Scab on potatoes: how to fight


Of all the diseases of potato scab at first glance seems to be the most harmless. In the initial stage of its development, many do not notice that the potato is sick with something. After all, for example, ordinary potato scab does not manifest itself during the growing season of the bushes. It usually affects only the tubers and is not very noticeable to the inexperienced eye. If nothing is done and continue to plant the infected potatoes, then you can soon be left without a crop. Moreover, the scab infection mostly lives in the ground and the situation needs to be corrected by an integrated approach.

Types of scab

Before you think about how to deal with scab on potatoes, it is necessary to understand that this disease has several guises, each of which is characterized by its own characteristics, which often differ greatly from each other. Accordingly, the measures taken to prevent and get rid of it can be completely different. There are the following types of potato scab:

  • Ordinary;
  • Powdery;
  • Black (also found under the name of rhizoctoniosis);
  • Silvery.

The most common in the fields and gardens ordinary scab. The causative agent of this type of disease is a fungus called Streptomyces scabies. Most often he lives in the soil, prefers dry, sandy soils with a reaction close to alkaline. It develops especially actively at air temperatures above + 25 ° + 28 ° С.

Signs of potato scab damage are quite diverse, but most often the disease begins with small, almost imperceptible brown sores, sometimes with a red or purple tinge. Sometimes the surface of the potato becomes rough and on it are formed barely noticeable grooves in the form of a grid. With a strong lesion, ulcers increase in size, harden, cracks appear along them and the tubers begin to rot intensively.

Attention! Most often, common varieties of potato with thin or red skin are affected by common scab.

As mentioned above, this type of disease almost does not spread to other parts of the potato, it lives mainly in tubers. Moreover, the potato is not able to become infected during storage, because under adverse conditions (low temperatures) the fungus falls into anabiosis, but does not die. But when applying to the soil of raw, not rotten manure or significant doses of limestone as fertilizer, the risk of common scab of potatoes increases. Therefore, it is necessary to treat, first of all, the land itself used for planting potatoes.

To counter the common scab, you can use potato varieties resistant to this disease: Domodedovo, Zarechny, Amber, Sotka.

Powdery scab, in contrast to the ordinary, usually appears as a result of prolonged rains on heavy, wetlands.

Comment! The fungus called Spongospora subterranean is very mobile and can move freely both in the plant itself and in the ground.

The disease manifests itself not only on the tubers, but also on the stems, as a rule, on their underground part. The stems are covered with small white outgrowths, while on the tubers a lot of warts appear in different sizes of red-brown color. Powder scab spores develop well in conditions of high humidity and at temperatures from + 12 ° C. They can be transferred both with organic residues and by air. During storage, the affected tubers usually shrivel, but if the storage has high humidity, they will quickly rot. The fungus can persist in soils up to five years or more.

Black scab of potatoes or rhizoctoniosis is one of the most dangerous varieties of scab. The only thing that facilitates diagnosis is the fact that the whole potato plant is affected - from tubers to stalks with leaves. But as a rule, the defeat of the above-ground part suggests that it will not be possible to save the plant - it is better to destroy it. The first signs of the disease appear on the tubers and have the appearance of small black or brown sores, which often merge into extensive spots.

A warning! It is necessary to be vigilant, since the inexperienced eye of the gardener can take them for pollution from the soil.

That's how black scab on potatoes looks in the photo.

If such tubers are accidentally used as planting material, the shoots will be very weak and, most likely, the bushes will not even live to bloom. The causative agent of this dangerous disease is Rhizoctonia solani. Spores of this disease also like the increased soil moisture (80-100%) and temperature from + 18 ° С. Soils prefer loamy and most often actively develop when spring is cold and rainy. In this case, black scab spores are able to penetrate the tubers in the period of germination, and such potatoes are doomed to death.

Due to the unpredictability and transience of the development of the disease, the fight against this type of potato scab should be as serious as possible, including the use of strong chemicals. Moreover, unfortunately, at the moment there are no varieties of potatoes that are completely resistant to this type of scab.

The silver scurf of the potato got its name due to the grayish-silver spots on the tuber, which can occupy up to 40% of the area of ​​the tuber.

