Black and brown spots on potato tops: how to treat them

Potato

Potatoes are a highly disease-prone crop. Potato diseases can develop due to viruses, fungi, bacteria, and nematodes. Planting material can become infected due to improper storage conditions, neglect of disinfection of available materials, the use of unrotted compost, and failure to adhere to agricultural practices and cultivation techniques.

Growing potatoes is a labor-intensive process. To avoid losing a harvest this fall, it's important to identify crop diseases early and take measures to combat them. Preventative measures to maintain the health of the potatoes themselves and the soil in which they are grown are equally important.

Description of the causes of diseases

Disease

Pathogen

Distribution mechanism and features

Late blight

Phytophthora fungus. White spores are usually found on the underside of leaves.

Spores can fly over long distances, damaging plants.

Favorable weather for late blight: humid, rainy, temperatures up to 25°C.

Black scab

Rhizoctonia solani JG Kuhn (mycelial stage).

Thanatephorus cucumeris (AB Frank) Donk (sexual stage of the fungus).

The mycelium and sclerotia of the fungus persist on tubers and in the soil.

The disease most often progresses on loamy soils with acidity from 5.5 to 6.5 pH at a temperature of approximately 17°C and a relative humidity of 65%.

Silver scab

Helminthosporium solani Durieu et Mont.

The mycelium is located in the tubers' skins. The color is initially light, but turns brown as it develops.

The fungus spreads by non-motile spores at high humidity and temperatures, most often in light soils.

The pathogen can be detected on tubers during their formation or during harvesting in the fall.

Powdery scab

Pseudogungum Spongospora subterranea (amoeboid without an envelope).

The fungus attacks tubers by parasitizing their cells. It spreads via spores. An infected tuber can be identified by spore-filled nodules. When the nodule bursts, a cavity remains in the tuber, and the mature amoeboids migrate through the soil, infecting new tubers.

The main condition for spreading is waterlogged soil.

Under unfavorable conditions, the pseudofungus becomes covered with a shell and becomes dormant.

Potato cancer

Synchytrium endobioticum (Schilb.) Perc.

The fungus does not affect potato roots. It reproduces by spores contained in the growing plant tissue. Spores can be spread through garden tools, water, and plant debris.

It can be preserved in the soil for up to 30 years.

Alternaria

The genus of fungi Alternaria.

The source of infection is mycelium that survives on plant debris, as well as small spores that are carried by the wind and penetrate plant tissue through the epidermis. Leaves, stems, and, less commonly, potato tubers are affected.

Early development is asymptomatic, manifesting itself during budding and flowering. Black circles or triangles may be visible on the plant. The tubers have dark, slightly sunken spots with distinct borders.

Factors in the development of the disease:

- dry weather;

— t>25°C;

- lack of potassium and nitrogen in the soil;

- excess phosphorus in the soil.

Alternaria often affects seed material infected with viruses and rhizoctonia.

Macrosporiosis

Macrosporium solani and Alternaria solani

Infection of plants with spores occurs through stomata and damage to the epidermis of leaves.

Blackleg

Pectobacterium.

The fungi live in the upper soil layers, feeding on dead plant tissue. They infect the crop upon contact with the root collar and roots. Pectobacterium can survive in seed tubers.

Pectobacterium is spread by aphids, Colorado potato beetles, wireworms, and cicadas.

From the stems, bacteria penetrate the stolons into the tubers, causing softening and rotting.

Breeding conditions: high humidity, t=21-26°C.

Ring rot

Corynebacterium sepedonicum.

The pathogen from infected tubers moves to the aboveground part of the plant and clogs the vessels.

Can be transmitted through non-disinfected instruments and knives.

Develops in high humidity and moderate temperatures.

Brown rot

Ralstonia solanacearum bacteria.

The source of infection is soil. Bacteria penetrate tubers through stolons, stomata, and various wounds on stems and roots. As they multiply, the bacteria fill the plant's blood vessels with a brown slime. The foliage wilts.

Develops at temperatures >27°C and high humidity.

Verticillium wilt

Soil fungi Verticillium albo-atrum Reinke et Berthold.

It appears at the end of flowering. The pathogen penetrates the plant and spreads along the xylem bundles, where mycelial development occurs. Brown spots are visible on cross-sections of the stems.

