Potato scab (rhizoctoniosis): description and treatment, how to fight

Potato scab is a generic name for a fungal disease that mainly affects the surface of potato tubers.

The infection is divided into several types:

potato rhizoctoniosis (black scab);
powdery scab;
common scab;
Scab tuberculate (oosporosis)
silver scab.

Fungus Damage:

potatoes are losing their presentation;
increasing amount of waste during cleaning;
yield is declining;
reduced keeping quality;
worse taste and nutrition;
tubers unsuitable for planting;
infested areas unsuitable for growing any nightshade family vegetables;
in severe cases, young potato seedlings die.

Black scab (rhizoctoniosis)

Rhizoctoniosis is considered one of the most dangerous types of potato scab. The fungus Hypochnus solani Prill excites her. et Delacr. In the basidial stage, it develops on the stems of potatoes, this disease is called the white leg. On infected plants, a characteristic off-white coating can be seen that covers the lower parts of the stems with a film.The fungus requires high humidity and warm weather to thrive.

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

Rhizoctoniosis symptoms

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

If infected tubers are used as seed, young seedlings will be destroyed.

Once in moist soil, the sclerotia of the fungus germinate with mycelium straight into the sprouts. Often, the plant dies before leaving the ground. The disease is especially serious when planting diseased tubers in poorly warmed soil.

Sprouted infected stems are girdled at the bottom with dark depressed spots.

Aerial green nodules begin to grow in the axils of the leaves.

Sick plants noticeably lag behind in growth, 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 full moisture capacity), the fungus grows rapidly and destroys young shoots.

Heavy loamy soils are primarily at risk of disease. Potato black scab prefers neutral soils but can thrive in the pH range of 4.5-8.

Rhizoctonia can be dormant in the soil for up to 3-4 years. Compliance with crop rotation is complicated by the possibility of infecting other crops with this fungus. (carrots, tomatoes, sugar beet, pumpkin and others).

How to deal with scab on potatoes?

The main way to deal with rhizoctonia is to prevent it. It is difficult to do something with diseased seedlings, it is easier to remove diseased plants to prevent the spread of the disease. You can only get rid of scab on potatoes 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 (Bactofit, Planriz, Integral, Fenoram, Maxim and others).
The area for planting potatoes should be “rested” from the nightshade family for 3-4 years. Ideal if flax, winter rapeseed, perennial cereal grasses, grain crops grew on it.
Potato plantings should receive timely top dressing with mineral and organic fertilizers.
Better to avoid very early planting dates.
A week before harvesting potatoes, it is better to remove the tops and remove them outside the site.
Don't delay the harvest. Choose warm, dry weather 6-7 days after haulm removal.

Scab powdery (dusty)

The causative agent Spongospora subterranea Wallr belongs to the lower fungi. In the 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 enters 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 full capacity. If the soil dries out, the amoeboid becomes covered with a shell and forms a cyst.In this form, pseudomushrooms can be for several years. A temperature of 12-18 degrees is optimal for development.

Symptoms

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

When the roots become infected, the fungus forms light, irregular growths called galls. Galls turn brown over time.

Slime mold tubers attack through the eyes and lenticels, forming light-colored pustules. Then the spots become larger and more prominent. The surface of the pustules cracks and you can see the dust, which contains spores. The keeping quality of tubers affected by scab is poor, often secondary infections (late blight and dry rot) join 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 occur.

Key infection prevention steps:

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

Common scab

Scabby 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, which gradually increase in size. Over time, the ulcers deepen and become corky. Especially "loves" common scab red and thin-skinned varieties. Tubers damaged by ulcers are easily subject to secondary infections of various rots.

Conditions for active growth of infection

Common potato scab loves well-warmed (25-30 degrees), not very wet (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 he althy tubers for planting, possibly with varietal resistance to the disease;
do pre-sowing germination of tubers in the light and dressing with fungicides (nitrafen, polycarbacin and others);
use of green manure (mustard, lupine, alfalfa, soy) and crop rotation;
full watering, during the growth of tubers (humidity 75-85%);
careful cleaning of the soil from all potato residues;

Silver scab

Silver potato scab becomes a frequent intruder on tubers. The causative agent of the disease is the fungus Helminthosporium solani. This disease grows in the peel of tubers and usually becomes noticeable only after 3-4 months of storage. Slightly depressed silver spots appear on the potatoes.

Sick tubers:

are exposed to secondary infections through the damaged surface;
have poor presentation;
give weak sprouts and rare seedlings.

Sick tubers and contaminated soil can become a source of infection. At high humidity and high temperatures, scab on potatoes grows very quickly.

How to deal with scab on potatoes:

seed tubers with fungicides (Maxim, Celeste, Nitrofen) before being sent for winter storage;
Selection of only he althy seed tubers;
sending well-dried potatoes for storage;
ventilation, temperature maintenance (+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 lenticels. Immediately the disease is not visible, it manifests itself closer to spring. Small brown tubercles are formed on the tubers, each of which has a depressed edging. Sometimes the bumps merge into large growths.

Sod-podzolic sandy soils are at risk. The disease is rare in peat bog soils.