Potato scab (rhizoctonia): how to combat it (photo)

Potato scab, or rhizoctonia as it's known scientifically, is a very common disease that can be found in virtually every potato crop (a photo of an affected crop can be seen below). This disease has the most negative impact on planting material. It's important to understand the disease and how to treat it. Timely treatment can save the harvest.

Potato scab (rhizoctonia) can affect not only the roots but also the stems of the plant. Younger crops are particularly susceptible to this disease, so they require special care. It's important to understand the disease and how to treat it. Undeveloped potatoes can quickly become unusable and die from scab (photo below). Furthermore, rhizoctonia can spread rapidly from one crop to another, potentially resulting in the loss of a significant portion of the future harvest, so prompt treatment is essential.

Causes of potato scab

The entire process of spreading and developing these types of diseases involves the active growth of spores, which primarily affect the plant's tubers. This bacterial activity is facilitated by external factors, such as the temperature and humidity of the potato crop. Soil conditions and structure can also influence this. It has been found that in loamy soils, the proliferation of parasitic bacteria is much faster and their activity significantly increases.

There's also the issue of soil acidity, as the pH most favorable for potato planting coincides with the pH at which bacterial growth is most active. In fact, the spread of potato scab occurs because the environment in which the vegetable grows is the most favorable for bacteria.

How to recognize the appearance of scab on potatoes

The most common signs of potato scab are dark, sunken spots and net-like necrosis. If your potato roots are gradually rotting and the leaves are showing signs of dying, this is a sure sign that your crop is at risk. There's a type of rhizoctonia called "black scab." It gets its name from the dark spots that spread across the surface of the tubers.

This disease typically doesn't cause significant damage because it's a dormant species. The main problem is that scab can penetrate the plant, initiating decomposition from within. Dark spots begin to form rings, and the plant itself turns brown. This is followed by necrosis, after which the plant gradually dies.

Signs of scab development on potatoes

The progression of harmful bacteria development on potatoes is accompanied by several phases:

1. Dormant phase. This is the stage of Rhizoctonia blight that occurs in the form of sclerotia. They are easily detected visually, as the sclerotia look like the same black spots that appear on affected potatoes. It is this visual appearance that gives these spots their name, "black scab." Keep in mind that if potatoes are affected at this stage, never plant this material, as it can infect neighboring root tubers.
2. The growth and parasitic phase. At this stage, the sclerotia grow very rapidly and form myceliums that rapidly encircle the potato tuber, which negatively impacts the tuber, as the planting material simply fails to germinate. Rotting occurs in an area unseen by human eyes, that is, still underground. However, there are cases where the potato may germinate, but its shoots will remain infected. This, in turn, can significantly impact the further development of the potato itself, as well as neighboring tubers.
3. The reproduction phase. This begins with the development of the so-called "white leg." This involves the proliferation and spread of fungal spores, which can only develop under favorable conditions, namely high humidity and an air temperature of 20 degrees Celsius. It is at this stage of development that we can confirm that the plant is completely infected and must be immediately destroyed before the harmful bacteria spread to neighboring potatoes.

Types of black scab

According to experts, there are several subspecies of black scab:

1. Common scab. Dry, highly acidic soils are most favorable for the growth of black scab. The bacteria also thrive in dry soil and in alkaline environments. The optimal temperature for common scab growth is 25-30 degrees Celsius. The pathogen can reside not only on potato tubers but also in the soil itself and organic debris within them. This type of scab can be recognized by the irregularly shaped ulcers that appear on the roots. If pressed, these ulcers feel abnormally hard. In advanced stages, potato roots can be completely covered with these ulcers.
2. Powdery scab. The pathogen in this case appears as a slimy lump that can change location on its own. This type attacks not only the roots but also the stems of the plant. When affected by powdery scab, the rhizomes dry out quickly, but if the planting material was stored in an area with excessive moisture, the tubers begin to rot quite quickly. Potatoes infected with this type of scab begin to develop raised, brown growths. If the soil is excessively wet, this creates favorable conditions for the development of powdery scab.
3. Lumpy scab. This type of rhizoctonia primarily infects the plant roots. The shape and appearance of the affected areas depend on the potato variety being grown, as well as the nature of the disease. As a result, the potatoes begin to lose their flavor, and their starch and protein content decrease. Poor-quality seed or improper cultivation practices are usually identified as the cause.
4. Silver scab. It gets its name from its silvery appearance. It typically appears in high humidity, after which it begins to actively migrate to healthy tubers. The paradoxical nature of this type of scab is that tubers begin to rapidly lose moisture, but no signs of rotting occur.

Methods of preventing infection

How to combat potato scab? This question concerns not only novice gardeners but also those who have been growing this vegetable for quite some time. It's much easier to prevent the disease in advance than to deal with it later.

First and foremost, potato prevention and treatment are essential. It's crucial to periodically sort through the planting material to monitor for signs of disease. If you notice even the slightest hint of rhizoctonia on one root, it's essential to immediately remove that planting material from the other roots.

Don't forget about agricultural practices either. You need to follow a crop rotation system. If scab was previously detected on potatoes, some bacteria likely remained in the soil. You shouldn't plant potatoes in this soil for at least four years. It's crucial to use good-quality seed and resistant potato varieties.

Preparations against rhizoctonia

There are many such medications. The most effective are:

1. Fitosporin. A highly effective treatment against fungal diseases on vegetables. Its action is almost immediate, immediately after use. Its effect is systemic, meaning it spreads throughout the entire plant.
2. Mancozeb is a wettable powder that adheres to the plant surface, creating a protective film that prevents the spread of scab.
3. Fenoram is super. It works exclusively on root crops. The effect is guaranteed to last until the end of the growing season.
4. Kolfugo. This product is applied to plants during the growing season. It is primarily used to combat rhizoctonia on seed potatoes. It remains effective even in heavy rainfall, as this does not affect the effectiveness of the treatment.

Reviews

Many gardeners claim that the effectiveness of these products has been proven in practice. Often, if gardeners try at least one of them, they subsequently use it universally. Some even expressed regret that they hadn't used them earlier, instead relying primarily on traditional treatments and preventative measures.