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Of all potato diseases scab at first glance seems the most harmless. At the initial stage of its development, many do not even notice that the potato is sick with something. After all, for example, ordinary potato scab 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 you can soon be left without a crop at all. Moreover, the scab infection mainly lives in the ground and the situation must be corrected with an integrated approach.
Types of scab
Before thinking about how to deal with scab on potatoes, you need to understand that disease this one 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 Rhizoctoniae);
- Silver.
Most widespread in fields and gardens common scab... This type of disease is caused by a fungus called Streptomyces scabies. Most often he lives in soil, prefers dry, sandy soils with a reaction close to alkaline. It develops especially actively at air temperatures above + 25 ° + 28 ° С.
Symptoms of common scab damage to potatoes are quite diverse, but most often the disease begins with small, almost imperceptible brown ulcers, sometimes with a red or purple tint. Sometimes the surface of the potato becomes rough and subtle grooves in the form of a mesh form on it. With severe damage, 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, since under unfavorable conditions (low temperatures) the fungus falls into suspended animation, but does not die. But when raw, not rotted manure or significant doses of limestone is introduced into the soil as fertilizer, the risk of common potato scab increases. Therefore, it is necessary to treat, first of all, the very land used for planting potatoes.
To counteract common scab, you can use potato varieties that are resistant to this disease: Domodedovsky, Zarechny, Yantarny, Sotka.
Powdery scab, unlike ordinary, usually appears as a result of prolonged rains on heavy, waterlogged soils.
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 growths, while the tubers develop a variety of warts of various sizes, reddish-brown. Spores of powdery scab 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 there is high humidity in the storage, they will rot rather quickly.The fungus can persist in soils for up to five years or more.
Black scab potato or rhizoctonia disease is one of the most dangerous types of scab. The only thing that makes it easier to diagnose is the fact that the whole potato plant is affected as a whole - from tubers to stems with leaves. But as a rule, the defeat of the aboveground part indicates 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 extensive spots.
This is how the black scab on the potato 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 live to bloom. This dangerous disease is caused by Rhizoctonia solani. Spores of this disease also like high soil moisture (80-100%) and temperatures from + 18 ° C. They prefer loamy soils and most often develop actively when the spring is cold and rainy. In this case, spores of black scab are able to penetrate the tubers even during the germination period, and such a potato is doomed to die.
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.
Silvery scab The potato got its name from the grayish-silvery spots on the tuber, which can occupy up to 40% of the tuber area.
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 type of scab - after all, the affected tubers are well stored and practically do not rot. But this appearance is deceiving.
In addition, during storage, dehydration occurs quickly, and the tuber can become dry and wrinkled by spring. Due to this, up to 40% of the yield is lost and such tubers are not suitable for use as planting material.
The pathogen of silvery scab is undemanding to soils, it feels good both on loam and on sandy loam 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 Rhizoctonia disease, 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 plot, then they will also be infected and there will be no end to this. So, how can you get rid of the scab on the potatoes and make sure that it does not appear on the site again?
Agrotechnical techniques
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 for planting potatoes every year. Moreover, no plants of the Solanaceae family (tomatoes, peppers, eggplants), as well as beets and carrots, can be grown on this site. They are also susceptible to this disease.
What can be done in this case is to sow the site with siderates immediately after harvesting the potato tubers. It is best to use mustard, but both legumes and grains will play a positive role. When the seedlings reach a height of 10-15 cm, the plot is dug up again, or at least mowed and the green manure is mixed with the ground. 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 fought with the scab and quite successfully. In the spring, before planting potatoes, you can also plant fast-growing green manures, 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.
Since spores of common scab develop especially well in alkaline soils with an insufficient content of manganese and boron, it is especially important to apply the following types of fertilizers in the spring before planting potatoes to combat this type of disease (application rate per 100 sq. M):
- Ammonium sulfate (1.5 kg);
- Superphosphate (2 kg) and potassium magnesium (2.5-3 kg);
- Trace elements - copper sulfate (40 g), manganese sulfate (20 g), boric acid (20 g).
Treatment with various drugs
Other methods of scab control include, first of all, preplant dressing of tubers with various fungicides. The use of Maxim or the microbiological preparation Fitosporin is quite effective and safe. The latter can be used in a variety of ways. It is not only intended for processing seed potatoes. To consolidate the effect, they are recommended to spray the 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 chemicals available to get rid of potato scab. For example, to destroy black scab and tubers, the plants themselves are treated with such powerful drugs as Mancozeb, Fenoram super, Kolfugo. Processed tubers are able to resist disease even under adverse conditions.
In order to cope with other types 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. If it is used twice, the disease can recede completely. 1 ml of zircon (1 ampoule) is diluted in 20-30 liters of water and the resulting solution must 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.
