Causal agent(s) and transmission
Common and netted scabs are caused by bacteria of the genus Streptomyces, belonging to the Actinomycetes group (heterotrophic bacteria with a filamentous structure and living in the soil).
Two main types of symptoms may be distinguished: common and netted scab, which, accord- ing to recent scientific findings, are considered as two different diseases with separate causal agents seemingly involved and with different responses to climatic conditions and cultivar susceptibility.
- Common scab (raised or pitted scab) is caused by several species of pathogenic Strep- tomyces. In France, at least three species are responsible for this symptom: Streptomyces scabiei, S. europaeiscabiei, S. stelliscabiei. Other species have been described (S. acidiscabies, S. turgidiscabies, etc.) as associated with the common scab symptom, in other parts of the world.
- Netted scab is caused by Streptomyces reticuliscabiei and by some strains of S.europaeiscabiei.
For both types of scab, the soil is the main inoculum source as Streptomyces may persist in the soil on organic matter or on roots or residues of host plants.
Common and netted scab are present wherever the potato is grown. Common scab is a serious issue for fresh ware potato production as it can impair the visual aspect of the tuber.
Symptoms on tubers
The symptoms vary greatly:
- Common scab characterised by shallow or deep-pitted corky lesions (pustules) which depend on various factors, including the type of strain, the cultivar and the climatic conditions. On susceptible potato cultivars, the lesions form craters (photo 1) in the tubers (photo 2 to 4) and necrosis may be observed on the stem base (photo 5) and on the stolons. On other varieties, attacks may be superficial corky lesions on the tuber surface (photo 6) or star-like corky lesions (photo 7).
- Netted (or flat or surface) scab is characterised by corky spots that usually coalesce to form a network of polygonal corky lesions on the tuber surface (photos 8 and 9). Necrosis on the stolons and on roots and a reduction of root system (photo 10) may also be observed on highly susceptible cultivars.
In the case of an early attack, the yield may be affected.
With both types of scab, infection is via the lenticels during tuber formation and the disease does not progress further during storage. Generally speaking, common and netted scabs are favoured by light, aerated soils.
- Common scab (raised or pitted scab)
The appearance of this disease depends on the cultivar (different levels of susceptibility), on the temperature (optimum temperature 19 to 24°C) and on soil moisture (the disease is favoured by dry, aerobic conditions).
The appearance of this disease depends mainly on the cultivar used as only a few of them are susceptible and most potato cultivars are resistant to netted scab. The disease thrives in a cool temperature (13 to 17°C) and wet soil.
The control of common and netted scab must combine different techniques before or during the crop season:
- try to arrange long rotations to limit the incidence of netted scab;
- limit the introduction of host plants (beet, carrot, radishes, etc.) in crop rotation with the potato;
- evaluate the risk to the crop, in relation to the level of soil infection, with the use of bait plants;
- avoid the use of inadequately decomposed organic matter;
- limit the plantation of potatoes in too light soils or after soil preparation giving light-textured soil;
- limit the application of lime-based soil amendments just prior to the cultivation of the potato.
During the growing season:
- Use of cultivars resistant to netted scab and relatively less susceptible to common scab;
- irrigation to saturate the soil during tuber initiation and during the 2-4 weeks after tuberisation is an important means of maintaining the soil moisture near the field capacity and thus limiting the conditions for tuber infection with common scab.