Ultimate Guide to Preventing and Treating Potato Scab Disease

Potatoes stand as a vital staple in diets worldwide, beloved for their versatility, nutritional value, and ability to thrive in various climates. However, these valuable crops face threats from diseases that can significantly diminish their quality and yield, with potato scab disease being a notorious culprit.

This fungal menace not only mars the potatoes with unsightly blemishes but also reduces their size, impacting both their market value and nutritional worth.

Understanding how to combat and prevent potato scab disease is crucial for gardeners and farmers alike to ensure their potato plants remain healthy and productive.

What Causes Potato Scab?

Potato scab is a problem that occurs in two main forms, common scab and powdery scab. Both are caused by tiny, harmful microorganisms that invade the potatoes growing underground. Specifically, common scab is triggered by several species of a bacteria-like organism called Streptomyces. On the other hand, powdery scab is caused by a pathogen known as Spongospora subterranean.

These pathogens attack the developing tubers, which are the swollen parts of stems or roots that plants use to store food. Potatoes are a well-known example of a plant that develops tubers. The scabs caused by these organisms appear as rough, scaly patches on the potato's skin.

They first show up in summer as the potatoes grow and can stick around on the harvested potatoes during storage, leading to diminished quality and appearance.

Symptoms of Potato Scab Disease

As a grower, knowing what to look for when inspecting your potato crops and early detection can make a significant difference in treating and controlling potato scab disease. Here are some of the most common symptoms to watch out for:

1. Superficial scabs: The first sign usually appears as small, corky raised spots on the tuber surface. The spots may be brown or tan.
2. Pitted scabs: As the disease progresses, the scabs become pitted or crater-like. The size and depth of the pits can vary.
3. Russeting: In severe cases, russeting (roughening) of the entire potato surface may occur, giving the potatoes a bumpy, unattractive appearance.
4. Reduced size of tubers: Infected potatoes may also develop a reduced size due to the disease's impact on the growth process of the tubers.

How To Prevent Potato Scab Disease

Preventing potato scab disease is far more cost-effective and practical than attempting to control it once it has infected your crops. A well-implemented prevention plan can protect your yield, maintain the quality of your potatoes, and save you from potential financial losses.

Proper Planting Techniques

Here are some steps and precautions to keep in mind:

1. Certified Seed: Always use certified disease-free seed potatoes for planting. These have been inspected and are guaranteed to be disease-free, including potato scab disease.
2. Crop Rotation: Rotate your crops annually. Avoid planting potatoes in the same spot where potatoes, tomatoes, or other nightshade family members were grown in the previous year. This helps to break the disease's life cycle and reduce the bacterial count in the soil.
3. Soil pH: Strive to maintain a soil pH of around 5.2. Potato scab disease thrives under alkaline conditions, and keeping the soil slightly acidic can significantly hinder its growth. Lime should be avoided as it can increase soil pH.
4. Irrigation: Ensure your potatoes are adequately watered, especially during tuber set. Dry conditions can increase the risks of a potato scab outbreak.
5. Planting Depth and Spacing: Plant potatoes at the correct depth (about 4 inches deep) and space (about 12 inches apart) to ensure the crops get essential nutrients and water and have enough room for healthy growth. This can help in reducing the chances of disease.

Remember, prevention is always better than cure. By implementing these proper planting techniques, you stand a better chance of growing healthy, scab-free potato crops.

Soil Management

Here are some practical soil management strategies that you can adopt:

1. Organic Matter: Adding organic matter like well-rotted compost or aged manure to your soil can help increase its moisture retention and nutrient content. However, avoid using fresh manure as it raises soil pH, creating a conducive environment for the bacteria causing potato scab disease.
2. Cover Crops: Planting crops such as rye or oats in the fall can help improve soil structure, control erosion, and suppress weed growth. These crops can be tilled into the soil in spring, adding organic matter and improving soil health.
3. Soil Testing: Regular soil testing is vital to monitor pH levels and nutrient content. If the pH is above 5.2, consider adding sulfur or spruce needles to make it more acidic.
4. Resistant Varieties: If potato scab has been a persistent problem in your soil, consider planting resistant potato varieties. Varieties such as 'Sebago', 'Kennebec', and some fingerling potatoes have shown resistance to scabs.
5. Green Manure Crop: Growing a green manure crop like mustard or oilseed radish can help suppress the bacteria that cause potato scabs. These plants can be turned into the soil about two weeks before planting potatoes.

Appropriate Watering Practices

Here are some suggestions to ensure your watering practices do not encourage the disease:

1. Consistent Moisture: Keep the soil moist, especially during the tuber formation phase. Underwatering at this stage can result in dry soil conditions ideal for the bacteria that cause potato scabs.
2. Avoid Overwatering: While maintaining moisture is crucial, overwatering can lead to waterlogged soil and the development of other fungal and bacterial diseases. Balance is the key.
3. Appropriate Watering Time: Early morning is the best time to water your potatoes. This allows the water to reach deep into the soil during the cooler days, reducing evaporation.
4. Drip Irrigation: Consider using drip irrigation or soaker hoses to water your potatoes. These methods deliver water directly to the root zone, reducing water waste and keeping the foliage dry, which helps prevent many foliar diseases.

Treatment of Potato Scab Disease

While prevention is the most effective strategy, a few treatment options can be employed to manage the disease and minimize its impact.

