Botrytis Blight or Gray Mold

Botrytis blight, commonly called gray mold, is caused by the fungus Botrytis cinerea. This is the most important disease on floral crops and it is also the most important above ground disease on poinsettias. Botrytis blight can occur on poinsettia during all stages of production.

Botrytis infected leaf.
Click on image for larger version. (image © Ron Jones, NCSU)

Botrytis infected bract.
Click on image for larger version. (image © Ron Jones, NCSU)

Botrytis infected bract.
Click on image for larger version. (image © Robert Eddy, Purdue University)

Botrytis infected stem.
Click on image for larger version. (image © Brian Whipker, NCSU)

Plant with Botrytis stem canker.
Click on image for larger version. (image © Brian Whipker, NCSU)

Plants infected with Pythium (left) and Botrytis (right). Note the healthy white root system present on the Botrytis infected plant.
Click on image for larger version. (image © Brian Whipker, NCSU)

This fungus causes brown lesions on leaf stem or bract tissue. Infection and initial disease development almost always occur on wounded, weakened or dead tissue of leaves, stems or bracts. Healthy tissues that come in contact with diseased tissue almost always become infected. Lesions on bracts are initially located near margins and are minute in size. First they are light tan in color but become darker brown as the lesion expands. Latex may be exuded on the underside of leaf lesions. Under high humidity, Botrytis produces grayish brown spore masses over the surface of the rotting tissue. Millions of spores may be produced on each rotting lesions and each spore is capable of causing a new infection. Spores are easily dislodged and carried by air currents or splashing water to healthy plants.

Botrytis cinerea attacks over 200 hosts and is not host specific. Epidemics of Botrytis blight are favored by high relative humidity (85% or above) and free moisture on the plant surfaces. The fungus is active over a wide temperature range. Growing plants with very tight spacing, plants with dense foliar canopies, plants with excessive shade, and plants with very succulent growth are also favorable for disease infection.

Management of Botrytis Blight

Management of Botrytis Blight is not too difficult on poinsettias if the plants can be grown under ideal, stress-free conditions. This is not always possible.

There are numerous cultural practices growers can use to minimize the threat from Botrytis blight on poinsettia:

1. Adequate spacing;
2. Good air movement;
3. Minimize wounding and bruising;
4. Do not water overhead;
5. Watch temperature and watering carefully to avoid stress;
6. Use fungicide sprays or fumigants. (see: Disease Management Strategies Table)

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