Aphanomyces Root Rot
Seedlings: 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.
- 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
- Infected plants may grow back slowly after harvest, and winter survival
may be reduced.
- 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:
Conditions Favoring Disease:
- All season, usually most damaging to seedlings.
- Wet, warm soil conditions and slowly-drained soils.
- Soil temperatures of 24-28° C are favored by this pathogen.
- 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.
Back to Alfalfa Diseases