Diseases and Pests of the Potato

The enemies of the potato

Qu they fly, crawl, or evolve underground or in the air, whether they suck, devour, or sting plants, insects are numerous in making potatoes their dish of choice. Whether they attack leaves, stems, shoots, or tubers, they are everywhere, ready to draw their weapon of war to poison your life. And when it comes to potatoes, one cannot overlook the Colorado potato beetle, even if other enemies lurk insidiously to devour your young plants.

The Colorado Potato Beetle

Description

Easily recognisable, the Colorado potato beetle has an oval and bulging body. Each of its elytra is marked with five black stripes on a yellow background. The larvae are reddish-orange. Both larvae and adults measure between 1 and 1.5 cm.

Colorado potato beetles can cause serious damage

The Colorado potato beetles spend the winter in the soil at a depth of 25 to 50 cm. When temperatures reach around 15 °C, they emerge and rush towards the young potato leaves. Sated, they lay eggs on the undersides of leaves that have escaped their appetite. And as soon as the larvae hatch, they continue their destructive work, jeopardising an entire crop. Then the larvae become adults and burrow into the soil to emerge again the following spring.

Symptoms

The first symptoms are holes in the leaves. Next, adults and larvae consume the entire leaf starting from the edge. Young adults also feast on the tubers.

Prevention

• Implement a crop rotation of at least three years.
• Remove all potato regrowth that appears in spring.
• Make every effort to welcome the predators of Colorado potato beetles, such as blackbirds, lizards, small mammals, and ladybirds that feed on the eggs.
• Sow a flower meadow made up of repellent annual flowers for Colorado potato beetles.

Natural Treatments

• By carefully observing the undersides of leaves, one can detect small yellow eggs grouped in tight clusters that need to be removed.
• Hand-picking adult Colorado potato beetles and larvae.
• At harvest time, stems and potato remnants should be destroyed.
• Spray nematodes against Colorado potato beetles.

Wireworms

Description

In fact, it is the larva of the wireworm, a small beetle of the genus Agriote, that one should fear. Slim, cylindrical, shiny yellow with a dark brown head, it is referred to as the “wireworm”. It appears from May to late August after living several years in the soil where it was born. It prefers moist soils.

Symptoms

The wireworm larva penetrates the potatoes, feeding on them, which leads to the wilting of the plant.

Prevention

• Strictly apply crop rotation.
• Thoroughly work and loosen the soil before planting potatoes.
• Add manure and compost in autumn.
• As wireworm larvae often develop in freshly turned soil, let the chickens out if you have any.
• Attract the natural enemies of wireworms, such as ground beetles, shrews, and birds.
• Apply lime, especially in acidic soils.
• Spray a fern manure that seems to be repellent.
• Sow castor or flax seeds that effectively repel Colorado potato beetles.

Natural Treatments

• As soon as you plant your potatoes, create baits with other potatoes cut in half. Bury the cut side 4 to 5 cm deep. Check your traps every two days. The wireworm traps are also very effective.

Potato Cyst Nematode

Description

Nematodes are small pale yellow or white microscopic worms capable of living many years in the soil where they were born. Females attach to the roots to lay eggs. Those that attack potatoes belong to the genus Globodera. They do not move, but the cysts can be spread by tools or shoes.

Attack of cyst nematodes on the root system of potatoes

Symptoms

The root system is attacked, and the plants become stunted, yellow, and wilted. Tissues are damaged by cysts, and potatoes may be dotted with fine cracks.

Prevention

• Crop rotation is essential.
• Avoid excess nitrogen.
• Incorporate a little powdered clay into the soil in autumn.
• Burn plants affected by nematodes.
• Carefully clean your tools.
• Plant marigolds, garlic, or onions between potato rows.

Natural Treatments

There is no effective treatment against nematodes.

Slugs

Description

There are slugs of various sizes and colours. Hermaphrodites, they fertilise each other and reproduce mainly in wet weather and in the absence of predators. They live only 9 to 12 months and are active from April to August.

Symptoms

The skin of the tubers is pierced. Slugs burrow into them and feed on the flesh. There is also noticeable defoliation of plants and trails of mucus.

Prevention

• Set up insect shelters, feeders, or wood piles to attract slug enemies such as hedgehogs, ground beetles, birds, and shrews.
• Monitor compost and remove any slugs that may be hiding there.
• Water only in the morning.
• Cover the soil with pine needles or crushed ferns.
• Place copper nets or bands or slug barriers, which are very effective in preventing gastropods from passing.

