Moldy appearance renders produce unappealing and unsafe.

Vulnerable Plants and Crops

Botrytis cinerea infects hundreds of different plant species. Some of the most common hosts susceptible to severe damage include:

Strawberries

Grey mold is a major scourge in strawberry production, infecting flowers, fruits, leaves, and crowns. The fungus thrives in the dense strawberry canopy and causes fruit rots later during storage and transport.

Grapes (Table and Wine)

Bunch rot and berry shrivel is a common problem in humid grape-growing regions. Tight clusters of grapes are ideal targets, and cracked berries are highly vulnerable to infection and leakage.

Tomatoes (Field and Greenhouse)

The humidity and condensation of greenhouses provides optimal Botrytis growth conditions. Field tomatoes can suffer blossom blight and fruit rot.

Stone Fruits (Peaches, Cherries, Plums, etc)

Brown rot blossom blight and fruit decays are serious issues, especially in wet seasons. Ripening fruits rapidly break down into brown mush once infected.

Lettuce and Leafy Greens

Leaf spots, tip burn, bottom rot, and generalized decay can make lettuce unmarketable. Infection often originates from dying outer wrapper leaves.

Onions and Garlic

Neck rot initiated in the field causes onions and garlic to rot during storage and transport. Leaks render bulbs unsaleable.

Nursery Stock and Bedding Plants

Seedling damping-off, cut flower blights, and leaf/petal spots on ornamental stock can cause major losses for growers.

Cultural and Biological Controls

Several cultural practices and biological treatments can help prevent or reduce Botrytis infections:

Provide Good Air Circulation

Adequate airflow keeps foliage dry and prevents the humid conditions Botrytis requires. Avoid overcrowding and prune dense growth.

Remove Infected Debris

Routinely remove diseased leaves, shoots, and fallen debris that harbor fungal spores. Compost plant trash deeply or discard offsite.

Avoid Overhead Watering

Water at the base of plants to keep foliage dry. Drip irrigation and soaker hoses help reduce humidity in the plant canopy.

Use Resistant Varieties

Select cultivars with genetic resistance when available. Check with seed companies and Extension resources for recommendations.

Apply Beneficial Microbes

Biofungicides with Bacillus subtilis, Bacillus amyloliquefaciens, or Streptomyces lydicus help suppress Botrytis and other fungal pathogens.

Minimize Plant Wounds

Handle plants carefully to avoid bruises, cuts, and scrapes that offer entry points for infection. Control insect pests that produce feeding injuries.

Using Fungicides to Control Botrytis

While cultural practices form the foundation of Botrytis management, fungicide sprays are often essential to protect susceptible crops:

Protective Applications

Apply broad-spectrum fungicides like mancozeb, chlorothalonil, or captan 7-10 days before favorable infection conditions and reapply according to label intervals.

Targeted Materials

Use FRAC group 7 (SDHI), 9 (AP), or 17 (QoI) fungicides that specifically target Botrytis when conditions warrant, such as during bloom, leaf out, or fruit set.

Rotation

Rotate between different modes of action to