Description and treatment of potato scab (rhizoctoniosis), modern control measures

Potato scab is a general name for fungal diseases that mainly affect the surface potato tubers.

The infection is divided into several types:

• potato rhizoctonia (black scab);
• powdery scab;
• common scab;
• tuberous scab (oosporosis)
• silver scab.

Damage from fungi:

• potatoes lose their presentation;
• the amount of waste during cleaning increases;
• productivity decreases;
• keeping quality decreases;
• worse taste and nutritional qualities;
• tubers are unsuitable for planting;
• infected areas are unsuitable for growing any vegetables of the nightshade family;
• in severe cases, young potato shoots die.

Black scab (rhizoctoniosis)

Rhizoctonia is considered one of the most dangerous types of potato scab. She is aroused by the mushroom Hypochnus solani Prill. et Delacr. In the basidial stage, it develops on potato stems; this disease is called white leg. On infected plants you can see a characteristic dirty white coating covering the lower parts of the stems with a film. Active growth of the fungus requires high humidity and warm weather.

More often this fungus is in the stage of the imperfect fungus Rhizoctonia solani J.G. Kuhn, from which the second name was taken potato diseases rhizoctoniasis.

Symptoms of rhizoctoniasis

Infected tubers are covered with black sclerotia, similar to dried soil particles. These formations are difficult to remove with a fingernail and they do not wash off. In this form, the fungus overwinters on potatoes without causing any noticeable harm.

If infected tubers are used as seed material, young shoots will be ruined.

Once in moist soil, the sclerotia of the fungus sprout as mycelium directly into sprouts. Often, the plant dies even before leaving the ground. The disease becomes especially serious when planting diseased tubers in poorly heated soil.

Sprouted infected stems are girded in the lower part with dark depressed spots.

Green aerial nodules begin to grow in the axils of the leaves.

Sick plants are noticeably stunted in growth and have withered foliage.

Conditions for active growth of infection

The most comfortable temperature for the development of black rot is 16–18 degrees.If it is accompanied by high humidity (60–70% of the full moisture capacity), the fungus grows quickly and destroys young shoots.

Heavy loamy soils are primarily at risk for the disease. Potato black scab prefers neutral soils but can develop in a pH range of 4.5–8.

In the soil, rhizoctonia can remain dormant for up to 3–4 years. Crop rotation is complicated by the possibility of infection of other crops by this fungus. (carrots, tomatoes, sugar beets, pumpkin and others).

How to deal with scab on potatoes?

The main way to combat rhizoctoniosis is to prevent it. It is difficult to do anything about diseased seedlings; it is easier to remove diseased plants to prevent the spread of the disease. You can get rid of scab on potatoes only by taking preventive measures:

• When choosing seed material, you need to make sure that it does not show signs of disease.
• Treat planting tubers with fungicides (Baktofit, Planriz, Integral, Fenoram, Maxim and others).
• The area for planting potatoes should be “rested” from the nightshade family for 3–4 years. Ideally, flax, winter rapeseed, perennial cereal grasses, and grain crops grew on it.
• Potato plantings should receive timely fertilizing with mineral and organic fertilizers.
• It is better to avoid very early planting dates.
• A week before harvesting potatoes, it is better to remove the tops and remove them from the site.
• Do not delay harvesting. Choose warm, dry weather 6-7 days after removing the tops.

Powdery scab (dusty)

The causative agent of the disease, Spongospora subterranea Wallr, belongs to the lower fungi.In a free state, it is an amoeba-like clot of protoplasm that moves in the upper layers of the soil in search of plants. The parasite penetrates the root cells and grows in them.

Conditions for active growth of infection

Powdery scab develops best in peat soils. Slime mold can only move in very wet soil, more than 70% of its total moisture capacity. If the soil dries out, the amoeboid becomes enveloped and forms a cyst. Pseudo-fungi can remain in this form for several years. A temperature of 12–18 degrees is optimal for development.

Symptoms

The slime mold infects the tubers, roots and stolons of the potato bush.

When the roots are infected, the fungus forms light, irregular growths called galls. Over time, the galls turn brown.

The slime mold attacks tubers through the eyes and lentils, forming light-colored pustules. Then the spots become larger and more prominent. The surface of the pustules cracks and dust containing spores can be seen. The keeping quality of potato tubers affected by scab is poor; secondary infections (late blight and dry rot) often appear at the site of damage.

Sources of infection

In the soil, powdery scab usually overwinters on the remains of roots and tubers, as well as in the form of dormant spores. In this state, the fungus can last up to 3–4 years until favorable conditions arise.

Basic steps to prevent infection:

• observe correct crop rotation;
• choose only healthy planting material;
• treat seed tubers with fungicides;
• do not over-moisten the soil, especially on cool days;
• select disease-resistant varieties.

Common scab

Common scab is caused by several species of fungi of the genus Streptomyces.The description of the disease is known to any vegetable grower; infected tubers become covered with ulcers that gradually increase in size. Over time, the ulcers deepen and become corky. Common scab especially “loves” red and thin-barked varieties. Tubers damaged by ulcers are easily susceptible to secondary infections of various rots.

Conditions for active growth of infection

Common potato scab loves well-warmed (25–30 degrees), not very moist (50–70%), slightly alkaline or neutral soil (pH 6–7.5). Common scab, like other fungal diseases, is easier to prevent than to treat.

If you increase watering during the formation and growth of tubers, you can significantly stop the development of this fungus.

Control measures:

• selection of only healthy tubers for planting, possibly with built-in varietal resistance to the disease;
• do pre-sowing germination of tubers in the light and dressing with fungicides (nitrafen, polycarbacin and others);
• the use of green manure fertilizers (mustard, lupine, alfalfa, soybeans) and compliance with crop rotation;
• full watering during tuber growth (humidity 75–85%);
• thorough cleaning of the soil from all potato residues;

Silver scab

Silvery potato scab becomes a frequent uninvited guest on tubers. The causative agent of the disease is the fungus Helminthosporium solani. This disease grows in the skin of tubers and usually becomes noticeable only after 3–4 months of storage. Slightly depressed silvery spots appear on the potatoes.

Diseased tubers:

• are exposed to secondary infections through the damaged surface;
• have a poor presentation;
• produce weak sprouts and sparse seedlings.

The source of infection can be diseased tubers and contaminated soil.At high humidity and high temperatures, scab on potatoes grows very quickly.

How to deal with scab on potatoes:

• treat the planting tubers with fungicides (Maxim, Celeste, Nitrofen) before sending them for winter storage;
• selection of only healthy seed tubers;
• sending well-dried potatoes for storage;
• availability of ventilation, maintaining temperature (+2...3 degrees) and low humidity during storage;
• observe the principles of crop rotation.

Lumpy scab (oosporosis)

The infection is caused by the fungus Oospora pustulans Owen. The disease spreads through the eyes, mechanical damage and lentils. The disease is not immediately visible, but appears closer to spring. Small brown tubercles form on the tubers, each of which has a depressed edging. Sometimes the tubercles merge into large growths.

Soddy-podzolic sandy soils are at risk. The disease is rarely found in peat swamp soils.

A soil temperature of 11–12 degrees stimulates the development of the fungus.

Treatment for oosporosis is the same as for silver scab.