How to Control and Prevent Potato Scab

Potato scab is a common tuber disease found in soils around the world. Learn how to control and prevent this issue in your garden using this complete guide by sustainability expert Huan Song.

Potato scab disease.

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One of the best parts of growing potatoes is the delight of harvesting them after months of waiting for the delicious tubers. Unfortunately, pests and diseases often get in the way. Because the crop is underground, the symptoms are not visible until we pull them out.

There’s nothing worse than waiting the entire growth period to learn a disease has stricken the crop. Potato scab, also known as common scab, is one of these diseases. 

While potato scab is not a fatal disease, it is pervasive across the world where potatoes are grown. This bacterial disease impacts a range of vegetables in addition to potatoes such as beets and parsnips.

So let’s talk about potato scab and how to deal with it to protect your crops and ensure a healthy harvest.

What Is Potato Scab?

Pile of newly harvested russet potatoes with potato scab disease.
The scab disease causes dark brown lesions on the surface of vegetables.

Potato scab or common scab is a soil-borne disease caused by several Streptomyces species of bacteria. The most widespread is Streptomyces scabies. This bacterial disease is related to another scab-causing pathogen, S. acidiscabies, although S. acidiscabies can survive under much more acidic conditions.

S.clavifer, S.fimbriatus, S.carnosus, and S.craterifer are additional bacterial pathogens that can cause scabs on potato tubers. 

As its name suggests, the potato scab causes dark brown lesions on the surface of these vegetables which greatly diminishes their sale value and marketability for commercial growers. There is no known cure for this disease. However, it can be managed through integrated techniques like crop rotation and soil management.

What Causes Potato Scab?

Red potatoes on table in sunshine with signs of disease.
Soil moisture can impact the severity of the common scab infection.

Potatoes are particularly vulnerable to infections of potato scab during their early tuber development stage. The tubers are more easily damaged or pierced by garden tools.

The severity of the infection also increases as soil pH increases. The ideal condition for this bacteria is in dry, alkaline soil with moderate to warm temperatures.

Environmental conditions, such as soil pH levels and soil moisture, can impact the severity of the common scab infection.

Potato scab should not be confused with powdery scab. Powdery scab is a fungal disease caused by Spongospora subterranea. This fungal pathogen is entirely different from the bacterial pathogens mentioned above. 

Life Cycle

Yellow potato with brown lesions on the skin.
This pathogen spreads through soil.

S.scabies is an unusual bacteria in that it has fungal-like filaments. It survives in the soil in its vegetative mycelial form or as spores, much like fungi do.

The pathogen spreads through the soil and can also be transported by water or even on insects and nematodes. It penetrates the tuber surface through wounds, lenticels, or raised pores on woody stems, and the stomata, or the pores in leaves, stems, and other plant organs.

Once inside the plant, S.scabies releases a toxin that breaks down cell walls and helps it to invade neighboring cells. Eventually, plant cells die and are replaced by cork cells which protrude to form scab lesions. The pathogen sporulates in the scabs which then repopulates the soil, continuing the cycle. 

What Plants Does Potato Scab Effect?

Red potato with signs of potato scab.
Many vegetables can be affected by this disease.

Potatoes are the main hosts for S.scabies, but other root crops can also be affected. These vegetables include beets, radishes, turnips, carrots, parsnips, and rutabagas.

Different Streptomyces species impact different hosts. For example, S.ipomoeae is most associated with scabs on sweet potatoes and S.tumuli with sugar beets. 

Symptoms

Potato with small brown bumps or lesions on the skin.
The scabs first start as small brown spots with a margin before expanding.

The potato scab disease results in visible lesions or pits on potato tubers. The symptoms vary depending on the crop variety.

The scabs first start as small, raised brown spots with a margin before expanding and taking on a cork-like texture. They are smaller in earlier tuber development, merging and becoming one larger scab as the tubers mature.

As the disease progresses, the scabs become necrotic and sporulation begins to perpetuate the cycle. 

Control

Almost all soils have some population of S.scabies existing within them. Therefore, it is not possible to eliminate this bacteria. However, there are several options for control.

Crop Rotation

Healthy potato plants in the garden.
Use crop rotation to improve soil conditions.

Some of the best methods for controlling the S.scabies population is by practicing crop rotation with non-host species like grains and cereal crops. Crop rotation can also help with managing other potato pests such as the Colorado potato beetle.

Corn, rye, and alfalfa are a few examples of crops able to control common scab bacteria proliferation in the soil. If you’re worried about potato scab affecting upcoming crops, use crop rotation to ensure a better harvest.

Soil Management

Gardening managing soil conditions in garden beds.
Monitor soil pH and soil moisture closely.

Other control measures include managing potato soils. Planting potatoes in acidic soil with pH between 5.0 and 5.2 greatly suppresses the common scab. However, it is important to keep in mind that soil nutrients are typically more available when the soil pH is around 6.5, impacting plant growth.

Maintaining soil moisture (especially during the 2-6 weeks following tuber initiation) can make a significant difference. Keeping soil moist and water stress at a minimum during the tuber initiation phase is a key control method. Do not plant potatoes in areas that are known to dry out quickly.

When you build your soils, provide materials that retain moisture, and those that help it drain easily. Ensure your potatoes receive 1 to 2 inches of water per week to avoid stress from over or under-watering. 

Don’t Use Fresh Manure

Shovel with fresh manure ready to use in the garden.
Compost manure before use to avoid problems with potato scab.

Applying fresh manure can increase the number of crops infected by scab. Streptomyces bacteria are part of the process of breaking down organic matter in soils, so the bacterial population would increase under these conditions.

Check with the source of your manure to ensure it composts long enough before applying it to your garden. If you obtain fresh manure, let it sit at least 3 to 4 months in ideal conditions. Longer is a safer bet. Not only does using fresh manure put your crops at risk of disease, but the intensity of nutrient density in manures can burn your plants and inhibit growth. 

Prevention

Box of several newly harvested potatoes with sprouts.
Choose scab-resistant varieties to limit issues each season.

Prevent potato scab from spreading in your soil by planting scab-resistant potato varieties or certified disease-free seed potatoes. Infected seed tubers can quickly spread the pathogen, which will remain in the soil for many years.

Although scab resistance is not yet well understood, there are some tried and true cultivars such as Superior, Keuka Gold, Lehigh, Pike, and Marcy. Chieftain, Eva, Reba, Andover, and Russet Burbank are also moderately scab-resistant potatoes. 

Frequently Asked Questions

What causes scabbing on potatoes?

Potato scab or common scab can be caused by several soil-borne bacteria including Streptomyces scabies and a few other species of the same genus. It occurs most often during initial tuber development. There are other pathogens that cause diseases with similar symptoms.

How do you get rid of potato scabs?

The soil bacteria can be found all over the world and anywhere potatoes are grown so it is very difficult to get rid of once it is in the soil. Plant potatoes that are scab resistant or practice crop rotation with non-host plant species.

Is it OK to eat scabby potatoes?

Yes, the potato scabs are cosmetic damages on potatoes. They can be peeled away and the potatoes would still be edible. The scab disease causes damage that reduces the marketability of potato crops and other susceptible plants.

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Sunlight bathes a verdant potato plant, its leafy emerald fronds reaching skyward from a burlap sack in a garden bed. Deep green hues and smooth edges tell a tale of health and vigor, promising a bounty of tubers beneath the soil.

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