Gentry County

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Potato scab –by Tim Baker, Horticulture Specialist, University of Missouri Extension, Daviess County

A gardener asked me recently why her potatoes had developed scab lesions. And how could potato scab be prevented? Those are good questions. While most of the tubers are still edible, potato scab is unsightly and may reduce the yields you get from your garden by the time you cut out the scabs. 

There are several forms of potato scab, which are caused by soil-borne bacteria-like organisms. These same organisms are also known to cause scab lesions on beets, mangels, turnips, rutabagas, radishes, carrots, salsify, and parsnips.

Two forms of scab are seen. Common scab occurs in soils with a pH above 5.5. There is also an acid scab, which occurs in soils with a pH below 5.5. Acid scab is rare.

Scab symptoms can vary. The lesions are usually circular, and in light cases may only cause additional russeting, with the development of corky tissues. Heavy cases may result in deep pitting, which may be up to one half inch deep. There are no above ground symptoms of the disease.

Sometimes insects and other organisms may be attracted to the scab lesion, causing further damage. These include white grubs, wireworms, millipedes, scab gnats, and others.

Ok, so you have potato scab in your soil. What can you do to help alleviate the problem?

One thing to try would be crop rotation. Because the scab-causing organisms live in the soil, crop rotation is a good idea. If you have plenty of garden space, it is a good idea to avoid planting potatoes in scab producing soils for several years. Unfortunately, this may not entirely eliminate the problem, since these organisms are thought to have alternative hosts, such as organic matter in the soil. Scab has been known to resurface again in fields which have been out of potato production for many years. If you decide to try crop rotation, wait at least three years before planting potatoes in that garden spot again. 

For gardens with a known history of potato scab, a fungicide seed treatment when you plant your seed potatoes may help to reduce scab infections.

Some varieties are known to be less susceptible to potato scab. If you have a severe problem with scab each year, you might consider switching to a different variety.

Scab seems to be worse in dry soils. Keep your soil moist, but not too wet. This is especially important during early tuber development, when potatoes are likely to become infected.

Soils with high levels of organic matter tend to encourage the organisms which cause potato scab. While manures are often good sources of nutrients and organic matter, they should be avoided if your garden has a history of potato scab. If you have a choice, finer soils are better than coarser soils, because of the finer soil’s greater water-holding capacities.

The best way to reduce infections caused by common scab is to lower your soil pH. The place to start is a soil test. This will tell you if your soil is alkaline or acid. You may need to apply sulfur to lower the pH. The soil test will tell you how much sulfur to add. In this situation, you will need to avoid soil amendments that raise pH. This includes lime and wood ashes.

Did I say avoid wood ashes? Wood ashes are full of good nutrients, but they can be overdone. Through time, they can raise your soil’s pH, which can lead to many problems, including potato scab. It’s always a good idea to monitor your soil when using wood ashes, and quit using them when the pH rises too high.

Finally, even if you don’t have potato scab in your garden, be sure to buy disease-free seed potatoes. Scab-causing organisms can be introduced on infected tubers when you plant your garden. This is a good reason to be sure that you only use certified, disease-free tubers.

If you need more information, please contact your local Extension Center. We have some guide sheets on potato scab which we’ll be happy to send to you.

 

Local Resources

Home and Garden Resources for Northwest Missouri

    Horticulture in Season by Specialist Tom Fowler.

Northwest Missouri Extension News You Can Use (Regional Newsletter) Taste of Harvest Newsletter provides Northwest Missourians who produce and preserve food with timely, research-based information so they can ... plant ... pick ... prepare ... preserve ... and eat it right.  Do it right with University of Missouri Extension information! Jump to these articles: Making and Preserving Apple Cider Drinking Cider Safely Making Sauerkraut at Home Cabbage Culture Blackberries and Black Raspberries Berry Delicious When They're Hot They're Hot

Missouri-wide Home and Garden Resources

• University of Missouri Extension Home & Garden main page
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• Integrated Pest Management Diagnose and manage garden pests.
• Master Gardeners Share your gardening expertise.
• Missouri Environment and Garden Newsletter Get year round gardening advice.
• Garden 'n Grow Summer volunteer gardening program developed for 9 to 13- year-old youths.
• Plant Diagnostic Clinic Diagnosis plant and pest problems.
• Soil and Plant Testing Lab Analyze your soil, plants and fertilizer use.
• MU Horticulture Department
• Kemper Center for Home Gardening, Missouri Botanical Gardens Find answers to your gardening questions 24 hours a day.

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		University of Missouri Extension Gentry County gentryco@missouri.edu Site maintained by Shawn Deering  deerings@missouri.edu Updated 07/21/06