Agriculture: Alfalfa Pest Management Guidelines

Phymatotrichopsis Root Rot (Texas Root Rot)

  • Phymatotrichopsis omnivora (=Phymatotrichum omnivorum)
  • Symptoms and Signs

    The first symptom of phymatotrichopsis 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

    Phymatotrichopsis root rot is also called, Texas root rot, phymatotrichum root rot, and cotton root rot. The disease is limited to certain areas in the deserts of Southern California (Palo Verde and to a lesser extent the Imperial and Coachella valleys), 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. omnivora infestation is most common on alkali calcareous soils in southern humid areas 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 mycelia 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.

    Text Updated: 03/17