N O V E M B E R / D E C E M B E R 2 0 1 7 12 POTATO GROWER Every year as harvest approaches there is excitement about getting the crop out of the ground. This is where all the hard work of raising a crop pays off. However, sometimes when the potato crop is lifted, there are unwanted ugly, scabby lesions— blemishes that spoil the appearance of the tuber and make them unde- sirable to consumers or difficult to process (Figure 1). This all too com- mon problem is generally referred to as common scab. I get multiple calls each year about common scab and what can be done to control this disease. For this article, I want- ed to summarize the biology of common scab and discuss what research has found to reduce com- mon scab on potatoes. As we all start thinking about the 2018 crop, I hope this article will help us all understand common scab better. Common scab comes from a soil- borne bacteria Streptomyces scabies or Streptomyces species that seem to be found in most soils. There are hundreds of bacterial species in the Streptomyces genus. This could explain why a potato cultivar may not be affected by common scab in one area, but have severe scab when planted in another geography. This wide spread disease expresses itself as raised, superficial and/or pitted lesions on the tuber (Figure 1 and 2). It can be difficult to visually determine the difference between common scab and powdery scab. A laboratory analysis is always a good idea to get the correct diagnosis. The NDSU Plant Diagnostic labora- tory can test for scab types for a nominal fee. The bacterium from Streptomyces species is spread by spores on seed, in the soil, in soil water and can hitch a ride on nematodes or insects. The spores enter the tubers through wounds and lenticels. Young lenticels are thought to pro- vide entry to Streptomyces species. Once in the plant, Streptomyces sca- bies produces a phytotoxin called thaxtomin that breaks down cell walls and penetrates rapidly grow- ing cells. As the potato plant cells die, they produce cork cells that push outward and form a scab lesion. As these cork cells continue to develop, the lesions grow larger. The type of scab (raised, superficial or pitted) varies based on the potato cultivar, environment and soil microbial community. Tubers are most susceptible to infec- tion of Streptomyces species during the first three to four weeks after tuber initiation, when compared to six to eight weeks after tuber initia- tion. Early infection can lead to deeper scab lesions on the tuber. However, there is less information available about susceptibility of tubers to the infection and develop- ment of common scab during late season tuber bulking. There are a variety of means that research has shown to reduce com- mon scab. However, few studies have shown effective and consistent control of common scab over many locations and years. Cultural and Andy’s Advice: Common Scab By Andy Robinson, Extension Potato Agronomist, NDSU/UMN Figure 1. Common scab of potato causes tan to dark brown rough scab- like lesions on the tuber surface. Figure 2. Common scab can be sunken causing what is often referred to as pitted scab.