True, such spots appear already in the stage of significant development of the disease. And it all begins with small pale "pimples" with a black dot in the middle. The causative agent of this type of scab is Helminthosporium solani. From the outside, it seems that this is the most innocent kind of scab - after all, the infected tubers are well kept and practically do not rot. But this appearance is deceptive.

Comment! The silver scab is the most insidious because its spores retain the ability to live even at + 3 ° C, which means that during storage it can infect neighboring tubers.

In addition, during storage, dehydration occurs quickly, and the tuber may become dry and wrinkled by spring. Due to this, up to 40% of the crop is lost and such tubers are not suitable for use as planting material.

The causative agent of silver scab is undemanding to soils, it feels good both on loams and on sandy loamy soils. Like almost any fungus loves conditions of high humidity, from 80 to 100%. Therefore, the disease progresses during flowering and tuberization.

Means of prevention and control

Potato tubers affected by all types of scab except rhizoctoniosis are quite suitable for human consumption. Probably for this reason, gardeners, as a rule, do not pay enough attention to the treatment of this disease. But to fight it is necessary because both the taste and nutritional value of such potatoes is minimized. And if even healthy, but not specially treated tubers are planted on infected land, they will also be infected and there will be no end to it. So, how can you get rid of the scab on potatoes and make it not appear on the site again?

Agrotechnical receptions

The main way to resist scab is a crop rotation. If you do not plant potatoes on contaminated land for 4-5 years, the infection may have time to die. But not everyone can afford to change every year the land for planting potatoes. Moreover, in this area you can not grow any plants of the family solanaceous (tomatoes, peppers, eggplants), as well as beets and carrots. They are also affected by this disease.

What can be done in this case, it is carried out immediately after harvesting the potato tubers sowing the site with green manure. It is best to use mustard, but legumes and cereals will play a positive role. When the shoots reach a height of 10-15 cm, the plot is again dug up or at least mowed and mixed with green mans with the ground. Being in the ground, the remains of siderats contribute to the formation of saprophytic fungi and bacteria, which are natural enemies of scab pathogens. Thus, our great-grandfathers fought the scab and quite successfully. In the spring, before planting potatoes, you can also plant fast-growing siderats, or at least sprinkle future beds with mustard powder and shed. Mustard significantly reduces the number of fungal and viral infections in the soil, and also protects against many pests: thrips, wireworms, slugs.

Important! When preparing the plot for planting potatoes, no fresh manure should be introduced into the ground. This can cause a significant outbreak of the disease.

Since spores of common scab especially develop well in alkaline soils with insufficient content of manganese and boron, it is especially important to apply the following types of fertilizers in the spring before planting potatoes (application rate per 100 square meters) to combat this type of disease:

  • Ammonium sulfate (1.5 kg);
  • Superphosphate (2 kg) and potassium magnesia (2.5-3 kg);
  • Trace elements - copper sulphate (40 g), manganese sulphate (20 g), boric acid (20 g).

Treatment with various drugs

Other methods of dealing with scab include, first of all, preplant treatment of tubers with various fungicides. It is quite effective and safe to use Maxim or Fitosporin microbiological preparation. The last tool can be used in different ways. It is intended not only for processing seed potatoes. To consolidate the effect, they are recommended to spray potato bushes three times during the growing season. To obtain a working solution, one packet of the drug is diluted in three liters of water.

There are many chemicals to get rid of scab on potatoes. For example, to destroy black scab and tubers, and the plants themselves are treated with such strong-acting drugs as Mancozeb, Super Fenoram, Colfugo. Treated tubers are able to withstand disease, even under adverse conditions.

In order to cope with other varieties of scab, it is not necessary to use such strong chemicals. For example, various growth regulators are suitable for inhibiting the development of common scab, primarily zircon. In the description it is noted that the harmfulness of the disease decreases even with a single treatment with this drug. With its double use, the disease can retreat altogether. 1 ml of zircon (1 ampoule) diluted in 20-30 liters of water and the resulting solution to process the bushes of potatoes after germination and at the beginning of flowering.


Scab on potatoes is an unpleasant phenomenon, but it is quite possible and necessary to cope with it if you follow all the recommendations outlined above.