Progresses at temperatures of 21-24°C in humid conditions.

Fusarium wilt or dry rot

Fusarium oxysporum Schl.

It can appear at any time during potato growth, most often during the flowering period.

The mycelium penetrates the plant and develops within the vascular system. If the stem is cut, brown vessels or vascular rings will be visible.

The stored crop is affected from the stolon portion, when spores penetrate. As the mycelium grows, the potato skin wrinkles, becomes rotten, and sunken. Sporulation appears as light, raised pads.

The disease develops very quickly if the plant has mechanical damage or is infested with wireworms. Favorable conditions for dry rot include high soil moisture and hot weather.

Anthracnose (darthosis)

Colletotrichum atramentarium Berk, et Broome.

A receptacle forms on the pathogen-infected surface. It resembles small, bristly, black pads. Condia and condiphores are small, colorless, and covered with sclerotic thickening. Sclerotia containing the pathogen are present singly or in groups.

Anthracnose most frequently affects early-ripening varieties through soil (the main source) during the second half of the growing season. Tubers can become infected during storage or harvest. Spores can be spread by insects, wind, and water during irrigation.

The fungus spreads through the stolon end. A dark spot appears on the tuber, followed by an increase in the number of sclerotia, and the tuber becomes soft and foul-smelling.

Favorable weather for the pathogen: dry, hot (t>22°C), acidic soil, with a lack of phosphorus.

Potato diseases and their control

When combating potato diseases, it is important to follow recommendations and not use toxic chemicals that are hazardous to health.

Fungal diseases

Late blight

Brown spots appear on the leaves above, with a white coating of spores underneath. The foliage dies and falls off. On the tubers, the spots are initially gray, then gray-brown.

It is recommended to apply double doses of fertilizers containing potassium and phosphorus.

Fungicide treatment:

during the period of top closing, apply the preparations “Ridomil MC” / “Oxyhom” twice;
after flowering with copper oxychloride / the drug "Kuproksat";
during the period of tuber formation, twice with the drug "Alufit".

Black scab

On tubers, scab appears as black sclerotia, net-like necrosis, or pitted spots. Brown ulcers and dry rot appear on the stems. Bushes are often stunted, with foliage curling from the edges toward the center.

Chemical control methods:

A single soil treatment with Quadris when planting potatoes. Consumption rate: up to 200 liters per hectare;
Treatment of seed potatoes with the drug "Maxim".

Silver scab

The lesion appears as spots of varying sizes without shine.

If infected potatoes are left in storage, silver scab will infect the entire crop by spring. By this time, the spots on the tubers will take on a metallic sheen, and the skin will wrinkle.

For soil treatment before planting tubers, we recommend using Azoxystrobin. Apply once, at a rate of up to 200 liters per hectare.

Powdery scab

Potatoes affected by scab look and store poorly. "Warts" (pustules) develop on the surface, which dry out and crack after a while. A dry mass—spores mixed with destroyed potato tissue—spills out of the pustules. White growths can be seen on the roots, which later darken.

Severely affected tubers should not be eaten. Potatoes should not be planted in contaminated soil for up to 7 years.

Potato cancer

Quarantine dangerous disease.

A plant with cancer has numerous growths of varying sizes. On the tubers, the growths are initially white, then darken and rot. On the stems and leaves, the growths are greenish.

Important! Under no circumstances should you eat these potatoes!

Chemical treatment of the soil with chloropicrin is carried out by a special quarantine inspection. All plants are immediately burned.

Disease-resistant varieties:

Spark;
Belarusian early;
Detskoselsky;
Pavlinka;
Candidate;
Table;
Gatchina.

Even when growing cancer-resistant varieties, it is important to maintain crop rotation.

Alternaria

The most resistant potato varieties to this disease are:

Volzhanin;
Snow White;
Fairy tale;
Resource;
Lina;
Bryansk delicacy;
Bronnitsky.

Important! There are no varieties that are 100% resistant to Alternaria.

Treatment with chemicals to combat the pathogen is carried out according to the following scheme:

at the first signs (dry brown spots on leaves, yellowing and dying of foliage, almost black spots on stems) or during the growing season, spray with Ridomil Gold MC (0.5% solution, consumption 2.5 kg/ha) or Folman (consumption 3 kg/ha);
During the period of tuber formation, spray three times with the preparation “Bravo KS” (consumption 3 l/ha) at intervals of 10 days.

