This is still true today: in 70 years, despite the progress made by potato researchers in controlling other important diseases, there is not a practical control method available, except genetic resistance, that consistently controls scab. While there are varieties tolerant or resistant to scab, many varieties preferred by the consumer are susceptible. So the search continues for management practices that will reduce the incidence and severity of scab.
Scab symptoms vary greatly. In mild cases, there is only netting on the skin. More severe infection results in superficial or raised areas of rough, corky tissue. Pitted scab has cavities that may be as deep as a centimetre. Lesions also vary in size and shape. They may be few and scattered or may cover most of the tuber surface. As tubers increase in size, lesions expand and scab severity increases. The types of lesions that develop are probably related to the aggressiveness of the Streptomyces strain, the time of infection and the environmental conditions.
The incidence and severity of scab varies from year to year and from field to field. The incidence of the disease is generally greater in sandy than in heavier soils. Warm, dry soils favour scab development. The addition of lime usually increases scab severity.
It is important to mention the term 'consistent control' because, in Europe, common scab incidence is greatly reduced by keeping high soil moisture for four to five weeks, starting at tuber initiation. This management practice has proven to be erratic in Ontario with some growers reporting a higher incidence of scab after following this practice. Many other management practices evaluated to control common scab in Canada have also given inconsistent results; what works in one field may be ineffective in other.
It seems that control methods should be custom-tailored to individual fields. One of the explanations given for erratic scab control is that soil physical characteristics, especially soil pH, play a role in the survival of the common scab bacterium. Another important factor that may contribute to the erratic control is the population dynamics of the scab bacterium. Besides S. scabiei, there are other species of Streptomyces that cause common scab. Reports from Finland indicate that S. turgidiscabies causes mainly pitted scab and S. aureofaciens causes netted scab. Another species, S. acidiscabies, causes common scab at pH lower than 5.2, but this pathogen only survives for two or three years in the soil.
To compound the problem, S. scabiei has many strains, some more aggressive than others. All the Streptomyces species and strains that cause common scab produce thaxtomin, a toxin that causes scab lesions. Control methods are now being investigated that would interfere with thaxtomin synthesis by the scab bacterium or inhibit the effects of thaxtomin on potato tubers.
Scab bacteria are introduced into healthy fields by planting infected seed, by spreading contaminated cattle manure, by spreading infected grade-out tubers in clean fields and by carrying infested soil on farm equipment.
Several researchers are focussing on the use of antagonistic micro-organisms able to suppress the growth of the scab bacterium. Agricultural soils suppressive to soilborne plant pathogens occur worldwide. Two classical types of suppressive soils are known:
- General suppressive soils. They suppress a pathogen due to the total diverse microbial population in soil. Their suppressiveness is not transferable between soils because of the complexity of the microbial population and their interactions.
- Specific suppressive soils. They suppress a pathogen due to the effects of individual or select groups of micro-organisms. This suppressiveness is transferable between soils.
An example of common scab suppressive soil occurred at a research plot in North Dakota. To conduct common scab trials, potatoes were planted continuously in that plot for many years. Initially scab incidence and severity were very high, but as potatoes were grown year after year, probably beneficial micro-organism populations started to increase. Scab incidence and severity decreased to a level too low to keep conducting trials.
Researchers have identified some bacteria that are antagonistic to the scab pathogen. The challenge is to match the prevalent scab bacterium strains in the field with the right antagonistic organisms. A practical application method and a reasonable cost are also important factors to consider.
As the potato researcher mentioned 70 years ago, the scab bacterium lives indefinitively in most soils. Once it is introduced to the field, it will not go away. Certified seed is more reliable and freer of scab than non-certified seed. However, even healthy seed can carry the scab bacterium on the skin or in the lenticels.
Management practices recommended to reduce scab incidence and their efficacy
- Recommended practices: Use of healthy seed. *Results: Not 100 percent reliable. *Comments: Even healthy seed can carry the scab bacterium on the skin or in the lenticels.
- Recommended practices: pH lower than 5.2. *Results: The bacterium S. acidiscabies causes scab at soil pH below 5. *Comments: Not a practical method for many fields. At soil pH lower than 5 some nutrients are tied up in the soil.
- Recommended practices: High soil moisture for four to five weeks beginning at tuber initiation. *Results: Inconsistent results. It might work in some fields but not in others. *Comments: Many growers have tried this method but the incidence of scab has not decreased.
- Recommended practices: Crop rotation. *Results: Not effective. *Comments: The scab bacterium can survive for many years in soil in the absence of potatoes. Even an eight year rotation proved to be ineffective.
