Among the multitude of soil-inhabiting, saprophytic Streptomyces species are a growing number of plant pathogens that cause economically important diseases, including potato scab. Streptomyces scabies is the dominant pathogenic species worldwide, but is only one of many that cause very similar disease symptoms on plants. Molecular genetic analysis is beginning to identify the mechanisms used by plant pathogenic species to manipulate their hosts. The nitrated dipeptide phytotoxin, thaxtomin, inhibits cellulose biosynthesis in expanding plant tissues, stimulates Ca2+ spiking, and causes cell death. A secreted necrogenic protein, Nec1, contributes to virulence on diverse plant species. The thaxtomin biosynthetic genes and nec1 lie on a large mobilizable PAI, along with other putative virulence genes including a cytokinin biosynthetic pathway and a saponinase homolog. The PAI is mobilized during conjugation and site-specifically inserts in the linear chromosome of recipient species, accounting for the emergence of new pathogens in agricultural systems. The recently available genome sequence of S. scabies will accelerate research on host-pathogen interactions.

Evolution of plant pathogenicity in Streptomyces.

Common scab (CS) is an important disease and quality problem in potato crops worldwide. CS degrades the appearance of the potato tubers, thereby diminishing market value. Knowledge of CS has expanded considerably over recent years, enabling improved detection of the causal pathogens and increased understanding of mechanisms of pathogenicity, and providing potential methods of modulating pathogen response for disease resistance. However, effective control of this disease remains elusive, and will require increased understanding of both the host and the pathogen. Traditional control strategies such as irrigation and reduced soil pH are not sufficient and often fail. Optimizing environmental conditions for reduction of CS can also lead to favorable conditions for other diseases. The most desirable control method would be disease-resistant potato cultivars. However, no currently available commercial potato cultivar has been shown to be completely resistant to CS. In this review, we provide an overview of potato CS caused by plant pathogenic Streptomyces species, recent research on mechanisms and management of the disease, and knowledge gaps that limit successful control of this ubiquitous and troublesome disease.

In Search of Better Management of Potato Common Scab

Potato common scab is a widespread disease in which scab-like lesions develop on tubers. The disease is caused by pathogenic Streptomyces species, which synthesize the phytotoxin thaxtomin. The txtAB operon, responsible for thaxtomin production, can be used as a marker to identify pathogenic strains of the bacterium. Screening methods to assess scab susceptibility in breeding programs are time-consuming and can produce variable results. Management practices to control the disease vary and include crop rotation, tolerant varieties, monitoring soil pH, avoiding low soil moisture at tuber initiation, and application of soil- and/or seed-applied pesticides. There is a wide range in levels of tolerance among potato varieties. Many public research programs are committed to breeding for scab-tolerant varieties and evaluating management methods. Topics reviewed target readers focused on breeding and disease management objectives to reduce the incidence and severity of potato common scab.

Potato Common Scab: a Review of the Causal Pathogens, Management Practices, Varietal Resistance Screening Methods, and Host Resistance

Chemical profiling of extracts from a mud dauber wasp-associated fungus, Aspergillus sp. (CMB-W031), revealed a remarkably diverse array of secondary metabolites, with many biosynthetic gene clusters being transcriptionally responsive to specific culture conditions. Chemical fractionation of a jasmine rice cultivation yielded many known fungal metabolites, including the highly cytotoxic (−)-stephacidin B and an unprecedented nonribosomal peptide synthase derived nitro depsi-tetrapeptide diketopiperazine, waspergillamide A (1). All structures were assigned by detailed spectroscopic analysis and, where appropriate, chemical degradation and Marfey’s analysis.

Waspergillamide A, a Nitro depsi-Tetrapeptide Diketopiperazine from an Australian Mud Dauber Wasp-Associated Aspergillus sp. (CMB-W031)

The phylum Actinobacteria encompasses Gram-positive bacteria with a high DNA G+C. In soils, they present multiple lifestyles: rhizosphere saprophytes, endophytes, facultative symbionts and obligate phytopathogens. They play either beneficial or adverse effects towards plants. Numerous studies focused on their taxonomy and preservation. Also, adequate strategies were developed to enable their isolation. Phenotypically, the Actinobacteria is one of the most diverse phyla within bacteria. Their presence was once monitored by classical culture dependent techniques and phenetic characterization. Development of culture independent methods including chemotaxonomic and genomic approaches such as 16S rRNA gene analysis allowed to detect the so called unculturable bacteria. Major works on their diversity combine culture dependant and independant techniques. This chapter focuses on the phenetic and genetic diversity of Actinobacteria in plant-soil systems. It begins with some knowledge of their biology and their taxonomy, followed by a brief overview of the methods that have facilitated advances in the understanding of their diversity. It also discusses diversity of ecologically important plant associated Actinobacteria (rhizosphere and phyllosphere colonizers, endophytes, symbionts and phytopathogens).

Diversity of Plant Associated Actinobacteria

A relatively homogeneous group of streptomycete isolates was obtained from netted scab lesions of potato tubers collected from a potato field in Hokkaido, Japan. Based on 55 phenotypic data of 72 Streptomyces strains selected from these isolates together with spectral data on their soluble pigments and with data of a PCR analysis, using species specific primers, these netted scab causing pathogenic organisms were identified as S. acidiscabies. S. acidiscabies had previously been isolated from deep (common) scab lesions in the USA and reported as thaxtomin A producer. In contrast, our S. acidiscabies strains were not able to induce deep scab symptoms on potato minitubers in pot test, did not produce the phytotoxin thaxtomin A and did not contain the pathogenicity related gene, nec-1.

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Isolation and identification of pathogenic strains of Streptomyces acidiscabies from netted scab lesions of potato tubers in Hokkaido (Japan).

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Isolation and identification of pathogenic strains of Streptomyces acidiscabies from netted scab lesions of potato tubers in Hokkaido (Japan).