Common Scab

Streptomyces spp.

Common Scab

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A close up picture of common scab disease Streptomyces scabies on potato
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Common Scab is a plant disease caused by a specialised group of bacteria called Actinobacteria. Their life cycle shows similarities to fungi. The symptoms produced by Common Scab resemble those of Powdery Scab. However, unlike the latter, Common Scab thrives in dry conditions; typically occurring following dry, hot summers. The disease is most aggressive when soils are sandy and light with higher lime content. The main symptoms involve patches of 'corky' tissue on tubers, roots and stolons. Although, symptoms typically aren't noticed until harvest. Overall yields are rarely impacted severely, although, tubers will require additional peeling to remove the lesions. Potatoes which are grown for shows can be spoiled.
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Identification

Irregular-sized patches, or margins, of 'corky' tissue on tubers. Some of the lesions may be pitted. Pockmarks or small darkened craters on tubers. Similar symptoms may arise on roots or stolons.

Growth factors

The disease-causing bacteria thrives in dry, sandy soils with over average lime content.

Lifecycle

Common Scab Disease is caused by a group of microscopic organisms called Actinobacteria. They're a specialised type of bacteria that share similar behaviours of fungi. Unlike bacteria, Actinobacteria produce mycelium and spores. Spores or hyphae enter the plant from the soil through natural openings, such as the pores of the epithelium (stomata). The infection develops in time with tuber formation or enlargement, but the timing may also be influenced by soil moisture and pH. With growth, irregular lesions, or scabs begin to form which are where the bacterial spores are produced. They're released into the soil where they can pesist and reinfest new plants.

Prevention

The more resistant potato cultivars include: 'Accent', 'Ostara', 'Swift', (First Earlies) 'King Edward', 'Nadine', 'Wilja' (Second Earlies and Main Crop). The varieties 'Desiree', 'Majestic' and 'Maris Piper' are highly susceptible and should be avoided if growing conditions are dry and the soil is sandy and alkaline. The drier the soil, the more severe the disease can be. Proper irrigation can be beneficial in controlling Common Scab. Green organic matter, e.g. lawn cuttings, can improve light soils. Around three buckets of mowings or green organic matter should be applied per every square meter of garden/ allotment, making sure it's dug in well. Avoid using lime if on scab infested land. Don't dispose of any diseased material in compost heaps. If Common Scab is expected, the severeity/ occurrence of the disease may be reduced if plants are watered right before the tubers are about to develop (during flowering) and when the soils begins to dry.
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