How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Phymatotrichum Root Rot

Pathogen: Phymatotrichum omnivorum

(Reviewed 11/06, updated 11/06)

In this Guideline:


SYMPTOMS AND SIGNS

The first symptom of Phymatotrichum root rot on alfalfa is a rot of the outer surfaces of the roots, followed by bronzing of leaves and sudden wilting. Plants die quickly when taproots are girdled. A sheath of soil clings to the roots, and white-to-tan mycelial strands are found on the root surface. The disease appears as somewhat circular spots (fairy rings) within the field, which increase in size following rain or irrigation. The disease develops in late spring as soil temperatures increase.

COMMENTS ON THE DISEASE

Phymatotrichum root rot, also called Phymatotrichopsis root rot, cotton root rot, and Texas root rot, is limited to certain areas in the deserts of southern California (Palo Verde and to a lesser extent the Imperial and Coachella Valleys) and to Texas and Arizona. The causal fungus has a host range of more than 1800 species of plants.

Extensive research in Arizona and Texas indicates that a P. omnivorum infestation is limited to certain soil types and that the infestation was most likely originally associated with native desert flora.

The fungus survives many years in soil as sclerotia, as deep as 6 feet or more. Sclerotia produce mycelial strands that grow through soil and eventually contact a root. The growth of the fungus is favored by moist soil conditions. The temperature range for growth is 59 to 95°F with an optimum of 82°F. The fungus is more prevalent in heavy alkaline soils than in acidic soils.

Because the causal fungus is almost impossible to eradicate and could affect the value of the land, have the diagnosis confirmed by an expert diagnostician.

MANAGEMENT

Crop rotation with resistant crops such as corn, sorghum, or onion can help prevent the infestation from spreading within a field and reduce the level of inoculum, but it will not eliminate the infestation. For more information, see CROP ROTATION. No resistant cultivars are available.

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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