Gray mold in the flower garden

Quick facts

Gray mold is a fungal disease that blights flowers, leaves and stems of many different flowering plants.
Wounded and old plant tissue and flowers are easily infected by gray mold.
Gray mold is common in wet or very humid conditions.
The best way to prevent gray mold is to space out plants so they have room to dry out after rain or watering.
Remove infected flowers, leaves and stems. Infected plant parts should be buried or composted.
Many flowering plants can recover from gray mold when warm, dry conditions return.

Gray mold causes a dark brown to black blight of flowers, buds, leaves and stems.
Infected flowers and flower buds turn completely brown or black.
On plants with large petals, brown spots may form on petals.
Leaf spots often form near a wound or where an infected petal has fallen onto the leaf.
Leaf spots are large round brown blotches with target like rings of darker brown.
In wet weather, leaf spots can grow to turn the entire leaf brown.
Flower and leaf infections can progress into stems. Infected stems are brown and may have target like darker brown rings.

Gray mold is caused by the fungus Botrytis cinerea.
The gray mold fungus is able to infect a large number of plants including many common flowering plants, fruits, and vegetables.
It can also live on dead leaves and other plant debris.
As a result the gray mold fungus can be found in most gardens in Minnesota.
Infection often starts on wounded or old plant tissue. Flowers are also easily infected by the gray mold fungus.
Powdery spores are produced on all infected plant parts. These spores spread by wind or splashing water to new plants.
The gray mold fungus thrives in wet weather or high humidity.

Gray mold can show up anytime wet weather occurs during the growing season.
Space your plants properly to encourage good air circulation between plants.
Split or thin overgrown perennials.
Keep your plants dry.
Avoid overhead watering.
Use drip irrigation or a soaker hose.
Don't water late in the day. Give your plants time to dry off after watering them.

Infections usually begin on weak, dying or wounded plant tissue.
Handle plants carefully when transplanting and pruning.
- Gray mold usually attacks wounded plants, so avoid harming your plants.
- Prune plants later in the day when plants they are dry.

Remove infected plants or plant parts in a paper bag. Bury or compost all infected plant parts.
Compost stems, leaves, and flowers that are removed for deadheading and other garden maintenance.
Many moths, butterflies, bumble bees and other wildlife rely on leaf litter and plant debris for winter protection.
- Allow fallen leaves and other plant debris from healthy plants to remain in the garden over winter.
- Remove and compost leaves, stems, flowers and plants that have been infected with gray mold or other fungal diseases.

Michelle Grabowski, Extension educator

Reviewed in 2019