Common Scab: A Possible Solution in the Soil

Photo courtesy of Agriculture and Agri-Food Canada

Potato producers have been hoping scientists would find a way to fight common scab—researchers have found a champion in the soil.

Common scab (Solanum tuberocum) is a serious problem that has plagued potato growing regions around the world for many years; however, a solution may be just around the corner.

After screening hundreds of soil bacteria, Agriculture and Agri-Food Canada molecular bacteriologist Claudia Goyer has found a naturally occurring Bacillus species that can inhibit the growth of common scab. “We are conducting research to find a way to control this disease naturally and reduce the impact on human health and the environment,” she explains.

Common scab is caused by a bacterium named Streptomyces scabies, which occurs in the soil naturally. Most, if not all, potato soils have a resident population of Streptomyces scabies, which increases with successive potato or other host crops. The disease infects potatoes when they start forming (tuberization), causing reddish-brown lesions and roughened areas on the tubers. As the tuber continues to grow, the areas of these spots also enlarge.

Once harvested, the quality of the infected potatoes is usually not good enough for the table market, resulting in yearly economic losses for farmers. “Common scab produces brown lesions on the tubers and, while not dangerous for human consumption, the affected potatoes will not make it to the table market because of the unsightly appearance of the tubers,” explains Goyer. Therefore, finding a solution to this disease will help producers’ bottom lines.

Disease Control

Common scab can live on decomposing material in the soil and does not need a potato or root crop to remain alive. Nevertheless, the disease will become more aggressive if the same crop is grown year after year without rotation. The presence of common scab and its severity varies by season and field to field. Cropping history, soil moisture, soil acidity, and soil texture are largely responsible for the variability of the disease in any given year.

Once a potato producer has common scab in the soil, there isn’t much the producer can do as no single measure provides effective control of the disease. “The disease is difficult to control because there is no chemical method of containment available in Canada,” explains Goyer. “A new control method for common scab would be very good for farmers and the environment.”

Currently, the disease must be managed through a combination of proper soil condition management techniques, such as irrigation and acidity levels in the soil, soil type, and crop rotation methods. For example, dry soil conditions during tuberization dramatically increases common scab severity, acidic soils with lower pH levels can help to reduce the severity of infestation, and crop rotation can be very important to help reduce the level of inoculums in potato fields. The use of resistant potato cultivars is also recommended to control common scab. Chemical control methods have met with limited success.

Research Discoveries

However, Goyer has been successful in testing several of the Bacillus species in greenhouse trials with a noted percent reduction in the severity of scab. “We are tapping into the natural organisms already in the soil that have the ability to inhibit this disease,” says Goyer. “In a controlled environment [such as a greenhouse] we have seen a 30% to 40% reduction in common scab infestation.”

Next year, for the first time, Goyer is conducting larger-scale field trials in which she hopes to see significant reductions in the occurance of common scab. “We are hoping to see a 15% to 20% reduction in the field,” states Goyer.

The researchers at Agriculture and Agri-Food Canada are still working on how best to use this Bacillus species in the field and they are conducting further research to find the best formulation. For example, researchers are testing liquid and pellet formulations of the Bacillus species, the right time to apply it, and what quantity would work best to fight the disease. “Through research into biocontrols, like the Bacillus species, we are able to develop biological systems, techniques, and strategies that can reduce soil diseases and thus decrease the impact of threats to Canadian food production,” explains Goyer.

A new way to control common scab will be welcome news for the potato industry with the promise of greater profitability and competitiveness for farmers.

Factors Influencing Common Scab Severity

  • Varietal resistance—Some varieties offer different levels of resistance, which can be an effective tool for management of common scab.
  • Soil acidity—The severity of common scab can be significantly reduced in soils with lower pH levels.
  • Soil moisture—High soil moisture levels during tuberization dramatically decreases common scab infection.
  • Soil type—Light-textured soils and those with high levels of organic matter are favourable to common scab infection.
  • Crop rotation—Crop rotation reduces the inoculum levels in potato fields, but Streptomyces scabies can survive for many years within the soil, even if potatoes have not been grown for some time.

Shannon Schindle