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Of all the potato diseases, scab at first glance seems to be the most harmless. In the initial stage of its development, many do not even notice that the potato is sick with something. After all, for example, the common scab of potatoes does not manifest itself in any way during the growing season of the bushes. It usually affects only tubers and is not very noticeable to the untrained eye. If you do nothing and continue to plant infected potatoes, then soon you can be left without a crop at all. Moreover, the scab infection mainly lives in the ground and the situation needs to be corrected with an integrated approach.
Types of scab
Before thinking about how to deal with scab on potatoes, you need to understand that this disease has several guises, each of which is characterized by its own characteristics, which are often very different 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 rhizoctoniosis);
- Silver.
Most widely used in fields and gardens. common 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. Especially actively develops at air temperatures above +25°+28°С.
Signs of damage to potatoes by common scab are quite diverse, but most often the disease begins with small, almost imperceptible brown sores, sometimes with a red or purple tint. Sometimes the surface of the potato becomes rough and barely noticeable grid-like grooves form on it. With a strong lesion, the ulcers increase in size, harden, cracks appear along them and the tubers begin to rot intensively.
As mentioned above, this type of disease almost does not spread to other parts of the potato, it lives mainly on tubers. Moreover, potatoes are not able to become infected during storage, because under adverse conditions (low temperatures), the fungus falls into suspended animation, but does not die. But when raw, not rotted manure or significant doses of limestone are introduced into the soil as fertilizers, the risk of common potato scab increases. Therefore, it is necessary to treat, first of all, the very land used for planting potatoes.
To counter scab, you can use potato varieties that are resistant to this disease: Domodedovsky, Zarechny, Yantarny, Sotka.
Poroshistaya parsha, unlike the ordinary one, usually appears as a result of prolonged rains on heavy, waterlogged soils.
The disease manifests itself not only on tubers, but also on stems, as a rule, on their underground part. The stems are covered with small white growths, while on the tubers there are many red-brown warts of various sizes. Powdery scab spores develop well in conditions of high humidity and at temperatures from +12°C. They can be transmitted both with organic residues and by air. During storage, the affected tubers usually shrink, but if the storage is high in humidity, they will rot fairly quickly. The fungus can persist in soils for up to five years or more.
black scab potato or rhizoctoniosis is one of the most dangerous varieties of scab. The only thing that makes the diagnosis easier is the fact that the whole potato plant is affected – from tubers to stems with leaves. But as a rule, the defeat of the aerial 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 precisely on the tubers and look like small black or brown sores, which often merge into large spots.
This is how black scab on potatoes looks in the photo.
If such tubers are accidentally used as planting material, then the sprouts will be very weak and, most likely, the bushes will not even survive until flowering. The causative agent of this dangerous disease is Rhizoctonia solani. The spores of this disease also like high soil moisture (80-100%) and temperatures from +18°C. Soils prefer loamy soils and most often actively develop when the spring is cold and rainy. In this case, black scab spores are able to penetrate the tubers during the germination period, 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, up to the use of strong chemicals. Moreover, unfortunately, at the moment there are no potato varieties that are completely resistant to this type of scab.
silver scab potato got its name from the grayish-silvery spots on the tuber, which can occupy up to 40% of the area of the tuber.
True, such spots appear already at the stage of significant development of the disease. And it all starts 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 variety of scab – after all, the affected tubers are well stored and practically do not rot. But this appearance is deceiving.
In addition, dehydration occurs quickly during storage, 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 loamy and sandy soils. Like almost any fungus, it loves conditions of high humidity, from 80 to 100%. Therefore, the disease progresses during the period of flowering and tuberization.
Means of prevention and control
Potato tubers affected by all types of scab, except for rhizoctoniosis, are quite edible. Probably, it is for this reason that gardeners, as a rule, do not pay due attention to the treatment of this disease. But it is necessary to fight it, since both the taste and nutritional value of such potatoes are minimized. And if you plant even healthy, but not specially treated tubers on an infected land, they will also be infected and there will be no end to this. So, how can you get rid of scab on potatoes and make sure that it does not reappear on the site?
Agricultural practices
The main way to resist scab is crop rotation. If you do not plant potatoes on contaminated land for 4-5 years, then the infection may have time to die. But not everyone can afford to change the land every year for planting potatoes. Moreover, no plants of the nightshade family (tomatoes, peppers, eggplants), as well as beets and carrots, can be grown on this site. They are also affected by this disease.
What can be done in this case is to sow the site with green manure immediately after harvesting the potato tubers. It is best to use mustard, but legumes and cereals will also play a positive role. When the seedlings reach a height of 10-15 cm, the site is again dug up or at least mowed and mixed with green manure. Being in the ground, the remains of green manure contribute to the formation of saprophytic fungi and bacteria, which are natural enemies of scab pathogens. Thus, our great-grandfathers also fought with scab and quite successfully. In the spring, before planting potatoes, you can also plant fast-growing green manure, or at least sprinkle future beds with mustard powder and spill. Mustard significantly reduces the number of fungal and viral infections in the soil, and also protects against many pests: thrips, wireworms, slugs.
Since common scab spores develop especially well in alkaline soils with insufficient manganese and boron content, it is especially important to apply the following types of fertilizers in the spring before planting potatoes (application rate per 100 sq. m.):
- Ammonium sulfate (1,5 kg);
- Superphosphate (2 kg) and potassium magnesia (2,5-3 kg);
- Trace elements – copper sulfate (40 g), manganese sulfate (20 g), boric acid (20 g).
Treatment with various drugs
Other scab control methods include, first of all, pre-plant dressing of tubers with various fungicides. The use of Maxim or the microbiological preparation Fitosporin is quite effective and safe. The latter remedy can be used in a variety of ways. It is intended not only for the processing of 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 package of the drug is diluted in three liters of water.
There are many chemical preparations for getting rid of scab on potatoes. For example, to destroy black scab, both tubers and the plants themselves are treated with such strong drugs as Mancozeb, Phenoram Super, Kolfugo. Treated tubers are able to resist the disease even under adverse conditions.
In order to cope with other varieties of scab, the use of such strong chemicals is not necessary. For example, to suppress the development of common scab, various growth regulators, primarily zircon, are suitable. In its description, it is noted that the harmfulness of the disease is reduced even with a single treatment with this drug. With its double application, the disease can recede completely. 1 ml of zircon (1 ampoule) is diluted in 20-30 liters of water and the resulting solution should be treated with potato bushes after germination and at the beginning of flowering.
Conclusion
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.
