Other meanings of Streptomyces scabies
Plant Pathology
Streptomyces scabies is a Gram-positive bacterium that causes common scab, a widespread disease affecting potatoes and other root crops, characterized by corky, scab-like lesions on tubers and roots. It is a member of the genus Streptomyces, known for its filamentous growth and production of numerous secondary metabolites, including the phytotoxin thaxtomin A, which is the primary virulence factor. The disease leads to significant economic losses due to reduced marketability of produce, although the pathogen does not typically reduce yield. S. scabies is notable for its ability to survive in soil for extended periods, making management challenging.
Streptomyces scabies is a filamentous, soil-dwelling actinobacterium that forms branching hyphae and sporulates, producing spores that are resistant to desiccation and can persist in soil for years.1 It infects plant roots and tubers through natural openings or wounds, and its primary virulence mechanism involves the production of thaxtomin A, a phytotoxin that inhibits cellulose synthesis in plant cell walls, leading to cell death and lesion formation.2 The pathogen is also known to produce other secondary metabolites, including the plant hormone auxin (indole-3-acetic acid), which may contribute to symptom development. Disease severity is influenced by environmental factors such as soil moisture and temperature, with dry, warm conditions favoring infection.
Common scab affects a broad range of root and tuber crops, including potato, beet, carrot, parsnip, and radish, as well as some non-root crops like turnip. On potato, lesions appear as raised, corky patches that can be superficial or deep, reducing the aesthetic quality and market value of tubers, particularly for fresh market and processing industries. While the disease does not typically reduce total yield, the economic impact is substantial, with losses estimated at 10% of global potato production annually.3 In addition to direct crop damage, the presence of scab lesions can increase susceptibility to secondary infections and reduce storage life.
Management of common scab relies on an integrated approach, as no single method is fully effective. Cultural practices include maintaining adequate soil moisture during tuber initiation, using resistant cultivars, and avoiding the introduction of infested soil or seed tubers. Soil pH management is critical: acidifying soils to pH below 5.2 can suppress disease, but this may not be feasible for all crops. Crop rotation with non-host plants, such as cereals or legumes, can reduce pathogen populations, though S. scabies can survive on weed hosts and organic matter. Chemical controls, including soil fumigants and seed treatments, are available but are often costly and have environmental concerns. Biological control using antagonistic microorganisms, such as certain Bacillus and Pseudomonas species, is an emerging area of research.
Beyond its role as a plant pathogen, Streptomyces scabies has been studied for its evolutionary adaptations and genomic features. It was the first plant-pathogenic Streptomyces species to have its genome sequenced, revealing a large chromosome of approximately 10 Mb with a high G+C content and numerous genes involved in secondary metabolism.4 Interestingly, the pathogenicity island (PAI) containing the thaxtomin biosynthesis genes can be horizontally transferred to non-pathogenic strains, expanding the pathogenic potential within soil communities. The bacterium also produces geosmin, the compound responsible for the earthy smell of soil, which is a common feature of Streptomyces species. Additionally, S. scabies has been found to suppress plant defense responses by interfering with salicylic acid signaling, a mechanism that is still being unraveled.
This article focuses on the bacterial species Streptomyces scabies, the causal agent of common scab on potatoes and other root crops.
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