Every year about this time, a certain question pops up on the Kodiak Growers Facebook page. It’s worded differently, of course, but the concern remains the same. It goes something like this: “Can anyone tell me if these brown spots, scab-looking marks on the potatoes are OK? What causes it? Thanks!”
The question is usually posted with a close-up image of potatoes – robust and healthy, but for a few corky, warty-looking patches on the outer skin.
Kodiak gardeners are a friendly lot, and the answers appeared immediately below the concerned potato grower’s post.
“It’s called scab,” replied one gardener. “It will not hurt you, just cut or peel it off and eat the potato. Potatoes with scab do not store as long or as well, so use those first.”
Another grower offered, “My soil always produces these, to some extent, but newer soil causes less. A friend grew potatoes in straight seaweed and had clear skin, so try that if it bugs you.”
Here’s the lowdown: Common potato scab is a disease that occurs throughout the potato growing areas of the world, which is extensive, by the way, with China, India and the Russian Federation being the top growers.
Scab is not caused by the potato itself. It is caused by the microorganism called S. scabies, which is present in nearly all soils. You could say it’s ‘lying in waiting’ for the right conditions, which occurs when the soil pH climbs above 5.0-ish. The scab lesions develop on the skin of the potato, not the inside, and while it doesn’t affect how it tastes, spuds affected with it do not store well.
How to prevent scab
1. Rotate your crops: In other words, don’t plant potatoes in the same bed year after year. Alternate with other plantings of lettuce, members of the cabbage family or onions. It’s not the end-all solution, though, as the practice will reduce the S. scabies population, but not eliminate it.
2. Select resistant varieties: A potato expert in Palmer once told me, “Using resistant varieties is an effective tool for management of scab.” That was over 10 years ago. Ever since then, I try to select varieties that are resistant to scab.
3. Be pH aware: Scab is greatly reduced in soil with pH levels of 5.2 and below. Commercial potato growing operations are keen on maintaining a soil pH between of 5.0 to 5.2. That’s a pretty tight margin and one that is commonly maintained with chemical applications of products such as ammonium sulfate.
There is a downside to low pH values, though. Plant nutrients are most available at more alkaline soil pH levels near 6.5. Since acid soils are unfavorable for most vegetable and field crops such as broccoli, lettuce and so on, the number of them that can be grown in rotation with potatoes is limited. Maintaining soils near pH 5.0 can result in what agriculturalists describe as “phytotoxic levels of some minor elements.” Translated, that means that nutrients can accumulate to the point where they are toxic. While potatoes grown in soils near pH 6.5 produce higher yields, it also means more potential for scab formation.
4. Don’t overfeed soil with compost or manure, or high-nitrogen chemical fertilizer.
Speaking of things that go bump in the night, pay attention to bugs when you bring plants, such as geraniums, in for the winter. If you’re considering setting a geranium on your windowsill this year, triple-check soil, leaves, flowers and stems for pests.
Aphids are notorious for hitching rides into your home. Spider mites are another bad boy. What do spider mites look like? Well, let me say that before you see the mites, you are more likely to see the thin, white webs and the damage the mites cause: curled and distorted young leaves. Grab a magnifying glass and take a look. (Caution: Spider mites are creepy looking and are probably the source of more than one episode of Outer Limits).
If you suspect spider mites, immediately quarantine the infested plant. Then, dip or spray the plant with insecticidal soap. Misting your plants on a regular basis will help prevent mites, since they thrive in a warm and dry atmosphere.
Keep tarps, sheets, spruce branches and other frost covers handy.
Clean out seed-starting pots, trays, and containers.
Clean around strawberry plants: Weed around plants; remove and trim away dead leaves and unwanted runners. All this attention helps prevent crown rot, mold, and slugs and aphids from overwintering. While the season is still fresh in your mind, make notes to build your wish list for next year. Think about what varieties worked, and which didn’t. What would you do differently?
And finally, steam a pile of potatoes and make a batch of potato pancakes this weekend. Top them with applesauce. There’s nothing finer on a cool, autumn morning.
Marion Owen is co-author of the New York Times bestseller, Chicken Soup for the Gardener’s Soul, which is available through Amazon. To learn how to garden in Kodiak, sign up for Kodiak Growers Facebook group, and chat with folks at local retailers and farmer’s markets. You can find Marion Owen on Facebook, Instagram or visit her blog at https://marionowen.wordpress.com