A1822

Managing Lenticel Spot on tubers

Andy Robinson, Extension Potato Agronomist, NDSU/University of Minnesota Gary Secor, Professor and Plant Pathologist, NDSU

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 (Figure 1). Lenticels are breathing pores that appear as tiny slits on the tuber surface. They are the gateway for the (primarily to take in and exchange ) 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.

Figure 1. Lenticel spots on a potato tuber. (Andy Robinson, NDSU, U of M)

North Dakota State University, Fargo, N.D. December 2016 How Lenticel Spot Develops Lenticel spot is a common physiological disorder that Lenticel spot not only is unsightly; it also may allow occurs when lenticels enlarge. Potato tubers are living pathogen entry when lenticels are enlarged. This organisms, so when they are unable to obtain sufficient can lead to pathogen infection in and around the oxygen, the lenticels enlarge to acquire more oxygen. lenticels. Research has focused on bacterial soft rot When soil is waterlogged for a period of time, the (Pectobacterium carotovora) accumulation in lenticels, lenticels become enlarged and they can appear to be most likely because it is the most common pathogen like small white “popcorn” on the tuber surface (Figure found within the lenticels. 2). When the enlarged lenticels dry, they may appear as Lenticels have been reported to provide an entry point small scab lesions (Figure 3). for other pathogens of potato tubers, such as Dickeya Other conditions that cause reduced oxygen availability species, powdery scab (Spongospora subterranea), late and thus lead to enlarged lenticels are dry soil that blight (Phytophthora infestans), pythium leak (Pythium ultimum), pink rot (Phytophthora erythroseptica) and is highly compacted, elevated CO2 concentrations in storage or a film on the tuber after washing. common scab () (Figure 4).

Figure 2. Enlarged lenticels appear as “popcorn” as a result of too much water. (Andy Robinson, NDSU, U of M)

Figure 4. Multiple Figure 3. Dried infections in tuber enlarged lenticels lenticels. (Photo may appear as courtesy of John small scab lesions. Nordgaard, Black (Andy Robinson, Gold Farms, Grand NDSU, U of M) Forks, N.D.)

2 Effects of Lenticel Infections Reducing Lenticel Spot The infections of lenticels may result in visible lesions, Tubers being moved from the field to storage or a breakdown of tubers in storage or latent pathogens packaging facility likely will encounter changes in surviving and expressing themselves in the grocery temperature, moisture levels and air quality that may store or in seed. The spotted lenticels are a result of encourage lenticels to open and bacteria to multiply. In the latent phase caused when the lenticel closes and a wet year, many harvested potatoes are likely to have a corky layer forms over the bacterium, but the tuber lenticel spot. is not broken down. To mitigate problems with lenticel spot, avoid harvesting These small, localized areas of lenticel spot typically do low areas in the field. When piling tubers, be sure to not penetrate into the tuber more than 3 to 4 millimeters minimize bruising, and dry the tubers as quickly as (Figure 5) and typically are noticed a few days after possible to avoid lenticels staying too wet or being harvest or packaging. The spots with halos (Figure 6) exposed to elevated CO2 levels. Suberize potatoes around them are usually the result of bacterial soft rot for two to three weeks with fresh air that is 50 to 55 multiplying somewhat in the lenticel, causing sunken F and has 95 to 99 percent relative humidity. Proper and rotten tissue that is tan to dark brown, circular and suberization will close lenticels quickly, protecting the surrounding the lenticels in a halolike appearance. tuber from pathogen entry. Research has reported that Pectobacterium carotovora One situation likely to increase lenticel spots in the can survive in the lenticels throughout the storage packaging process is not sufficiently drying washed season. Rapid bacteria multiplication and rotting of the potatoes. Typically, washed potatoes are stored in a tuber is most likely to occur when oxygen is inadequate. cooler. This slows disease development, but it also slows When this happens, bacterial soft rot will occur quickly, drying time and suberization. especially in warm conditions. A water film around the tuber may lead to anaerobic conditions, causing lenticels to open and latent bacteria to multiply. Potatoes that are shipped with a water film can break down completely in transit under the right conditions and become a stinky mess; the warmer the shipping temperature, the more rapid the breakdown.

Figure 6. Rotten tissue can surround the lenticels in a sunken or halo-like appearance. (Andy Robinson, NDSU, U of M)

Figure 5. The depth of lenticel spot is shallow, only penetrating 3 to 4 millimeters. (Andy Robinson, NDSU, U of M)

3 Management Tips Selected References Managing lenticel spot can be challenging; however, here are some management tips to help reduce lenticel spot in the field and Inglis, D.A., B.K. Schroeder and D.A. in storage. Johnson. 2011. Bacterial Soft Rot and Lenticel Spot on Potato Tubers. Management in the field and at harvest: Washington State University n If possible, keep the available soil moisture below 90 percent. Extension Fact Sheet FS066E. n Harvest when temperatures are between 50 and 65 F. Pérombelon, M. 1972. The extent and n Avoid harvesting following heavy rains or in water-logged areas. survival of contamination of potato stocks in Scotland by Erwinia n Ensure that harvesting equipment is clean and sanitized. carotovora var. carotovora and E. n Minimize bruising. carotovora var. atroseptica. Annals n Properly suberize tubers. of Applied Biology 71:111-117. n Never wash tubers before placing in storage. Pérombelon, M. 1973. Sites of contamination and numbers of Storage management practices: Erwinia carotovora present in stored seed potato stocks in n Prevent the accumulation of CO2 and condensation in storage by ensuring good air movement. Scotland. Annals of Applied Biology 74:59-65. n Frequently clean and sanitize fluming systems and water. Scott, R.I., J.M. Chard, M.J. Hocart, J.H. n Clean water with an effective biocide. Lennard and D.E. Graham. 1996. n Clean and sanitize all equipment to prevent bacteria from Penetration of potato tuber lenticels spreading. by bacteria in relation to biological n Dry washed tubers with forced hot air as quickly as possible to control of blackleg disease. Potato remove the film of water on the tubers. Research 3:333-344. n Ensure that bags and shipping containers are well-ventilated.

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