What you see: Brown lines in an arch-like or ring pattern inside your potato
What it is: Evidence of a potato virus!
Eat or toss: Cut around the brown areas and eat. Potato viruses target potatoes, not people.
What are the weird brown lines inside these potatoes?
You may peer inside this potato and see curious brown bands, but potato expert Nora Olsen sees battle lines. The potato, sensing a viral invasion, killed off a strategic “fence” of cells with the hope of stopping a virus in its tracks.
“It’s a form of resistance,” she said. “‘We’re going to kill off a part of us and the rest can live.’”
Dramatic stuff! Sometimes, she said, the potato will destroy a ring of its own cells around a virus, resulting in a circular pattern. These brown arches, circles and lines, known as necrosis and sometimes, “spraing,” are classic potato virus symptoms.
Here at EatOrToss we more often discuss the destruction that bacterial and fungal invaders can cause (like in onions or berries). In those cases, the microbes are actively eating and digesting the food, which leads to things like squishiness, icky smells and fuzz.
But viruses aren’t quite alive the way bacteria and fungi are. They don’t need to eat; they just need a host, and they’ve chosen the potatoes depicted here, and possibly the one you’re currently squinting at in your kitchen. So while the potato killed off its cells, no microbes are generating toxins or icky-tasting substances as they break down and digest the tuber. The dead cells look brown due to chemical reactions that come about when cells die (similar to how apple slices turn brown). The potato is still edible.
You could eat the brown area if you wanted to; a potato virus wouldn’t find you to be a very good host, so there’s little risk in eating it. Olson says she’s eaten virus-afflicted potatoes, though when there’s more browning she’s likely to cut it out for aesthetic reasons. She added that the areas near the browning might have higher sugar concentrations (potatoes sometimes produce more sugars when they’re stressed), which would cause them to look darker after frying.
How does a potato get a virus?
Olsen explained that there are three main potato-invading viruses, which have increasingly alarming names: mop-top virus, tobacco rattle virus, and Potato Virus Y. While bacteria and fungi tend to gain entry via wounds, natural openings and weakened tissue, viruses are smaller and some of them can effectively hitch a ride from the above-ground stems and leaves into the tubers via fluids that naturally circulate within the plant. They can also enter through the potato’s outer skin, perhaps by lurking on the mouth-like parts of a tiny, potato-snacking creature, but only show symptoms in the middle of the tuber. Here’s a little more about the potato viruses farmers and scientists worry about:
Mop-top virus. Spread by a protist called spongospora subterranea (say that three times fast), this virus can manifest on the potato’s skin or within its flesh. Even without the virus, the protist can cause a condition called powdery scab on the potato’s skin. (By the way, remember protists from high school biology? They have their own kingdom, but, it’s complicated.) According to a North Dakota State University write-up, the virus earned its name because it stunts the growth of above-ground foliage, leading to a limp, mop-like mess of leaves and stems just above the soil. The symptoms vary by cultivar, with some types of potatoes more susceptible than others.
Tobacco Rattle Virus. While we’re reminiscing about high school biology, let’s talk about nematodes (remember those weird, worm-like organisms?), which spread tobacco rattle virus. The potato’s defensive game is often indicated by brown rings and dark blotches with a corky texture in the flesh of the potato. The disease, sometimes called “corky ring spot,” tends to show up in the nematode’s preferred soil type: sandy and loose.
Potato Virus Y. This virus is spread by aphids feeding above ground; they nibble on leaves of infected plants, then transmit the virus when they feed on the leaves of healthy plants (not so dissimilar from how mosquitos spread malaria in humans). The virus can cause a green and yellow pattern and other symptoms on the leafy, above-ground part of the plant, but can also travel down to the plant’s tubers where it leaves raised rings on the skin. Potato Virus Y targets members of the solanaceae family, including potatoes, tomatoes, pepper, tobacco and eggplants. Infections usually start when farmers plant infected seed potatoes (farmers grow potatoes from sprouted chunks of other potatoes, called “seed potatoes”). Farmers can fight back by planting a “cleansing barrier” of crops that don’t host the virus. The virus can’t stay viable on an aphid for very long, and that cleansing barrier helps prevent transmission.
Thanks to Shirley G., of England, for sending us the photo featured at the top of the post!
- Nora Olsen. Professor and potato specialist. University of Idaho.
- About Potato Virus Y (PVY). Potato Virus Initiative. University of Idaho.
- About Potato Mop Top Virus (PMTV). Potato Virus Initiative. University of Idaho.
- Tobacco Rattle Virus (TRV) Quick Facts. Management of Tuber Necrotic Viruses. College of Agriculture and Life Sciences. Cornell University.
- Tobacco Rattle. Rachel Zwieg* and Brian Hudelson, UW-Madison Plant Pathology. Wisconsin Horticulture, Division of Extension. University of Wisconsin – Madison.
- Potato Tuber Viruses: Mop-top Management. North Dakota State University Extension. Revised Sept. 2018.
- Spraing. Tobacco Rattle Virus (TRV) Bayer Crop Science. UK.
- Potato Virus Y Strains. Animal and Plant Health Inspection Service. U.S. DEPARTMENT OF AGRICULTURE.
- Potato virus Y in potato crops. Brenda Coutts. Agriculture and Food. Department of Primary Industries and Regional Development. Government of Western Australia.
- Potato Virus Y. Utah Vegetable Production & Pest Management GuideExtension. Utah State University.
- Potato Virus Y (PVY) Bayer Crop Science. UK.
- Structural basis for the multitasking nature of the potato virus Y coat protein. ANDREJA KEŽAR, LUKA KAVČIČ, MARTIN POLÁK, JIŘÍ NOVÁČEK8, ION GUTIÉRREZ-AGUIRRE, MAGDA TUŠEK ŽNIDARIČ, ANNA COLL, KATJA STARE, KRISTINA GRUDEN, MAJA RAVNIKAR, DAVID PAHOVNIK, EMA ŽAGAR, FRANCI MERZEL, GREGOR ANDERLUH, AND MARJETKA PODOBNIK.
- SCIENCE ADVANCES. 17 Jul 2019. Vol 5, Issue 7. DOI: 10.1126/sciadv.aaw380
- Detection of Tobacco Rattle Virus RNA in Processed Potato Chips Displaying Symptoms of Corky Ringspot Disease. James M. Crosslin, United States Department of Agriculture, vegetable and forage crops research unit. HortScience. 2009.
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