Cause Chlaropsis thielavioides (formerly Chalara thielavioides), a fungus. It grows on the ends of 'Manetti' cuttings (Rosa chinensis var. manetti) and on wounds left by disbudding, resulting in poor callus formation and, subsequently, poor root formation. It develops under bud shields on field-budded 'Manetti', thus preventing callus formation and causing the bud to die. The fungus also may damage rose roots if the plants are in storage too long before planting or if plants are overwatered after planting. The chief infection source is fungal spores in the dust and debris of storage houses where the plants are handled and cuttings prepared for planting.
Symptoms A white, granular, fungal growth that spreads rapidly over cut and bruised areas of stock and scion. Soon the growth turns black.
- Practice strict sanitation in greenhouses, storage bins, and storage houses. Wash all dust and debris from the walls and floors before disinfecting storage house and bins.
- Use only uncontaminated stock for planting.
Chemical control Although fixed copper was used in the past to disinfect greenhouses and storage areas, there are no current registrations to support this use.
Reference Milbrath, J.A. 1946. Control of Black Mold Fungus Chalaropsis thielavioides Peyr. on Manetti Rose. Oregon State University Technical Bulletin 8.