Natural Treatment Methods

If you notice the signs of potato scab disease in your crops, there are several natural treatment methods you can apply:

1. Crop Rotation: If the disease has already taken hold in your potato plants, switching to a different crop in your field or home garden may be best the following growing season. This helps break the disease's cycle and reduce the bacterial count in the soil.
2. Biocontrol Agents: Certain beneficial bacteria and fungi effectively suppress potato scab disease. These biocontrol agents, like Trichoderma and Streptomyces, can be introduced into the soil to compete with the pathogen-causing potato scab.
3. Wood Ashes: Wood ashes can be applied to the soil as a natural fungicide to control the spread of potato scab disease. However, since wood ashes can raise soil pH, they should be used sparingly and only under expert supervision.
4. Compost Tea: Compost tea is a rich source of beneficial microbes that can help combat potato scab disease. It can be made by steeping well-rotted compost in water for several days and then spraying it on the soil around the potato plants.
5. Plant-Resistant Varieties: If you live in a region where potato scab is prevalent, consider planting varieties resistant to the disease. Varieties such as 'Russet Burbank' and 'Russet Norkotah' have demonstrated resistance to common scab.

Remember, these methods may not eliminate the disease, but they can often reduce its severity and keep it from spreading.

Chemical Treatment Methods

Here are some chemical treatment methods you can consider:

1. Chemical Fungicides: Apply fungicides that are specifically designed to treat potato scab. Always read and follow the label instructions carefully to ensure effective and safe usage. Some fungicides are applied directly to the seed potatoes before planting, while others can be sprayed on the soil surface.
2. Ammonium Sulfate: This chemical fertilizer can lower the pH of alkaline soils, making them less favorable for the bacteria causing potato scab disease. However, it should be used sparingly and cautiously since it can burn the plants if over-applied.
3. Seed Treatment: Treat seed potatoes with approved chemical agents to eliminate potential pathogens. This measure can be especially effective if you have previously struggled with potato scab in your fields.
4. Chemical Rotation: In areas with high disease pressure, rotating different types of fungicides each year can help prevent the development of resistant strains of the pathogen.

Post Treatment Care

Maintaining vigilant care after treatment is crucial to successfully rehabilitating your potato crops. Below are some steps you can take:

1. Monitor Your Crops: Regularly check your potatoes for any signs of recurring disease. Keep a close eye on the health and growth of your plants, and be alert to any unusual changes.
2. Maintain Optimal Watering Practices: Continue to implement the watering practices discussed earlier. A well-balanced watering method is key to maintaining healthy growth conditions, thereby reducing the chances of disease recurrence.
3. Periodic Soil Testing: Regularly monitor your soil's pH level and nutrient content. By keeping the soil conditions optimal, you can prevent the resurgence of potato scab disease.
4. Cleanliness: Keep the area around your potato plants free of debris. Any infected tubers or plant residues should be disposed of properly to avoid the spread of pathogens.
5. Inspect Harvested Potatoes: Upon harvest, inspect your potatoes carefully. If any signs of disease are detected, discard those tubers immediately to prevent the bacteria from persisting in storage and affecting other potatoes.

How Potato Scab Disease Affects Different Types of Potatoes

It is important to note that potato scab disease doesn't discriminate between different types of potatoes. However, the severity and visible impact on the tubers vary between varieties.

• Russet Potatoes: These potatoes are highly susceptible to potato scab disease, often showing severe russeting and deep pitted scabs.
• Yukon Gold Potatoes: While this variety can also be affected, the symptoms are typically less severe than in Russet potatoes and are characterized by superficial scabbing.
• Red Potatoes: This type is somewhat resistant to scab. While they can be infected, the resulting scabs are usually shallow and affect a smaller surface area.
• Fingerling Potatoes: These potatoes show considerable resistance to scab disease. Any scabbing is typically minor and does not significantly affect the tuber's appearance or edibility.

Frequently Asked Questions

How do you prevent scab on potatoes?

Preventing scabs on potatoes begins with maintaining appropriate soil conditions, including optimal pH levels and adequate organic matter. It's also beneficial to plant resistant potato varieties and implement good watering practices. Regular crop rotation and the use of biocontrol agents also contribute to preventing this disease.

What is the best fungicide for potato scab?

The best fungicide for potato scab often depends on your specific situation, including the severity of the disease and the potato variety grown. However, products containing mancozeb or copper are commonly recommended due to their efficacy against the disease. Always read and follow label instructions to ensure safe and effective use.

Can you eat the skin of potatoes with scab?

Yes, potatoes with scab can typically be eaten safely. The scab mostly affects the potato's skin, and the affected areas can be easily cut off or peeled before cooking.

What are the first early potatoes resistant to scab?

The first early potatoes resistant to scab include varieties such as 'Pentland Javelin' and 'Rocket'. These types have shown great resilience to common scab, making them a popular choice among growers dealing with this disease.

The Takeaway: Treating Potato Scab

Tackling potato scab disease effectively starts with understanding the available prevention and treatment options.

From selecting resistant potato varieties to implementing good farming practices, there are many steps you can take to protect your crops. Remember, consistency and vigilance are key.

For more insights and detailed guides on managing potato scab and other crop diseases, make sure to visit our blog.