By: Kristi Waterworth
I say potato, but you may scream, “What are these giant white bumps on my potatoes!?!” when you unearth your crop this season. Swollen potato lenticels give a potato an overall uniformly bumpy appearance when they make their debut. Scary though they seem, they’re not a cause for serious concern. You should take note when you find them, though, because swollen lenticels on potatoes tell you a lot about your garden’s suitability for growing this root vegetable.
Lenticels are special pores in plant tissues that allow oxygen exchange with the outside world. Similar to stomas, lenticels appear on woody tissues like stems and roots instead of along more tender leaf tissues. So, you may ask yourself, “What causes potato lenticels to swell?”. The answer is moisture and lots of it.
Enlarged lenticels in potatoes can appear while the potatoes are still growing, or they can pop up when potatoes are in storage, giving a gardener a sudden surprise. As long as there aren’t signs of other problems, like fungal or bacterial disease, potatoes with swollen lenticels are perfectly safe to eat. They tend to go bad faster, though, so keep that in mind when sorting your harvest.
Swollen lenticels on potatoes appear in overly wet soils or humid storage environments, especially if oxygen availability is low. Choosing a well-draining site for your potatoes is the only effective way to prevent them.
When you’re prepping your bed next season, check the drainage carefully by digging a hole that’s 12 inches (30.5 cm.) deep and 12 inches (30.5 cm.) square. Fill it up with water and allow it to drain before filling it again. Allow your hole to drain for exactly an hour and check the water level. If your soil drained less than two inches (5 cm.) during that time, you have very poorly draining soil. You can choose another site and try again, or attempt to fix the one you have.
Increasing soil drainage is a lot easier than it would appear, especially if you usually mix your soil well before planting time anyway. Start by adding a layer of compost to your bed that’s equal to 25 percent of its depth, for example, if your bed is 24 inches (61 cm.) deep, you would mix in about six inches (15 cm.) of well-rotted compost.
Recheck the drainage after you mix your layer of compost into the soil. If the drainage is still very slow, it may be better to construct an above-ground bed, potato hills, or to simply plant your potatoes in large containers.
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Having your own potato patch ensures an ongoing supply of buttery baked or mashed potatoes, crispy hash browns, refreshing potato salad or a host of other satisfying dishes. Home-grown potatoes, however, sometimes have skins blemished with unsightly white bumps. Don't discard them these harmless bumps indicate that your perfectly edible potatoes are struggling with improper -- but easily remedied -- growing conditions.
Bartz JA, Kelman A. Infiltration of lenticels of potato tubers by Erwinia carotovora subsp. carotovora under hydrostatic pressure in relation to bacterial soft rot. Plant Dis. 1985. 69:69-74. doi:10.1094/PD-69-128 https://doi.org/doi:10.1094/PD-69-128
Dickey RS, Kelman A., Schaad NW. Erwinia, Carotovora or soft rot group. Laboratory Guide for the Identification of Plant Pathogenic Bacteria. 1988. 2nd ed. St. Paul, MN: APS Pr. p. 44-59.
Harrison MD, Franc GD, Maddox DA, et al. Presence of Erwinia carotovora in surface water in North America. J Appl Bacteriol. 1987. 62:565-70.
Powelson ML. Potato early dying disease in the Pacific Northwest caused by Erwinia carotovora pv. carotovora and E. carotovora pv. atroseptica. Am Potato J. 1985. 62:173-6. doi:10.1007/BF02852974 https://doi.org/doi:10.1007/BF02852974
Powelson ML, Apple JD. Soil and seed tubers as sources of inoculum of Erwinia carotovora subsp. carotovora for stem soft rot of potatoes. Phytopathol. 1984. 74:429-32. doi:10.1094/Phyto-74-429 https://doi.org/doi:10.1094/Phyto-74-429
Romberg MK, Davis RM, Nunez JJ, Farrar JJ. Sources and prevention of Erwinia early dying of potato in Kern County, California. Phytopathol. 2002. 92(Supp):S70-
Rowe RC, Davis JR, Powelson ML, Rouse DI. Potato early dying: Causal agents and management strategies. Plant Dis. 1987. 71:482-9. doi:10.1094/PD-71-0482 https://doi.org/doi:10.1094/PD-71-0482
Sessitch A, Reiter B, Pfeifer U, Wilhelm E. Cultivation-independent population analysis of bacterial endophytes in three potato varieties based on eu-bacterial and Actinomyces-specific PCR of 16S rRNA genes. FEMS Microbiol Ecol. 2001. 39:23-32.
Farrar J, Nunez J, Davis R. 2009. Losses due to lenticel rot are an increasing concern for Kern County potato growers. Calif Agr 63(3):127-130. https://doi.org/10.3733/ca.v063n03p127.
