Michelle Grabowski, UMN Extension Educator
Recent wet weather has provided perfect conditions for a common fungal disease of flowering annuals known as gray mold. Gray mold is caused by the fungus Botrytis cinerea
Photo 1: Gray mold from an infected flower moves to leaves M.Grabowski UMN Extension.
and can occur in a wide variety of annual flowers including impatiens, zinnias, geraniums and many more.
Photo 2: Brown leaf spot from gray mold infection M.Grabowski UMN Extension.
Often called Botrytis blight, gray mold causes a dark brown to black blight of flowers, buds, leaves and stems. Flower petals are especially susceptible to infection by the gray mold fungus. Brown spots may be seen on petals or the entire flower may turn brown. As flowers age, they fall off onto healthy leaves below. The gray mold fungus then infects the leaves. Removal of these rotted petals often reveals a brown target shaped spot on the leaf which quickly grows to rot the entire leaf. With high humidity a cloud of fluffy gray spores forms on old infected leaves and petals. These spores are easily blown or splashed to new flowers to start the infection cycle all over again.
Photo 3: Zinnia seedling from the center of a nine pack is killed by Gray mold M. Grabowski UMN Extension.
Gray mold thrives in wet crowded conditions. This time of year it can easily be found in over crowed annuals. Gardeners may have flats of annuals waiting to be planted. As these plants outgrow their small containers, humidity builds, and gray mold takes off. It is not unusual for a gardener to find the center plant of a six or nine pack completely blighted by gray mold.
Photo 4: Gray spores of gray mold on rotted leaves and petals M.Grabowski UMN Extension.
The best management strategy to minimize gray mold in annuals is to space out plants so that leaves and petals dry out quickly after rain or irrigation. If the annuals cannot be planted in the garden due to weather conditions or other factors, take pots out of the flat and space them out to allow good air movement between plants. If plants in multipacks cannot be planted quickly, transplant them into larger pots where they will have room to grow and will not crowd one another. When planting in the garden, place plants to allow room for the mature plant. Gray mold can show up anytime wet weather occurs during the growing season.
Spent flowers and infected leaves should be pinched off and removed. Fungal spores are formed on these old rotted plant parts, so do not leave them lying in the garden. Rather collect all infected plant parts in a paper bag and dispose of them. It is ok to compost leaves and flowers infected with gray mold because Botrytis cinerea is commonly found in soil and old plant debris. With a little care most plants can recover from gray mold once warm dry conditions return.