Symptoms and Signs
Damping off is a name given to a condition where seeds are killed before germination or seedlings are stunted or collapse and die shortly after emergence. Seeds destroyed before germination are discolored and soft. After seed germination, symptoms include brown necrotic lesions along any point of the seedling. Lesions that girdle the young root or stem lead to plant death. Partially girdled plants, as well as those subject to continued root tip necrosis, may be stunted and yellowish in color to varying degrees.
A discolored, constricted area near the soil surface may be seen in older seedlings. The magnitude of the dark discoloration is dependent upon the age of the seedling as well as the duration of environmental conditions favorable for disease development.
Pythium ultimum and P. irregulare cause both pre- and postemergence damping-off of alfalfa in California. When emerged seedlings are killed, the stem has a water-soaked appearance and no definitive demarcation between healthy and diseased tissue. Pythium violae causes root -tip necrosis and inhibition of lateral root formation. Rhizoctonia solani may cause preemergent death of seedlings but usually causes postemergent necrosis of the stem at or near the soil surface. Necrosis is marked by a distinct margin between infected and healthy tissue.
Phytophthora megasperma, another common soilborne pathogen, can be particularly devastating in poorly drained soils. See PHYTOPHTHORA ROOT ROT for more information.
Comments on the Disease
Pythium and Rhizoctonia are common in most soils where they persist indefinitely. Both fungi are transported by water, contaminated soil on equipment, and movement of infected plant materials. Both have wide host ranges. Damping-off is favored by poor growth of alfalfa seedlings resulting from such factors as unfavorable temperatures, excessive moisture, low light, or improper fertilization.
Damping-off caused by Pythium spp. usually occurs under cool soil temperatures in fields with poor drainage.
Damage by R. solani is often related to the amount of organic matter that remains in the soil from the previous crop, with damage increasing as the level of organic matter increases.
Planting high-quality seed under environmental conditions favoring rapid germination and seedling growth reduces the chance of infection.
- Avoid planting in November and December.
- Avoid excessive irrigation and compacted or poorly-drained soils.
- Purchase seed treated with an appropriate fungicide to protect seedlings from the damping-off pathogens.
Although crop rotations do not eliminate these pathogens because of their wide host ranges and longevity in the soil, rotations with crops like small grains may help reduce inoculum levels.