By C. M. Ocamb
Cause Several fungi or fungus-like microorganisms, including Pythium aphanidermatum, P. debaryanum, P. ultimum, Fusarium oxysporum and potentially other Fusarium spp., and Rhizoctonia solani. These microorganisms may overwinter on plant residues of many different crops or in soil and some may be carried on seed. Damping-off is more prevalent in poorly drained and excessively compacted, cold soils. Planting poor quality seed increases the problem as does planting seed too deep in wet, heavy soil.
Symptoms In cold soil, seeds may decay or hemp seedlings die before they emerge. In warmer soil, seedlings usually emerge but may show post-emergence dieback (damping-off) or are stunted and have brown, rotted roots and crowns.
Cultural control
- Select planting sites and dates to take advantage of warm soils.
- Plant seed of high vigor and good quality.
- For transplant production, make sure that pots and trays are clean; if re-using containers, wash pots to remove any used soil and plant debris followed by a 10% bleach treatment. Use of pasteurized growing medium is recommended. Do not re-use potting media from diseased plants. Avoid high levels of organic matter in the planting medium. When propagating by cuttings, ensure that the mother plants do not have any disease symptoms and that plants are healthy and vigorous. Place plants on benches rather than the floor inside greenhouse structures. Maintain warm soil temperatures during germination through plant emergence. Avoid high relative humidity inside greenhouses. Reduce the relative humidity by improving air circulation.
- Avoid establishment of hemp fields in soils where seed rot, damping-off, or root rot has occurred in the previous three years.
- Long rotations are required to reduce the population of pathogenic Fusarium species in infested fields; rotate out of hemp for at least three years.
- Clean tools and equipment between fields, rooms, or groups of plants so that there are no remaining soil/growing medium or plant residues that can lead to pathogen spread.
- Remove and destroy diseased plants while avoiding contact with healthy plants. Burn or deeply bury infected plants. Clean hands and gloves to ensure that no pathogens remain.
- Practice optimum water management, especially avoiding overwatering the first few weeks after planting.
Biological control
- Apply a bio-fungicide to suppress root rot and damping-off. Follow directions on the pesticide label. See the Oregon Department of Agriculture hemp pesticide guide list at https://www.oregon.gov/oda/shared/Documents/Publications/PesticidesPARC/....