AMES, Iowa – The potato is one of the most important vegetable crops in the world. The edible part is the underground swollen stem known as a tuber – which varies in size, shape, color, storability and culinary uses according to cultivar. ISU Extension and Outreach horticulturists explain potential potato skin problems and management of the issues causing them. To have additional questions answered, contact the horticulture hotline at [email protected] or call 515-294-3108.
Potato tubers are enlarged underground stems. When potato tubers are exposed to light (either in the garden or storage), their skin turns green due to the formation of chlorophyll. The chlorophyll itself is not a problem. However, higher levels of glycoalkaloids also develop in the green tissue. Green tubers have a bitter taste when eaten. They may also cause an upset stomach and more serious health issues. Green portions should be cut off and discarded before boiling or baking the potatoes.
When growing potatoes in the garden, hill the soil around the base of the potato plants to prevent the tubers from being exposed to light. After harvesting, store potatoes in a dark location.
Rough, corky patches on the surface of potato tubers is due to potato scab. Potato scab is caused by the bacterium Streptomyces scabies. Though unsightly, scabby potato tubers are still edible. Simply peel the potatoes before use.
Potato scab is most common in alkaline soils (soil pH above 7.0). However, lowering the soil pH is difficult and not a practical option for most home gardeners. The incidence of potato scab can be reduced by selecting and planting certified, disease-free potatoes in spring. Choose cultivars, such as ‘Superior’ and ‘Red Norland,’ that possess good resistance to potato scab. Also rotate the placement of potatoes in the garden. If possible, plant potatoes in the same area of the garden only once every three or four years.
Growth cracks typically form when a prolonged dry period is followed by heavy rainfall or irrigation. The rapid uptake of water by the tubers causes them to split or crack. Cracking can be reduced by maintaining an even moisture supply during tuber development. Water on a regular basis during dry periods.
White, raised spots on potato tubers are due to wet soil conditions. Potato tubers are enlarged underground stems. Lenticels are small openings in the tuber surface that allow for gas exchange. Saturated soils cause the lenticels to swell as gas exchange is impeded. Affected potatoes may not store as long as normal, but are safe to eat.
Q: What are the white spots on our russet potatoes that I recently dug? Are they still edible? — Marie Talley, Oxbow, N.D.
A: Thanks for the interesting photos. The white lesions on the tubers are symptoms of a disorder called lenticel spot. Lenticels are the natural breathing pores of the potato tuber which usually go unnoticed — until they’re disrupted. Potato Grower Magazine describes the situation and cause well.
"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. When soil is waterlogged for a period of time, the lenticels enlarge and can appear like small white popcorn on the tuber surface. When the enlarged lenticels dry, they may resemble small scab lesions. Another condition that causes reduced oxygen availability, and thus leads to enlarged lenticels, is soil that is highly compacted."
Put bluntly, the potato tuber’s lenticel breathing pores opened wide in an attempt to get oxygen while nearly drowning in this year’s saturated soil. The potatoes are fine to eat, but the enlarged lenticels can cause the tubers to have a reduced storage life.
Q: Our bushes desperately need trimming and I was hoping to do it before we leave for Arizona. Considering the cold weather we’re having, can I trim now? — Jim W., Casselton, N.D.
A: I have good news: You can relax, as there’s no need to trim shrubs in the fall. In fact, there’s not really any advantage to prune this time of year, and it might carry some risks.
Sometimes in our fervor to tidy up the yard and garden for fall, we feel like we should be trimming overgrown shrubs. I’m a strong advocate for waiting until spring to prune shrubs while they’re still dormant and before the buds begin to swell and open. Fall pruning leaves open wounds, and I’ve noticed fall-pruned shrubs are more prone to branch dieback, possibly from moisture loss and desiccation through cut branch ends, especially during test winters with strong winds, frigid temperatures and lack of protective snow cover. Delaying pruning until closer to spring reduces the risk. You now have one less task before heading south.
Q: Last year, our moss roses were colorful and beautiful. This year they reseeded themselves by at least 1,000 percent, but we were so disappointed when every single one of them bloomed white. What happened? — The tenants at Mission Court, Ada, Minn.
A: Thanks for the fascinating question. Moss roses, also called portulaca, are one of the annual flowers that seed themselves readily, and lend themselves to planting in their own little flower bed space so they can emerge the following spring undisturbed. As a boy, I remember our neighbor in Lisbon, N.D., grew moss roses in a small square flower bed surrounded on all sides by sidewalk, and they arose each spring from seed shed by the previous year’s flowers.