- Recommended practices: Sulphur containing fertilizers. *Results: The value of ammonium sulphate (+SO4) fertilizer as a method of controlling potato common scab often varies from field to field and from year to year. *Comments: Under Prince Edward Island field conditions, soil acidification with sulphate fertilizers during the second hilling operation stimulated the development of beneficial bacteria able to produce secondary metabolites with activity against scab.
- Recommended practices: Soil amendments. *Results: Pig manure, Ammonium lignosulphonate might work in a few fields but not in others. Fish oil did not decrease scab incidence in a 2005 field trial. *Comments: Too many factors have an impact on the effectiveness of soil amendments: soil pH, organic matter, previous crop residues etc. What works in one field is ineffective in other fields.
- Recommended practices: Green clay. *Results: Ineffective. *Comments: No effect on scab incidence or severity.
- Recommended practices: Quintozene (blocker). *Results: Quintozene is registered in the US but not in Canada. Poor scab control has been reported in the US. The label rate seems to be too low to obtain good control. *Comments: Not an environmentally friendly material.
- Recommended practices: Chloropicrin. *Results: A scab reduction of 60 to 80 percent has been observed after row fumigation with Chloropicrin. *Comments: It should not be used immediately after the application of manure or following corn. A very expensive method. Row fumigation costs approximately $300 per acre.
- Recommended practices: Use resistant varieties. *Results: Superior, Pike, Dakota Pearl, Liberator and others are resistant/tolerant varieties that perform very well in heavilyinfested soils. *Comments: Genetic resistance is the most reliable, cost effective method to control scab. Breeding programs around the world are concentrating their breeding efforts to develop table and processing varieties with genetic resistance to scab.
- Recommended practices: Biofumigation. *Results: About 52 percent reduction in scab incidence was obtained in South Africa with the incorporation into the soil of dried cabbage residues. White Chinese mustard was tested in Maine. A 20 percent reduction in scab incidence was detected. Rapeseed meal tested in Scotland with good results. *Comments: The roots of Brassicacea (cabbage, canola, mustards) release sulphur containing compounds that might be toxic to the scab bacterium.
- Recommended practices: Introduction of antagonists. *Results: Trials conducted in Quebec with S. melanosporofasciens applied as a seed treatment looked promising. Bacillus sp sunhua is a bacterium recently discovered in Korea. It reduced infection rate from 75 to 35 percent. *Comments: S. melanosporofasciens and Bacillus sp sunhua produce antibiotics that are effective against S. scabiei.
- Recommended practices: Streptomycin seed treatment. *Results: Reduced scab incidence on daughter tubers. *Comments: Antibiotic Streptomycin seems to be toxic to the scab pathogen.
- Recommended practices: Fungicides. *Results: Ranman reduced scab incidence when applied in-furrow. Allegro had no effect. *Comments: Expensive?
All researchers agree that at the present time, genetic resistance is the most reliable, cost effective control strategy for common scab. Susceptible varieties such as Yukon Gold, Envol, Nordonna to name just a few, should be grown in scab free soil. In scab infested fields, grow varieties that are tolerant to scab.
In 2005 and 2006, a total of 200 potato varieties/lines were evaluated for their susceptibility or tolerance to common scab. The trial was conducted in an Alliston, Ontario, plot heavily infested with the scab bacterium. The replicated trial was planted on May 25 and 26 and harvested on October 18. The trial was not irrigated. The total rainfall during the growing season was four inches.
At harvest, the varieties/lines were evaluated according to these ratings:
- Rating 1 - Quite tolerant. Very few superficial lesions.
- Rating 2 - Good tolerance. Some superficial scab lesions.
- Rating 2.5 - Medium susceptibility. Up to 25 percent of tuber surface covered with lesions.
- Rating 3 - Susceptible. Many superficial scab lesions and some pitted lesions. Up to 50 percent of the tuber surface covered with lesions.
- Rating 4 - Very susceptible. High incidence of superficial and pitted lesions
- Rating 5 - Extremely susceptible. Mostly pitted lesions.
*Eugenia Banks is a potato specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs in Guelph.
A complete listing of the Alliston scab trial evaluation, with tolerance/ susceptibility ratings of all varieties, is available from Eugenia Banks or the Ontario Potato Board.
More on scab
Common scab is one of the most important production problems of potatoes worldwide. Once this pathogen is introduced into the soil, it persists for long periods even in the absence of potatoes. Although scab does not reduce yield, it does reduce tuber quality rendering the potatoes unmarketable.
The International Potato Common Scab Conference in Guelph, Ontario, March 5 and 6, 2007, brings together expertise from around the world to discuss current and potential management practices to reduce the incidence of this disease. Information will benefit growers, researchers, crop consultations and extension personnel.
The conference will feature speakers from the US and Canada as well as Scotland, South Africa and Australia. They will discuss their experiences and advice on management of scab, reducing its incidence and the development of genetic resistance.