Knowing Your Roots

Blog 6

Like humans, potatoes develop scabs. Although they look similar – raised, rough, brown growth on the surface of the skin – they are fundamentally different. The scabs you got when you scraped your knee as a child are part of the human body’s natural healing process. When the scab finally falls away, new, unblemished skin is left underneath. In contrast, potato scabs are a disease that infect the tuber skin; these scabs don’t heal and can even get worse as the disease progresses.  A better name would be “potato ulcers.”  Ulcers are open wounds that are slow to heal, originally thought to be caused by factors such as stress or skin irritation but later, were shown to actually be caused by bacteria.

Potato common scab is caused mainly by the bacterium Streptomyces scabies. The scabs produced by these bacteria are largely cosmetic and limited to the surface of tubers, but they drastically reduce marketability and are rated as one of the top 5 potato diseases in the U.S. Common methods of controlling infection – using pesticides and increasing irrigation – are expensive and often ineffective. In organic production, scab can be the major cause of tuber rejection. This is a classic example where breeding a potato with natural immunity to common scab is the most effective and perhaps the only realistic approach to managing this serious disease threat. There are currently no varieties of cultivated potato that are immune to common scab, but wild potato species offer a wealth of genetic diversity and have long been viewed as a potential source of desired traits such as disease resistance. Researchers at the UW-Madison have identified a line of the wild potato species Solanum chacoense from South America that is highly resistant to common scab and a closely related line that is susceptible.   

In my research, I am exploring the genetics of these two plants to examine the nature and location of scab resistance.  To accomplish this, I cross-pollinate the lines creating plants with one set of chromosomes from the resistant parent and one from the susceptible parent. These plants can then be self-pollinated for successive generations, creating plants with differing resistance to common scab. The resulting plants are powerful tools for mapping genetic traits; we can determine where the specific gene(s) that confer scab resistance are located in the overall genome, then create a simple genetic marker that can be used to quickly identify resistant plants and greatly speed up the process of selecting resistant lines – using traditional methods, this would take multiple years to achieve.

These two highly similar lines of a little known South American wild potato that differ in their resistance to common scab are providing a great opportunity to explore the factors that contribute to resistance or susceptibility to common scab and allow further insight into a disease that, despite its economic importance, is not well understood. Ultimately, these studies will assist other researchers in developing new commercial varieties that are resistant to common scab and allow potato growers to manage this destructive disease naturally.

For more information contact Christina Allen,

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