HPIPM:Damping Off, Seedling Blight

Author: Howard F. Schwartz, David H. Gent, and William M. Brown, Jr.

Identification and Life Cycle

Many pathogens can cause damping-off and seedling blight of alfalfa seedlings in the High Plains region, including Pythium spp., Phytophthora spp., and Rhizoctonia spp. Other fungi that cause alfalfa damping-off such as Cylindrocladium crotalariae and Mycoleptodiscus terrestris seldom are found in the High Plains region. Damping-off is associated with cool (Pythium and Phytophthora spp.) to warm (Rhizoctonia spp.) soil temperatures and excess soil moisture. Pathogens associated with damping-off are common soil inhabitants and can survive in soil as dormant oospores or sclerotia, pathogenically on alternate hosts and weeds, and saprophytically on crop residues.

Plant Response and Damage

Damping-off can occur before or after alfalfa emergence. Pre-emergence damping-off results in a brown, gelatinous rotting within the seed coat. Radicles and cotyledons may become brown and soft after germination, but fail to emerge. Water-soaked, greasy lesions may also form on hypocotyls and roots after emergence when seedlings are infected with Pythium or Phytophthora spp., causing plants to collapse and wither. Post-emergence damping-off caused by Rhizoctonia spp. begins as elliptically-shaped, brick-red sunken cankers on roots and hypocotyls. Plants eventually become flaccid and die.

Biological Control

Kodiak (Bacillus subtilis strain GBO3) is registered for suppression of Fusarium, Rhizoctonia, and Pythium spp., but its efficacy in the High Plains is unknown.

Cultural Control

Plant high quality alfalfa seed in a firm, well-prepared seedbed at optimum pH, soil temperature, and fertility level for rapid germination and growth. Avoid compaction, poorly drained fields, and excess irrigation that can favor damping-off pathogens. High seeding rates can partially compensate for stand losses when disease is present.

Chemical Control

Chemical controls are most effective when combined with sound cultural practices.

Product List for Damping-Off and Seedling Blight:

Pesticide	Rate per 100 lb seed (ai oz)	Application Frequency (days)	Remarks
Captan
Captan 30-DD	6.5 fl oz (1.9)	Seed treatment	Seed treatment
Captan 400	5-8 fl oz (1.9-3)	Seed treatment	Seed treatment
Captan 400-C	5-8 fl oz (1.9-3)	Seed treatment	Seed treatment
Nu-Grow Captan 4000	5-8 fl oz (1.9-3.0)	Seed treatment	Seed treatment
Metalaxyl/Mefenoxam
Apron XL LS	0.64 fl oz (0.5)	Seed treatment	Suppression of Pythium and Phytophthora spp.
Allegiance-FL	0.75-1.5 fl oz(0.2-0.4)	Seed treatment	Suppression of Pythium and Phytophthora spp.
Allegiance LS	1.2-2.4 fl oz(0.2-0.4)	Seed treatment
Ridomil Gold EC	¼ to ½ pint (1.9 to 3.8)	Pre-emergence soil drench	Suppression of Pythium and Phytophthora spp.
Thiram
42-S Thiram	8 fl oz (3.4)	Seed treatment	Seed treatment
Thiram 50WP	8 oz (4)	Seed treatment	Seed treatment
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.