How to Prevent Scab on Your Potatoes This Year

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Potatoes are a super easy crop to grow, but even potatoes have a few issues that home growers would rather avoid. One of the most prevalent problems is scab. Basically, the potatoes look like they have big warts on them. They’re still edible, but you’re going to want to cut away all of the funky looking parts. Yes, it looks gross, but it’s not tremendously terrible and there are ways to manage it.

Preventing Scab in Potatoes

Scab is caused by Streptomyces bacterium that stays in the soil and infects the potatoes. It thrives in soils that are rich in organic matter, so don’t dump manure in the bed where you’re going to plant potatoes. It also does best in more alkaline soils (a pH higher than 5.5), just like what we have in the Intermountain West. As a result, scab is a frequent problem out here. I don’t think a season has gone by without having at least a few potatoes with scab; other times it’s the whole bunch of them.

There are a few things you can do to minimize the effects of scab if it’s an issue in your area.

Don’t Add Fresh Manure

As I mentioned, don’t add fresh manure to the bed where you’re going to plant potatoes. The bacterium passes through the animal, and will stay in the soil (in fact, it can stay in the soil forever). This applies to amending the soil with compost to close to potato planting, as well. Yes, this is counterintuitive to what we do as gardeners, but if you’re in a region with a high soil pH and a problem with scab, it’s wise to skip a year adding amendments.

You also don’t want to plant potatoes in the same bed as you planted potatoes, tomatoes, peppers, or eggplants the previous year.

Use a Legume Cover Crop

Raising a cover crop prior to potato planting is another good practice. The University of Vermont Extension service recommends avoiding legume cover crops because it actually increases the chance of scab formation. On the flip side, raising millet or other small grains the year prior can help raise a healthy crop.

Plant Scab-Resistant Potato Varieties

Choosing scab resistant varieties is a sound strategy. Thicker skinned russets are a good option if you’d rather not have warty looking spuds. Unfortunately, some of my favorites, such as Yukon Gold and Katahdin, are very prone to scab.

Add Sulfur to Potatoes

The best way to combat scab is to employ acidifying techniques. A friend of mine dusted her potatoes with a powdered sulfur last year, and noticed significantly less scab formation. It’s also a wise practice to modify the soil acid levels, if you are growing in a small area, making this a long term solution. Adding elemental sulfur takes time to change the acid level. Add it at least 10 months prior to planting.

When you remove a crop in the fall, add sulfur to the soil. It’ll take all winter for it to be effective anyways. To be extra on top of it, dust the seed potatoes with sulfur powder when you plant them.

I’ve also known people who add pine needles, along with pine bows from the Christmas trees, to the potato planting area. It takes a lot to change pH levels for any long term effects, so if you wish to employ this strategy, and since you don’t want to plant potatoes in the same area in consecutive years, you’re going to have to think ahead.

Keep Potatoes Evenly Moist

Finally, keep your potatoes evenly moist. The Streptomyces likes dry soil (another feature we have on a regular basis!) so by watering when you don’t have adequate rain helps keep it at bay.

Scab is one of those common problems of potatoes that throws a lot of new gardeners for a loop because it looks terrible. But, rest assured, it’s a problem even experienced gardens contend with implementing any, or all, of these techniques.

Meet Amy Grisak

Amy is a freelance author and photographer in Great Falls, MT who specializes in gardening, foods, and sustainable agriculture. She provides information on every kind…

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