By Eugenia Banks
Common scab recommendations can have inconsistent results.
By Eugenia Banks
Common scab occurs in most potato growing areas of the world. The causal organism
Streptomyces scabies has many strains, and some strains of this bacterium
are more aggressive than others. This strain variability may explain why the
practices recommended to control scab have been so inconsistent. The bacterium
survives in the soil for long periods of time even in the absence of potatoes.
Common scab occasionally affects other crops: carrots, beets, radish, turnips
There are two other species of Streptomyces, S. acidiscabies
and S. turgidiscabies, that cause common scab on potatoes, but these
species have a more limited geographic distribution. All three pathogens produce
thaxtomin, a toxin that is essential to produce scab lesions.
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.
A soil pH from 5.5 to 7 is most favourable for scab development. S. acidiscabies,
however, is acid tolerant and can infect tubers in soils with pH below 5.
|Table 2. 2005 scab trial: Evaluation of tolerance to scab.
Rating 1: Quite tolerant, very few superficial lesions only.
|Superior||White flesh, scab resistant standard variety.|
|MSL 211-3||White flesh, late blight resistant, line from MSU.|
|Lady Christi||Yellow flesh, very smooth skin.|
|Keuka Gold||Yellow flesh, round tubers.|
|Satina||Yellow flesh, High lutein content.|
|Cecile||Yellow flesh, long narrow tubers.|
|Amandine||Yellow flesh long tubers.|
|GoldRush||White flesh, excellent tasting variety.|
|Velox||Yellow flesh, new in the market.|
|Liberator||Released by MSU.|
Scab does not reduce yield but the lesions affect tuber quality, rendering
the potatoes unmarketable. Most scab infections take place at tuber initiation.
The pathogen enters newly forming tubers through immature lenticels stimulating
excessive cell growth and causing cell death.
|Medium susceptibility. Superficial scab and some pitted lesions.|
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 one 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
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.
|Very susceptible. High incidence of superficial and pitted lesions.|
Because too many factors influence scab incidence and severity (soil pH, organic
matter content, bacterial species and strain, soil texture, soil temperature,
soil moisture), management practices recommended to control scab have given
inconsistent results. No single method – except resistant varieties –
has proved reliable and effective. It seems that control methods should be custom
tailored for individual fields.
Management practices recommended to reduce scab incidence and their efficacy
are presented in Table 1. Also included in the table are promising control methods.
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 and in scab infested fields, grow varieties that are tolerant to scab.
Table 2 presents the results of a scab trial conducted in 2005 in Alliston,
Ontario. One hundred varieties/lines were grown in a heavily infested plot.
Scab rating was done at harvest. -30-
*Eugenia Banks is the potato specialist for the Ontario Ministry of Agriculture,
Food and Rural Affairs.
|Table 1: Management practices recommended
to reduce scab incidence and promising strategies being evaluated.
|Use of healthy seed||Inconsistent.||
Even healthy seed can carry the scab
bacterium on the skin or in the lenticels.
|pH lower than 5.2||
The bacterium S. acidiscabies
causes scab at soil pH below 5.
Not a practical method for many fields.
At soil pH lower than 5 some nutrients are tied up in the soil.
|High soil moisture for four to five weeks beginning
at tuber initiation
Inconsistent results. It might work in
some fields but not in others.
Many growers have tried this method but
the incidence of scab has not decreased.
|Crop rotation||Not effective.||
The scab bacterium can survive for many
years in soil in the absence of potatoes. Even an eight year rotation
proved to be ineffective.
|Sulphur containing fertilizers||
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.
Under Prince Edward Island field conditions,
soil acidification with +SO4 fertilizers during
the second hilling operation stimulated the development of beneficial
bacteria able to produce secondary metabolites with activity against S.
Pig manure, ammonium lignosulfonate might
work in a few fields but not in others. Fish oil did not decrease scab
incidence in a 2005 field trial.
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.
|Green clay||Ineffective.||No effect on scab incidence or severity.|
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.
|Not an environmentally friendly material.|
A scab reduction of 60 to 80 percent
has been observed after row fumigation with Chloropicrin.
It should not be used immediately after
the application of manure or following corn. A very expensive method.
Row fumigation costs approximately $300/ac.
Superior, Pike, Dakota Pearl, Liberator,
etc. are resistant/tolerant varieties that perform very well in heavily
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.
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.
The roots of Brassicacea crops (cabbage,
canola, mustards) release sulphur containing compounds that might be toxic
to the scab bacterium.
|Introduction of antagonists||
Trials conducted in Quebec with S.
melanosporofasciens applied as a seed treatment looked promising.
Baciullus sp sunhua is a bacterium recently discovered in Korea.
It reduced infection rate from 75 to 35 percent.
S. melanosporofasciens and Bacillus
sp sunhua produce antibiotics that are effective against S. scabies.