Important! Treatment must be carried out in a calm and dry environment. Wear protective clothing and strictly follow the instructions on the product packaging.

To avoid the disease, it is necessary to harvest the crop correctly (without damaging the skin) in the fall and then deeply plow the soil.

Only fully ripe tubers are suitable for harvesting; affected ones should be discarded immediately. After harvesting, the crop should be kept at room temperature and high humidity for 21 days.

Note! It is not recommended to plant tomatoes and potatoes next to each other, as Alternaria attacks both crops.

Macrosporiosis

The spots are small, grayish-brown, with concentric circles. They are located on the upper side of the leaf.

Potato plants are recommended to be sprayed with Bordeaux mixture, 1% solution, at a rate of up to 6 liters per 100 square meters. Increased potassium fertilizer application is necessary.

Viral diseases

Disease	Manifestations
Striped mosaic	Dark spots on potato leaves. A mosaic-like pattern, they appear on the underside of the foliage. Later, black dots appear on the potato leaves. Turgor is lost, the leaves wilt, and fall off.
Wrinkled mosaic	The leaves are corrugated and wrinkled.
Common mosaic virus	The disease is caused by several viruses. Light spots appear on the leaves, which darken over time.
Leaf curling	Observed when the crop is infected with virus M or L. In the first case, the leaves curl upward, in the second - along the central vein.
Mottling	Leaf spot virus X (PVX) is the causative agent of leaf spot disease. The virus is transmitted by the fungus Synchitrium endobioticum. Light, mottled spots appear on the leaves. If the strain is severe, tissue death occurs.
Gothic tuber or spindle-shaped	The pathogen is the Potato Spindle Tuber Viroid (PSTVd) RNA molecule. The foliage of affected plants is narrow, small, and detached from the stem at an acute angle. The bushes are elongated and stunted. The tubers are underdeveloped, small, and have prominent lenticels. The tubers are pear-shaped or very elongated.

Bacterial diseases

Blackleg

The foliage of diseased plants dries up and curls into tubes. Potato stems soften and rot, and may break off. The plant is easily pulled from the soil. Mucus oozes from the cut stem when placed in water. The skin of the tubers cracks and darkens.

Infected bushes should be destroyed in one of two ways:

burn;
bury to a depth of more than 100 cm, covering with bleach.

The disease can develop latent. It's important to maintain crop rotation, use healthy seed potatoes, treat them before planting, and apply fertilizer to ensure normal crop growth.

Ring rot

A diseased plant can be identified by wilting, whitening leaves, stems (which fall to the ground), and rotten rings inside the tubers. A whitish or light yellow mucus will ooze from the stem cut when dipped in water.

Affected plants should be dug up and burned along with the tubers. The tops should be mown down and removed three weeks before harvesting. Tools should be disinfected. It is important to prepare the seed and soil before planting. When sprouting in the light, tubers affected by bacterial blight can be identified.

Note! To prevent ring rot, it is recommended to plant green manure crops such as oats or mustard in the field. Sow the plants in early spring. Before planting, mow the potatoes and loosen the soil.

Brown rot

This is a quarantine disease. The main method for detecting bacterial rot is simple: cut the stem and place it in water, which will cause a brown exudate to ooze. White mucus spots or brown rotten circles may be visible on cut tubers. Bacterial mucus may also ooze from the stolons and eyes.

Varieties resistant to bacterial diseases:

Luck;
Resource;
Ulyanovsk;
Volzhanin.

Diseased bushes should be dug up and burned with extreme care. Before harvesting (2-3 weeks), be sure to mow and remove the tops.

Beets or wheat should be planted in the area where the infected potatoes were grown. Potatoes should not be replanted in the same area for at least 3-5 years.

Chemical methods of combating bacterial rot are not effective.

Note! It is recommended to treat planting material with TMTD or Planriz.

Other diseases

Verticillium wilt

This disease can be distinguished by the following signs:

the leaves begin to turn yellow, dry out and curl (usually the development progresses along the bush from the bottom up);
in the heat the leaves quickly wither and then curl up;
the eyes on the tubers rot, leaving indentations in their place;
on diseased leaves and stems there is a pink or grey coating;
Potato sprouts curl up and die.

