Abstract:
Common scab is a disease that results in corky lesions on the surface of potatoes. These lesions can be variable in size, shape and as a result of secondary infection may appear extremely cryptic. The disease is caused by Streptomyces, a group of Actinomycetes that are largely saprophytic. For this reason lesions may also contain saprophytic Streptomyces which makes isolation and detection of pathogenic species on potatoes difficult. Symptom expression is thought to be attributed to environmental factors and prevalence of pathogenic species.
Common scab causing Streptomyces isolates were collected from most of the potato producing regions in South Africa. These isolates were characterized based on morphology, physiology and sequenced to confirm identity and determine their closest related species. A diverse range of other Streptomycetes, similar in morphology and physiology to Streptomyces scabiei (Lambert & Loria), were frequently isolated during this study from common scab lesions. Most Streptomycetes isolated from common scab lesions were S. scabiei or Streptomyces europaeiscabiei (Bouchek-Mechiche). No Streptomyces acidiscabies (Lambert & Loria) or Streptomyces turgidiscabies (Miyajima) were found. Streptomyces stelliscabiei (Bouchek-Mechiche) appears to be more geographically restricted in South Africa.
The presence of txtAB, tomA and nec1 genes were investigated within pathogenic as well as non-pathogenic isolates associated with common scab lesions. Some isolates were pathogenic but lacked any of the pathogenicity / virulence genes; while others were non-pathogenic but showed the presence of at least one of the three genes. However it seems that the combined presence of all three these genes is a good indicator of the pathogenicity of an isolate when compared with the tuber slice assay for pathogenicity.
Although common scab does not decrease yield, it results in smaller progeny tubers. A complex interaction exists between common scab causing Streptomycetes and their soil environment – increasing soil pH from 6.5 to 8.5 decreases disease incidence and severity. Symptoms were not found to be linked to species, soil pH or soil moisture. An increase in initial inoculum contributes to an increase in disease incidence and severity.
In future uncertainty regarding the pathogenicity gene makeup of the South African Streptomyces complex must be resolved. This will include sequencing the most important genes related to pathogenicity and comparing South African isolates with the isolates from other countries. The detection of these genes with Thaxtomin A expression (HPLC) should also be examined. Should these two factors co-exist it will then be possible to design primers specific to South African pathogenic Streptomyces spp. that could possibly be used for quantitative detection pre-plant.
