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Gray Mold (Botrytis) of Ornamental Plants

June 27, 2001
For those of you in the wetter parts of the state, here is a disease that you may have seen or will be dealing with soon. Botrytis blight is a fungal disease, often called gray mold due to the fluffy gray mold that develops on infected plant parts in humid conditions. You can confirm the presence of this fungus by placing infected tissue in a plastic bag with wet paper toweling. Seal the bag and observe the foliage. Within 12 to 24 hours, it is covered with a fluffy gray growth (mycelium and spores) of this fungus. Infection is worst in warm, wet, and humid conditions like those we see in the summer evenings following a rain. This disease infects in the evening after plants have been watered. For this reason, we try to encourage watering early in the day so that plants dry more quickly. Flowers, foliage, and even stems may be infected. Although any plant part may be infected, tender young growth or old tissues are most susceptible. Wounds are also likely sites of infection, but this pathogen can infect natural openings as well.

Disease control is similar to that for powdery mildew. Remove old blossoms and fallen plant debris, improve air circulation in the planting by pruning or thinning plants and using recommended spacings, and scout for the disease before using fungicides. The pest management handbooks list many fungicide options. Be sure to read the label of your chosen products carefully for host clearance, timing, rates, toxicity warnings, and the like. Consider alternating different chemical formulations to avoid development of resistant fungi. Botrytis was common in Illinois in 2000 on begonia, petunia, peony, rose, salvia, hosta, lilies, snapdragons, and marigolds.

For pictures of the disease, as well as details concerning the infection process, host plants, and disease management, consult Report on Plant Disease (RPD) no. 623, "Botrytis Blight or Gray Mold of Ornamental Plants," available on the Web or in Extension offices. (Nancy Pataky)

Author: Nancy Pataky


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