The pathogen persists for a long time in the soil and tubers, so it's important to treat and discard seed before sowing. The disease can be transmitted through the root system to mechanically damaged bushes; careful loosening and weeding of the field is recommended.

Potato bushes showing signs of wilting should be carefully dug up and burned along with the tubers.

Important! Fungicides are only active in the early stages of the disease.

Spraying is carried out with solutions of the following preparations in a concentration of 0.2:

"Topsin-M"
"Previkur";
"Fundazol";
"Benlat".

Fusarium wilt

The most resistant varieties to this disease are:

Detskoselsky;
Berlichingen;
Priekulsky early.

Infected plants are easily identified by yellowing and rot with a pink bloom on the leaves. Fusarium wilt is "treated" mechanically by destroying infected bushes. The plant should be removed with the soil and burned. Shoes and tools must be disinfected.

After culling, the remaining plants should be sprayed with a solution of potassium permanganate, and the soil should be sprinkled with powdered sulfur and ash.

Liming the soil with dolomite flour or chalk is highly effective. Mulching the soil significantly reduces the activity of the pathogen.

Medicines against the disease:

"Agat-25K";
Fitosporin-M;
"Baktofit";
"Fundazol";
"Topsin-M"
"Maxim" (for treating planting material).

Note! Potassium humate fertilizer has a fungicidal effect and can be used to combat the disease (applied to the soil and treated with seed).

Anthracnose

Reddish-brown spots on the leaves, darker or yellowish at the edges, gradually spread. The plant's tops die, and the tubers rot during storage and the growing season.

It is necessary to destroy all infected plants and disinfect vegetable storage facilities and tools.

Effective drugs:

"Previkur";
"Skor";
"Fundazol";
"Acrobat MC";
Fitosporin-M.

Copper oxychloride can be used. Treatment should be performed three times, 7-10 days apart, following the package instructions. These products can be alternated for greater effectiveness.

Potatoes can be planted in the same area after four years. Be sure to remove weeds and plant debris from the soil. The soil should be dug over without loosening in the fall.

Prevention and treatment of potatoes from diseases before planting

Processing technology

Step 1. One month before planting, the tubers are removed from storage, manually sorted, and culled based on size and weight. Healthy tubers with no signs of disease or rot are selected.

Step 2. The tubers are placed in boxes or on shelves in 2-3 layers.

Step 3. The boxes are placed in a bright location with a temperature of 17-20°C. The tubers are turned periodically until sprouts appear.

Step 4. The tubers are treated with insectofungicidal preparations:

"Maxim" solution: 20 ml per 1 liter of water. For 100 kg of tubers, you'll need up to 10 liters of solution;
Prestige. Add 100 ml of the product to 5 liters of water. This solution is sufficient for spraying 100 kg of potatoes. The suspension should be applied to three-quarters of the tuber. Not suitable for early potato varieties.
"Cruiser". Consumption: 70 ml per 100 kg of potatoes.

Crop rotation is essential. It's recommended to rotate potato growing locations every three years. Growing potatoes in low-lying areas or on heavy soils is not recommended. Before harvesting, mow down the tops, and after harvesting, completely remove any plant debris from the field, followed by deep digging. Potatoes should be harvested only in dry weather.

Watering should be moderate to prevent the soil from drying out or becoming waterlogged. It is not recommended to water potatoes with ice-cold water in hot weather or spray it on the foliage. After watering, carefully loosen the soil.

Potato storage facilities must be carefully prepared:

cleared;
the walls are painted with freshly slaked lime;
the premises are disinfected with a 3% solution of copper sulfate;
sufficient ventilation is provided.

Recommendation! Place a disinfectant barrier—a mat soaked in a 5% copper sulfate solution—in front of the storage facility entrance.

Reviews

Alexander B.

I've been using Maxim for over three years and have no plans to change. It's a versatile product, and I'm very pleased with its performance. I use it when planting potatoes and spray the crop before storing. It's very economical. The main advantage is that the potatoes are safe to eat after treatment.

Maria S.

My favorite product is Fitosporin M. It's not just a fungicide, it's biodegradable. It fights fungi and late blight well. I use the paste (I prefer it that way) and water the soil before planting. The potatoes grow healthy and large. I've never had blackleg.