We have all sorts of molds and mildews that cause infectious diseases on plants, often causing confusion to growers trying to establish a disease management plan. Botrytis blight is one of the fungal diseases that causes a rather distinct type of sporulation. The Botrytis fungus forms gray masses of spores on the infected plant part. The spore mass is often very fluffy and dusty. Color may range from green to brown but always appears dusty because of the mass of spores produced by the fungus. Gray mold is an appropriate common name for the disease.
We have seen gray mold on garden flowers, but this fungus has a broad host range, including herbaceous ornamental plants, vegetables, fruits, and greenhouse crops. Infection may occur on buds, flowers, foliage, stems, bulbs, and even roots. You will see sporulation on the most tender, newest growth as well as on the tissues that are injured or dying. Botrytis may be found on the new buds and dying flowers on the same plant. Refer to Report on Plant Diseases No. 623 for a list of plants commonly infected by this fungus. Some of the more common hosts infected with gray mold are zinnia, peony, marigold, phlox, rose, snapdragon, strawberry, tulip, and geranium.
Gray mold is prevalent in cool weather, so why are we seeing the disease now? The fungus thrives in cool, wet, humid weather. We have had hot, humid (or wet) days, but nighttime temperatures have been conducive to fungal development in some locations. This disease can spread rapidly, so long periods of cool weather are not needed. The time from infection to spore production is short, and spore production is prolific compared to many other fungal pathogens.
Fungicides are effective against Botrytis but only as protectants. To manage the disease, we need to produce conditions that do not favor fungal development. Avoid overhead watering, which splashes spores from plant to plant. Keep fruit (strawberries) from touching bare soil. Follow seed-packet recommendations for plant spacings to allow good air movement in the garden. Avoid overfertilization and constantly wet mulches. Know how to identify this disease so that you can stop it quickly. Fungicide recommendations can be found in the Illinois Homeowners’ Guide to Pest Management or the Commercial Landscape and Turfgrass Pest Management Handbook 2000.