common scab
Beginner
Scientific Name
Streptomyces scabies
Identification
Most scab infections take place at tuber initiation. Scab lesions are quite variable. The type of lesion— superficial, pitted or erumpent—is probably determined by variety tolerance, aggressiveness of the bacterium strain, time of infection and environmental conditions. The incidence and severity of scab varies from year to year and from field to field.
- Superficial scab or russet scab. Superficial, corky lesions
- Netted scab. Net-like, superficial lesions.
- Pitted scab. Severe lesions forming cavities as deep as 1 cm. The tissue around the interior of the pith is corky
- Erumpent scab. Lesions are raised, rough and corky.
- If tubers are dug when the soil is wet, a thin, whitish layer of spores may be present on scab lesions.
Often Confused With
Powdery
scab
Period of Activity
Most scab infections take place at tuber initiation.
Scouting Notes
- There are no above-ground symptoms
- Common scab lesions develop shortly after tuber initiation. Assessment of scab incidence can start when daughter tubers are about nickel size.
Thresholds
Different tolerances exist for processing and fresh market crops.
Advanced
Scientific Name
Streptomyces scabies
Identification
Most scab infections take place at tuber initiation. Scab lesions are quite variable. The type of lesion— superficial, pitted or erumpent—is probably determined by variety tolerance, aggressiveness of the bacterium strain, time of infection and environmental conditions. The incidence and severity of scab varies from year to year and from field to field.
- Superficial scab or russet scab. Superficial, corky lesions
- Netted scab. Net-like, superficial lesions.
- Pitted scab. Severe lesions forming cavities as deep as 1 cm. The tissue around the interior of the pith is corky
- Erumpent scab. Lesions are raised, rough and corky.
- If tubers are dug when the soil is wet, a thin, whitish layer of spores may be present on scab lesions.
Often Confused With
Powdery
scab
Biology
The scab bacterium is a common soil inhabitant that survives for long periods of time, even in the absence of potatoes. The strains of the bacterium that cause common scab produce a toxin called thaxtomin that enables the bacterium to cause lesions on tubers.
Most scab infections take place at tuber initiation. The bacterium enters newly forming tubers through immature lenticels. As tubers increase in size, lesions expand and scab severity increases
Planting scab-infected seed or spreading contaminated cattle manure will introduce the bacterium to scab-free fields. Tubers that look healthy may carry the bacterium on their skin. A soil pH from 5.5 to 7.5 is favourable for common scab development. The acid-tolerant scab species (S. acidiscabies) can infect tubers in soils with pH as low as 5.
Warm, dry soils favour scab development.
Period of Activity
Most scab infections take place at tuber initiation.
Scouting Notes
- There are no above-ground symptoms
- Common scab lesions develop shortly after tuber initiation. Assessment of scab incidence can start when daughter tubers are about nickel size.
Thresholds
Different tolerances exist for processing and fresh market crops.
Management Notes
- Plant scab-free seed. Infected seed should be discarded because it introduces the pathogen into healthy soils.
- The most reliable control method is growingonly resistant varieties in scab infested fields.Varieties such as Superior, Dakota Pearl, Norland, GoldRush, Gemstar are resistant to scab.
- Scab incidence and severity may be reduced by maintaining available soil moisture at 80% of field capacity for about 3 weeks starting at tuber initiation until tubers are golf ball sized.
- Side dressing ammonium sulfate has been reported to reduce scab severity.
- Do not use manure of farm animals that have fed on scab infected tubers.
- Avoid moving infested soil on farm equipment from contaminated to healthy fields.
