Gray mold of bedding plants
Gray mold (causal agent = Botrytis cinerea) is one of the most common diseases affecting bedding plants. Hosts commonly include, but are not limited to: begonia, carnation, chrysanthemum, cyclamen, geranium, impatiens, marigold, petunia and zinnia. Fortunately, gray mold is one of the easiest diseases to manage. By changing cultural conditions so that they are unfavorable to fungal growth, the disease can be successfully controlled.
Gray mold often infects the plant through flowers or leaves.
Photo: Chad Behrendt
Proper diagnosis is necessary prior to undertaking any control strategy. Because gray mold affects numerous host plants, care must be taken in examining affected plants. Symptoms of gray mold depend on both host and the environmental conditions. Commonly observed symptoms include: bud blast, leaf spots, flower blight, stem canker, and/or crown rot. (Fig. 1). If conditions are severe, plant death can result. Infected tissue is soft and brown, and may appear water-soaked. A key sign of Botrytis infection is the proliferation of gray mold covering the diseased plant.
Gray mold can quickly kill plants when conditions are favorable.
Photo: Chad Behrendt
Infection often begins at the site of the flower or bud. Flower blight of bedding plants is one of the first symptoms of this disease. The fungus establishes itself in the petals. If humidity remains high and temperatures are warm, the fungus spreads from the flower into the pedicel/peduncle. Eventually, the fungus invades the stem, leading to plant death. Symptoms of flower or bud blast begin as irregular gray/brown spots on the petals, whereas buds turn brown and/or have a water- soaked appearance. Infected buds may not open, or they may fall off.
A key sign for diagnosis is gray mold covering the blighted tissue.
Photo: Janna Beckerman
Leaf spot caused by Botrytis infection occurs in young, tender seedlings or when infected flower petals contact the plant leaves. Leaf spots commonly appear water-soaked, are tan in color, and have irregular margins. If warm temperatures and high humidity persist, the fungus can spread into the main stem and form cankers. When this occurs, plants can die quickly. (Fig. 2).
One of the key diagnostic features is the presence of the gray-brown mold growing over the affected area. (Fig. 3). When temperatures are warm (75° F) and humidity high (85%), the fungus will readily produce spores that spread the disease. Although the sign of Botrytis is a key diagnostic feature, you must remember that Botrytis may also appear as a secondary decay fungus. Care must be taken in observing that symptom development is consistent with Botrytis infection. If you observe unusual symptoms (e.g., wilt, angular leaf spot, deformity) of plant disease that are inconsistent with gray mold, and Botrytis is recovered, it should be assumed that Botrytis was acting as a decay agent and did not cause plant death.
Despite its voracious nature, gray mold is easily controlled by cultural techniques. Upon identification of gray mold, infected plant material should be removed and disposed of. This reduces the inoculum source and minimizes the possibility of infection. Equally essential for control of this disease is manipulating the environmental conditions that contribute to the growth and sporulation of the fungus. Adequate plant spacing, maintaining the relative humidity below 85% and providing good air circulation provide excellent control for this disease. By providing proper plant spacing, lowered humidity and good air circulation, the need for chemical controls can be minimized or eliminated entirely.