How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Seedling or Damping-Off Diseases

Pathogen: Pythium ultimum, P. irregulare, P. violae, Phytophthora megasperma, and Rhizoctonia solani

(Reviewed 11/06, updated 11/06)

In this Guideline:


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. 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 postemergent damping-off of alfalfa in California. 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, which 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).

COMMENTS ON THE DISEASES

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.

MANAGEMENT

Planting high quality seed under environmental conditions favoring rapid germination and seedling growth reduces the chance of infection. Therefore, avoid planting in November and December. Also avoid excessive irrigation and compaction or poorly drained soils. Purchase seed treated with an appropriate fungicides to protect seedlings from the damping-off pathogens. Although crop rotations do not eliminate these pathogens because of their wide host ranges, rotations with crops like small grains may help to reduce inoculum levels.

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430

Diseases

  • R. M. Davis, Plant Pathology, UC Davis
  • C. A. Frate, UC Cooperative Extension, Tulare County

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