Red Stele
Beginner
Scientific Name
Phytophthora fragariae
Identification
- Infected plants are stunted, with few runners and few fruit.
- New leaves are with bluish-green and may wilt.
- Older leaves may be reddish orange to yellow tinged.
- Roots rot from tip to crown.
- Lateral roots completely disintegrate leaving only the diseased main taproot (rat-tail).
- Interior core or stele of the root appears red surrounded by healthy white cortex tissue.
Often Confused With
Black root rot
Verticillium wilt
White grubs
Root weevils
Period of Activity
Red stele is most visible in spring between bloom and fruit ripening. Infections take place in spring or the previous fall, or anytime soils are cool and wet.
Scouting Notes
To confirm the presence of red stele, carefully dig up several wilted plants and slice the main roots lengthwise. If the interior core is brick-red or brownish-red, the plant has red stele disease. In a normal plant, the core is yellowish-white. The reddish discoloration is most evident in spring and in fall.
Varieties vary in their resistance to red stele. Susceptible varieties include: Evangeline, Veestar, Glooscap, Governor Simcoe, Honeoye, Jewel, Kent, Micmac, St. Pierre. Resistant varieties include: Annapolis, Brunswick, Cabot, Cavendish, Mira, Sable, Sparkle. Wendy and Mohawk are moderately resistant.
Thresholds
None established.
Advanced
Scientific Name
Phytophthora fragariae
Identification
Infected plants occur in irregular patches, associated with areas of poor drainage. They are stunted and produce few runners with little, or no, marketable fruit. New leaves may be a dull bluish-green in colour. Older leaves sometimes turn red, orange or yellow. During periods of plant growth or drought stress the plants wilt and die. Fine lateral roots disintegrate; main fleshy roots have a “rat-tail” appearance. Roots rot from the tip upward to the crown. The root core, or stele, is dark red, surrounded by healthy white tissue.
Often Confused With
Black root rot
Verticillium wilt
White grubs
Root weevils
Biology
The fungus attacks only strawberries. It is introduced into new fields on infected plants, contaminated soil, equipment or in surface water. Once introduced, red stele is able to persist for years in soil, even when strawberry plants are absent. It is most destructive in heavier soil types that are often saturated with water. The symptoms are most obvious in the low-lying, poorly drained areas but may spread to the entire field.
During the spring when soils are cool and have abundant free water, the fungus produces motile zoospores. These zoospores swim through saturated soil pores and are chemically attracted to the tips of strawberry roots. They penetrate the tips of young feeder roots and the fungus grows up the stele. As the disease develops, lateral roots die, giving the characteristic “rat-tail” appearance. During the summer, when soil becomes warm and dry, the fungus forms dormant oospores in the soil or dead root residues. Infected plants that have not totally collapsed grow new healthy roots.
During the fall, when the soil becomes cool and moist again, oospores germinate and zoospores are released into the soil. The fungus overwinters in infected roots and produces more zoospores in the spring. Zoospores may swim to the soil surface where they can be carried long distances in surface run-off. Zoospores are most active in soil between 7°C and 15°C (45- 60°F).
Period of Activity
Red stele is most visible in spring, between bloom and fruit ripening. Infections take place in spring or the previous fall, or anytime soils are cool and wet.
Scouting Notes
To confirm the presence of red stele, carefully dig up several wilted plants and slice the main roots lengthwise. If the interior core is brick-red or brownish-red, the plant has red stele disease. In a normal plant, the core is yellowish-white.
The reddish discoloration is most evident in spring and in fall.
Varieties vary in their resistance to red stele. Susceptible varieties include: Evangeline, Veestar, Glooscap, Governor Simcoe, Honeoye, Jewel, Kent, Micmac, St. Pierre. Resistant varieties include: Annapolis, Brunswick, Cabot, Cavendish, Mira, Sable, Sparkle. Wendy and Mohawk are moderately resistant.
Thresholds
None established.
Management Notes
- Choose well-drained sites and do not replant in sites known to have had the disease in the past.
- In high risk areas, choose resistant varieties.
- There are several races of Phytophthora fragariae; not all varieties are resistant to all strains of the fungus.
- Fungicides can be applied in the spring and/or fall to prevent spread of the disease.
- Try to eliminate spread of contaminated soil to new plantings.
- Obtain plants from a recognized plant certification program.
- Planting strawberries on beds raised at least 25 cm (10 in.) high will raise the root system above the water table and prevent high levels of infection with red stele.
