Potato Grower

December 2017

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42 POTATO GROWER | DECEMBER 2017 The appearance of potato tubers is a key characteristic determining consumer buying on the fresh market. Blemishes may discourage purchases or entice the consumer to switch to a different market class. One unsightly problem affecting potatoes is lenticel spot or lenticel rot. Lenticels are breathing pores that appear as tiny slits on the tuber surface. They are the gateway for the gas exchange (primarily to take in oxygen and exchange carbon dioxide) in the tuber necessary for growth and development. Potato tubers have 28 to 274 lenticels per tuber. Lenticel numbers will vary by cultivar and tuber size. As tubers rapidly grow, the rupturing of the periderm often forms more lenticels. HOW LENTICEL SPOT DEVELOPS Lenticel spot is a common physiological disorder that occurs when lenticels enlarge. Potato tubers are living organisms, so when they are unable to obtain sufficient oxygen, the lenticels enlarge to acquire more oxygen. When soil is waterlogged for a period of time, the lenticels become enlarged and they can appear to be like small white "popcorn" on the tuber surface. When the enlarged lenticels dry, they may appear as small scab lesions. Other conditions that cause reduced oxygen availability, and thus lead to enlarged lenticels, are dry soil that is highly compacted, elevated CO 2 concentrations in storage or a water film on the tuber after washing. Lenticel spot not only is unsightly; it also may allow pathogen entry when lenticels are enlarged. This can lead to pathogen infection in and around the lenticels. Research has focused on bacterial soft rot (Pectobacterium carotovora) accumulation in lenticels, most likely because it is the most common pathogen found within the lenticels. Lenticels have been reported to provide an entry point for other pathogens of potato tubers, such as Dickeya species, powdery scab (Spongospora subterranea), late blight (Phytophthora infestans), pythium leak (Pythium ultimum), pink rot (Phytophthora erythroseptica) and common scab (Streptomyces scabies). EFFECTS OF LENTICEL INFECTIONS The infections of lenticels may result in visible lesions, breakdown of tubers in storage or latent pathogens surviving and expressing themselves in the grocery store or in seed. The spotted lenticels are a result of the latent phase caused when the lenticel closes and a corky layer forms over the bacterium but the tuber is not broken down. These small, localized areas of lenticel spot typically do not penetrate into the tuber more than 3 to 4 millimeters and typically are noticed a few days after harvest or packaging. The spots with halos around them are usually the result of bacterial soft rot multiplying somewhat in the lenticel, causing sunken and rotten tissue that is tan to dark brown, circular and surrounding the lenticels in a halo-like appearance. Research has reported that Pectobacterium carotovora (bacterial soft rot) can survive in the lenticels throughout the storage season. Rapid bacteria multiplication and rotting of the tuber is most likely to occur when oxygen is inadequate. When this Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In DISEASE MANAGEMENT | By Andy Robinson & Gary Secor Photos by Andy Robinson Managing lenticel spot on potato tubers Spotting the Problem 42 POTATO GROWER | DECEMBER 2017 infection in and around the lenticels. Research Multiple infections in tuber lenticels. Photo courtesy John Nordgaard, Black Gold Farms, Grand Forks, N.D. Lenticel spots on a potato tuber 42 POTATO GROWER | DECEMBER 2017

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