Why did yours all bloom white? In many annual flowers, white is a predominant color, often more vigorous than other colors. For example, when planting different shades of petunias, I use only one white petunia plant for every two or three other colors to prevent the vigorous white from outcompeting the rest.
The white moss roses in your original mixture likely produced seed more rapidly than the other colors, dropping a larger quantity of viable seed. The other colors might not have produced any viable seed, or in a lesser amount, and when all seedlings emerged the following spring, the predominant white had won the survival-of-the-fittest battle.
Potato plant showing early wilting symptoms of blackleg.
Potato plant showing stem collapse due to blackleg.
Potato showing blackleg symptoms.
Cause The bacterium, Pectobacterium atrosepticum (syn. Erwinia carotovora subsp. atroseptica ), is usually associated with blackleg and soft rot in storage while P. carotovorum subsp. carotovorum (syn. E. c . subsp. carotovora ) is associated with aerial stem rot, lenticel rot, and soft rot. Moist, cool (below 70°F) conditions enhance blackleg while warmer conditions (70°F to 80°F) are optimal for soft rot. P. parmentieri and P. carotovorum subsp. brasiliense are also known to cause soft rot disease in the PNW but their abundance and economic importance is not currently known. Another bacterium, Dickeya dianthicola, is associated with blackleg on the east coast, resulting in poor emergence under warmer temperatures and decayed daughter tubers in affected fields. A Dickeya species, likely D. dianthicola , has been infrequently detected in diseased potatoes in the PNW.
Soft rot bacteria can be splashed onto plants by rain or irrigation causing aerial stem rot, and if present in surface irrigation water, can cause infections of leaf petioles and plant stems. These bacteria can survive on many weedy plants (nightshades, lambsquarters, pigweeds, purslane, etc.). They also reside in tuber lenticels without causing symptoms but can cause disease if tubers with lenticel populations are used for seed. The principal source of inoculum for blackleg is contaminated seed tubers. The bacteria can be spread among seed pieces by machinery and handling, moving from diseased to healthy seed pieces during seed cutting.
Symptoms Blackleg-leaves curl up foliage gradually yellows and a wilt, due to root and stem rotting, generally happens prior to or at row closure. Often at the soil line or above, mushy light brown-to-blue to inky-black lesions develop. Belowground stems are the same color. Aerial tubers also may form on stems. Tuber rot usually begins at the stem end and results in a black, slimy rot. Lenticel rot occurs after washing prior to packing in fresh-market potatoes. These bacteria are present in soil and water. The problem is most often seen when packing potatoes directly from the field when pulp temperatures are warm.
Aerial stem rot-symptoms occur in the upper canopy and do not involve the stems below ground. Lesions range from light brown to colorless. Stems still get very mushy and hollow and may be filled with mucilaginous slime. Aerial stem rot can be exacerbated by inclement weather that damages plant stems (e.g., hail, high winds, etc.). This disease can be a secondary infection associated with stem infections by late blight.
Soft rot of tubers-on tubers, soft rot bacteria can cause disease in an area as small as a single eye or involve the entire tuber. Tuber tissues becomes soft, wet, rotted and are cream to tan in color. Upon exposure to air, tissues may turn brown to black in color around the infected areas, creating a delineation between infected and healthy tissues. Although rot of the soft-rot bacterium is relatively odor free, secondary organisms usually cause a foul smell.
Lenticel rot-the area around affected lenticels on tubers is usually swollen and appears wet but will dry to create small, sunken areas around each colonized lenticel.
Biological control Efficacy unknown in Oregon.
Reference Strand, L. 2006. Integrated Pest Management for Potatoes in the Western United States, 2nd edition. UC Agriculture and Natural Resources Publication 3316. 167 pp.
The reds (can’t remember the variety offhand) are reasonable but covered in small white spots. These aren’t yet another problem thank goodness, but lenticels. Lenticels are minute organs used by the tuber to breathe but in waterlogged soils they become blocked and swell up.
Normally you don’t see them at all, but you can see them when swollen (see photo at the end). It’s not good news though, the swollen lenticels means the potato is more susceptible to soft rot and, at best, you’ll need to use quickly and check frequently for rot in store.
At least we don’t have potato stem rot which is similar to blight in appearance, unlike the tenants of one allotment site who emailed me saying:
“While not as serious as Potato Blight this is still a highly infectious fungal disease that is spread by windborne spores. These can live in the soil for years and will infect potatoes planted again next year given the same weather conditions. Plotholders are to be advised to cut down the stems immediately, remove every piece of the plant, take this home dry it and then burn it. As you will realise the chances of everyone doing that are nil!! They will take it away and put it in the Brown Bin and so spread it even further. The potatoes in the ground will be unaffected and may be left there for a few weeks.”