Symptoms and Signs
The above ground symptom of Phytophthora root and crown rot is plant wilting, typically occurring quite suddenly and often followed by plant death. The belowground symptom is tan-to-brown lesions on taproots, especially where a lateral root emerges. Lesions eventually turn black while the center of the root becomes yellow. In the root interior, orange-to-reddish streaks spread up several inches from the rotted end of the roots. Lesions can appear at any depth where water drainage is impeded. Occasionally, the disease may spread to the crown from the taproot.
Comments on the Disease
Phytophthora root rot is a cool-season crown and root disease, most often occurring in the San Joaquin Valley from March through May. It is important where soil water is excessive and can affect large areas of a field. Root and crown rot is common at the tail end of flood-irrigated fields where water collects.
The causal organism survives in soil as mycelia in infected plant tissue or as thick-walled oospores. The fungus also produces thin-walled sporangia that release motile zoospores in the presence of free water.
If the crown becomes infected, the plant will likely die. If infection is limited, the plant may continue growing at a reduced rate, and it will be susceptible to other pests and diseases. In these survivors, the taproot is usually killed and lateral roots develop to compensate. However, lateral roots will never grow as vertically or as deep as a taproot, resulting in plants only being able to take up water from a limited depth. Root and crown rot can be injurious to seedling stands, but is more common in established fields.
Proper water management, use of resistant cultivars, and crop rotation are key to keeping Phytophthora root rot under control in alfalfa production. Take the following steps to decrease the amount of time that soil is saturated with water:
- Till deeply to reduce compaction
- Reduce the length of flood irrigation runs
- Shorten irrigation time
- Level land before planting
- Install a tailwater ditch to remove excess water
- Plant on beds to help alleviate disease severity
Be careful when using return water because this and other pathogens (and nematodes) can be carried in recirculated irrigation water. Cultivars resistant to Phytophthora root rot are listed in the National Alfalfa & Forage Alliance website. Use resistant cultivars with sound cultural practices in fields known to have problems with Phytophthora.