Common Scab

Streptomyces scabiei species complex

Common Scab is a soilborne bacterial disease of potato and several other underground storage organs caused by pathogenic Streptomyces species in the S. scabiei complex. The disease is primarily a quality problem: rough, corky, raised, netted, or deeply pitted lesions reduce marketability and can increase trimming losses, although affected tubers usually remain firm rather than becoming soft and watery. The same pathogen group may produce scablike lesions on beet, radish, carrot, parsnip, and other susceptible roots, but host expression and economic importance differ. Infection occurs while young tuber or root tissues are expanding. Dry soil during early tuber set, soil pH above the crop-specific optimum, fresh undecomposed manure, and a history of susceptible hosts increase risk. Lesion appearance alone cannot reliably distinguish common scab from powdery scab, silver scurf, black scurf, or mechanical russeting; intact unwashed organs and production history are needed.

Identification

Operational role: bacterial disease complex. Confirm the host and determine whether lesions are superficial, raised, netted, or pitted. Common scab lesions are typically dry and corky, with irregular margins; underlying flesh is usually sound unless pits are deep or secondary organisms enter. Examine multiple organs from different plants because severity varies sharply within a field. Submit unwashed tubers or roots representing early and advanced lesions, plus a healthy comparison, soil pH, irrigation history during initiation, cultivar, seed source, rotation, and amendments. Laboratory isolation or molecular testing can support a Streptomyces diagnosis, but pathogenic and nonpathogenic Streptomyces coexist in soil, so detection without compatible symptoms is not proof of causation. Powdery scab tends to produce pustules that rupture and may involve roots or stolons; Rhizoctonia black scurf forms removable dark sclerotia; silver scurf causes silvery skin patches rather than corky pits.

Symptoms / Signs

Lesions begin during expansion of underground organs and become more obvious as the surface matures. They may remain shallow and russetlike, rise above the surface, form a netted cork pattern, or extend inward as pits. Affected potato tubers generally stay firm, and foliage may appear normal because the principal loss is cosmetic and grading-related. Severe pitting can increase dehydration and provide entry courts for secondary decay in storage. Uneven distribution often follows soil texture, pH, irrigation coverage, or previous amendment patterns. Do not interpret every rough potato skin as disease; physiological russeting, growth cracks, wireworm injury, powdery scab, and Rhizoctonia require separate consideration.

Life Cycle / Process

Pathogenic Streptomyces persist in soil, infested organic material, and lesions on seed tubers or susceptible roots. They produce durable spores and filamentous growth that can move with soil, contaminated equipment, irrigation water, transplants, and infected planting stock. Infection is most consequential during the several weeks after tuber initiation, when developing periderm is susceptible. Thaxtomin toxins produced by pathogenic strains disrupt expanding plant cells and contribute to corky lesion formation. Relatively dry soil during initiation often favors common scab, whereas maintaining adequate moisture during that narrow period can suppress disease in some production systems. The pathogen population may persist for years, and short rotations with nonsusceptible crops reduce risk but rarely eradicate it.

Prevention

Use certified or well-characterized seed tubers and avoid moving soil from infested fields on equipment, bins, or footwear. Select resistant or less-susceptible cultivars where their market class and local performance are acceptable. Test soil before planting; avoid liming a scab-prone potato field immediately before production unless pH correction is agronomically necessary. Do not apply fresh manure or poorly matured compost shortly before planting. Plan irrigation capacity so soil moisture can be kept even during tuber initiation without waterlogging the root zone. Rotate away from susceptible root crops for multiple seasons and control volunteer potatoes. Record field-level lesion severity, pH, cultivar, amendment, and irrigation data because risk is strongly site specific.

Management / Control

There is no curative treatment for lesions already formed. Confirm that the problem is common scab before changing soil pH or irrigation because those interventions can create other nutrient or disease problems. During the susceptible initiation window, maintain consistent soil moisture where water supply and soil drainage permit; avoid alternating drought and heavy irrigation. Segregate severely affected lots at harvest, handle gently, and store under conditions appropriate for sound potato tubers while monitoring pitted lesions for secondary decay. Clean soil from harvest and grading equipment before entering clean fields. For future crops, combine cultivar resistance, longer rotation, seed health, amendment discipline, and field-specific pH management. Any pesticide or biological product must be registered for the crop, pathogen, site, and application method; efficacy is variable and does not replace cultural control.

Procedure Steps

1. Map affected rows and collect at least 10 unwashed organs spanning lesion types and severities. 2. Photograph whole organs, close lesion margins, and a cut through representative pits. 3. Record cultivar, seed lot, soil pH, irrigation during initiation, manure or compost use, and rotation history. 4. Compare with powdery scab, Rhizoctonia black scurf, silver scurf, wireworm injury, and physiological russeting. 5. Submit intact samples and history to a diagnostic laboratory when identity affects seed, market, or field decisions. 6. Do not attempt to disinfect harvested lesions. Segregate heavily affected lots and watch for secondary decay. 7. Build the next-crop plan around resistant cultivars, clean seed, rotation, pH discipline, equipment sanitation, and even moisture during initiation.

Safety Notes

Common scab is not known as a human infection hazard from ordinary handling, but diseased produce may have cracks or pits that harbor soil and secondary decay organisms. Follow food-quality standards and discard tissue that is soft, foul-smelling, or otherwise decomposed. Wear gloves when handling treated seed or concentrated disinfectants, and follow all pesticide labels. Avoid acidifying or liming soil without a test-based plan because aggressive pH correction can injure crops and contaminate runoff.