Scouting and treatment of gray mold in greenhouse crops

While March 2021 was one of the sunniest on record here in Michigan, cloudy and cool weather inevitably rolls in during April and May. Peak color in the greenhouse coincides with cloudy, damp weather that creates a favorable environment for the fungus Botrytis cinerea, which causes gray mold on many greenhouse ornamentals. Unlike other diseases that require intensive scouting, inspection, and root examination, Botrytis only affects the above-ground portions of plants and is comparatively easy to find. It can cause symptoms such as leaf spots, blight, and stem canker.

Botrytis’ calling card includes the large masses of gray conidia or spores that are produced. It is this gray conidial fuzz that gives this disease the name “gray mold.” When scouting for Botrytis, growers should look in the lower canopy of plants and examine any leaves (alive and dead) resting on the surface of the growing media. Botrytis is very common during peak flower season in the greenhouse. Spent flower petals that drop onto leaves serve as a food base for Botrytis, allowing the pathogen to easily infect leaves and stems of nearby plants. A small leaf spot or dead leaves/flower petals sitting on the soil surface can quickly spread to infect a large region of plants. Botrytis behaves similarly to dust and can travel through air currents in a greenhouse. Take precautions when moving large numbers of plants around the greenhouse because it is likely that spores will be spread with movement.

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