Phytophthora Root Rot
Symptoms:
- Phytophthora root rot can kill seedlings and may damage and kill established plants in poorly drained soils.
- Seedlings are often infected as they emerge, then they wilt and collapse (‘damping-off’). The pathogens Aphanomyces and Pythium causes similar symptoms.
- Established plants wilt, become stunted, and leaves often develop a yellow-reddish brown color.
- Taproots infected with Phytophthora develop a red-brown to black rotting on the surface, and lesions may enlarge until the root is mostly rotted or girdled. Plants pull easily from the ground due to a short taproot and few lateral roots. Infected lateral roots usually have few or no Rhizobia nodules.
Pathogen Involved:
- Phytophthora medicaginis (a soil borne, fungal-like pathogen)
- The fungus is believed to survive for many years in the soil, and may attack alfalfa after long rotations to other crops.
- Phytophthora often occurs in the same fields as Pythium and Aphanomyces.
Time of Occurrence:
- All season
Conditions Favoring Disease:
- Disease is often seen during or after wet periods and can occur anytime during the growing season in slowly drained soils.
- It is most common in low-lying fields, those with poor drainage, and in soil with high clay content.
- Phytophthora root rot can also be damaging in well-drained areas during periods of heavy rainfall.
Disease Management:
- Plant alfalfa cultivars that have resistance to Phytophthora root rot.
- Treating seeds with fungicides containing active ingredients such as metalaxyl or mefenoxam will help protect seedlings from Phytophthora and Pythium damping-off for several weeks after planting.
- Water and soil management are key to disease control, these include selection of well-drained fields and improvement of water drainage by tiling,careful irrigation, and deep tillage to break up compacted soil.
- Maintenance of adequate soil fertility to promote lateral root growth.
- Harvesting during times that will minimize plant stress may also reduce disease.