Common scab is one of the oldest known potato diseases of worldwide distribution. For about 100 years, it was not considered to be of economic importance in many potato production areas. However, the incidence and severity of this disease have increased over the last decade to the point where common scab is now an important potato disease worldwide. Common scab does not reduce yield, but affected tubers are unmarketable because of the superficial, raised or pitted corky lesions. Growers have usually reported common scab in fields after spreading lime or contaminated cattle manure, or in fields where infected seed was planted. Lately, growers are reporting common scab in fields that had grown clean crops or in fields that had never grown potatoes.

In a 2007 survey of 50 Ontario growers/researchers, four reasons were cited for the steady increase in common scab:

There are two common cultural practices that favor the development of scab: liming and spreading cattle manure. Common scab worsens as soil pH rises from 5.5 to 7. Thus, the incidence and severity of common scab increases after liming. For many years, growers have applied cattle manure to increase the organic matter and the water holding capacity of soils. If the cattle manure is from cows that were fed scabby tubers, the pathogen passes unharmed through the cattle intestinal tract and infests healthy fields or increases the inoculum in infested fields. However, tests with cattle manure from cows that were not fed potatoes did not aggravate scab severity and often reduced it.

The scab bacterium grows well over a wide range of temperature, from 50° to 90° F (10° to 31°C). This may explain why scab is a problem in what is considered a "cool" or "warm" summer. Management practices recommended to control common scab have given inconsistent results because there are too many factors that influence scab incidence and severity. With so many factors involved, the incidence and severity of scab varies from year to year and from field to field. The soil complexity plays an important role in the erratic results of management practices. It appears that control methods need to be tailored to individual fields.

Table 1: Management practices recommended to reduce the incidence and severity of common scab.

Stable genetic resistance is the most reliable and cost-effective control strategy, but scab-resistant varieties are not available for all markets and growing areas. Susceptible varieties such as Yukon Gold, NorDonna and Snowden, to name just a few, should be grown only in common scab-free soil. Varieties that are resistant or tolerant to scab like Pike, Superior, Keuka Gold, GoldRush, Norland should be the preferred varieties in infested fields.

Many potato breeding programs in the US now include resistance to common scab as an important trait in variety development. Several advanced lines from Michigan State University and the University of Wisconsin have shown high tolerance to common scab in Ontario trials. In Ontario, the potato breeding program of Dr. Peter VanderZaag and Dr. Zenaida Ganga is also developing varieties resistant to scab. Potato breeders are screening lines for scab resistance at different locations, to ensure that resistance to common scab is stable.

Common scab screening trials in Ontario in a heavily infested research plot have shown that there is good resistance to common scab in many new varieties: Dakota Diamond, Rio Grande Russet, Brandon Russet, Premier Russet, Blazer Russet, Stampede Russet and Gemstar.

Lately, soil fumigation with chloropicrin has been used in the US and Ontario to reduce the incidence of scab. In Ontario in the spring of 2003, a row-fumigation trial conducted in a heavily infested area of a field showed that the high rate of chloropicrin (80lb/acre) reduced common scab by 66% when compared to the untreated plots. In 2005, Dr. Doug Rouse from the University o f Wisconsin evaluated row fumigation with chloropicrin (80lb and 100lb/acre) and got similar results. He concluded that fumigation is a viable option when cultural practices are inadequate. In fields where common scab is not widespread, precision technology like GPS will allow growers to map areas of fields where common scab occurs and to fumigate only those hot spots. This should reduce the cost of fumigation, a major concern for growers.

The use of green manures appears to be a promising control method. Brassica crops such as canola and mustards are being evaluated as soil amendments because they release glucosinolates, which is transformed when incorporated into compounds similar to the active ingredient of the fumigant Vapam.

As the search continues for practical, cost-effective, environmentally friendly control methods that work consistently, growers are still struggling with this old disease. There is no doubt that a multiple approach is needed to achieve control of common scab.