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Slippery character: The potato disease that evades consistent control

Image result for common scab potato“Common scab is notoriously difficult to consistently manage with chemical or biological treatments – varietal resistance needs to be a fundamental strategy in building an integrated management approach for successful control,” dr Amanda Gevens told a gathering of industry people who attended the Potato Session at the International Crop Expo in Grand Forks, North Dakota yesterday. Dr Gevens is Associate Professor & Extension Specialist in the Plant Pathology Department at the University of Wisconsin-Madison. “Common scab of potato is an intractable disease of the crop, and as you all know, this often results in severe reductions in tuber quality and often storability due to disrupted periderm that show up as pithy patches on tubers,” she said. 

“The disease is caused by soilborne, filamentous, Gram-positive bacteria in the genus Streptomyces. The bacteria produce spores and behave much like fungal pathogens. Management is very problematic. Whilst quite resistant to chemical and biological treatments, cultural practices – including water and pH management – can often mitigate disease severity. No single practice seems to completely controls the disease.”

“In pathology we often take a more pathogen-focused, or one can even say simplified approach to characterizing pathogens so that we can better understand direct pathosystem interactions,” dr Gevens told the meeting. “However, for a complex soilborne pathogen like Streptomyces, we need to be more thoughtful and respectful of the complete biology of the soil to best understand disease and agronomic responses. This biological component is critical within the trinity of soil health: biology, chemical, and physical components.”

In her presentation, Gevens offered updates on common scab lesion phenotypes; preliminary evidence of microbial characters in association with common scab suppression and tuber yield potential – from Dr. Richard Lankau’s collaborative effort, and summary of 9 years of product trials for common scab control in northern Wisconsin on a susceptible cultivar.

In summary, Gevens made the following conclusion when it comes to a control strategy: “Our most consistent treatment schemes for managing common scab have included Blocker, or PCNB (the soil fungicide pentachloronitrobenzene) and chloropicrin. Variable results have been seen with biological and biopesticide treatments.”
Amanda Gevens can be reached at for further information.

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