Field Crop Diseases
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Aphanomyces Root Rot


: this disease is most damaging and readily diagnosed on seedlings
  • Infected seedlings usually develop yellow cotyledons, wilt, and turn yellow with a purple tint.
  • Infected seedlings tend to be stunted and remain upright until severe chlorosis occurs prior to plant death and collapse.
  • Roots and hypocotyls are an off-white color in early stages of infection, and then become a yellow color, soft and water-soaked.
  • Large or small number of seedlings may die within 2-4 weeks of planting.

Established Plants
  • Plants can be stunted and chlorotic, and stands may be at low population levels due to death of many plants.
  • Clear, diagnostic symptoms are frequently undetectable.
  • Roots are often reduced in size and may have light brown, soft lesions, lateral roots tend to be rotted, and Rhizobia nodules are diminished or absent.
  • Infected plants may grow back slowly after harvest, and winter survival may be reduced.

Pathogen Involved:
  • Aphanomyces euteiches, a fungal-like pathogen
  • Two known races (1 and 2) have been identified that survive as oospores in soil or in infected roots. Symptoms are the same for both races.

Time of Occurrence:
  • All season, usually most damaging to seedlings.

Conditions Favoring Disease:
  • Wet, warm soil conditions and slowly-drained soils.
  • Soil temperatures of 24-28° C are favored by this pathogen.

Disease Management:
  • Plant alfalfa cultivars with resistance to Aphanomyces root rot. Most cultivars are resistant only to race 1, but a few cultivars with resistance to race 2 are available in some areas.
  • Avoid slowly drained fields and fields with a history of root rot or difficulties with seedling establishment.
  • Fields should be selected and managed to ensure good drainage of surface and subsurface water.
  • Rotate out of alfalfa for several years, and avoid pea, clover, and snap bean in rotations.

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