Natural Treatments

• Hand-picking slugs early in the morning.
• Place cardboard, tiles, or boards around the garden. Slugs take refuge there, and in the morning, simply collect them to take them far away.

Aphids

Description

Four types of aphids are most commonly found on potatoes: Aphis nasturtii, Myzus persicae, Macrosiphum euphorbiae, Aulacorthum solani depending on the climate. They are not dangerous in themselves to potatoes but are mainly vectors of various viruses.

Symptoms

Leaves turn light in colour, become crinkled, and die.

Prevention

The best preventive solution is to attract predators such as hoverflies, lacewings, and ladybirds by planting flowering plants.

Natural Treatments

• Spray a solution made from black soap.
• Spray nettle manure.

Viral diseases of potato

Essentially transmitted by aphids or, from generation to generation, by tubercles, viruses affect potato crops to varying degrees. There are many types:

• Leaf curl virus is transmitted by the peach aphid (Myzus persicae) and is characterised by leaf curling, chlorosis of the leaves, and the appearance of necrotic spots.
• Mosaic: small yellow spots appear on the lamina of the leaves, forming patterns reminiscent of Aucuba japonica. Sometimes, spots develop on the tubercles.
• Marbling virus is transmitted by beetles. It is recognised by the marbling that appears on the leaves.
• Frizzled: the leaves become crinkled and rough.
• Variegation: necrotic brown spots develop on the older leaves of adult plants.

Prevention

• Control the virus vector, often aphids.
• Carefully clean your tools.

Natural treatments

Remove and burn all diseased parts of the potato.

Fungal diseases of potato

Provoke by fungi, several cryptogamic diseases can affect the potato. Foremost among them is the formidable late blight. This disease was, in fact, responsible for the severe famine that affected Ireland in the mid-19th century.

Late Blight

Description

The fungus Phytophthora infestans is responsible for this disease. It requires humidity and mild temperatures ranging from 10 to 25 °C to develop. Successive thunderstorms and fogs promote its evolution.

Symptoms

From May onwards, brown spots with a slightly oily appearance appear on the aerial parts of potato plants, on both the upper and lower surfaces. A white fluff is often also visible.

The characteristic dark brown spots of late blight

Prevention

• Choose varieties that are naturally less susceptible to late blight, such as Mona Lisa, Charlotte, and Désirée.
• Avoid watering the foliage.
• Do not plant tubers too closely together to aerate the plot.
• Weed and hoe regularly.
• Implement crop rotation.
• Spray with Bordeaux mixture or a horsetail decoction.

Natural Treatments

• Remove and burn infested parts of the potatoes.

Black or Brown Scab

Description

The fungus Rhizoctonia solani is responsible for this disease, which infects the sprouts and roots of potatoes in spring. It develops in cold, damp conditions at the start of the growing season.

Symptoms

Dark brown spots form, and the stems die. The plants develop poorly. After hilling, the leaves curl and lighten. In humid weather, a whitish sheath may appear at the base of the stems. Crusts form on the tubers.

A potato root affected by black scab

Prevention

• Do not plant too early or too deeply; wait until the soil is well warmed.
• Implement crop rotation every 3 years.
• Avoid leaving tubers in the ground for too long.

Natural Treatments

No specific treatment is required. Simply destroy infested plants.

Common Scab

Description

The fungus Streptomyces scabies is present in most soils but thrives particularly in sandy or overly calcareous soils when temperatures are between 15 and 24 °C.

Symptoms

Brown, cracked, flat, hollow, or raised spots appear on the tubers. Potatoes do not store well.

Prevention

• Choose less susceptible varieties such as ‘Apollo, Sirtema, Roseval, Béa, Ratte, Rosa…
• Sow green manures such as vetch, lupin, rye, or mustard before planting potatoes. These green manures have an inhibiting effect.
• In case of drought, water at the beginning of tuber formation.
• Maintain a 4-year interval between two crops.

Natural Treatments

No natural treatments available.

→ Learn more about common scab of potatoes in our advice sheet!

Alternaria

Description

Also known as brown spot disease, Alternaria is caused by two fungi, Alternaria solani and Alternaria alternata that attack the leaves of potatoes. They thrive in humid conditions.

Symptoms

Brown then black spots appear on the upper surface of the leaves. They form concentric circles. The leaves dry out or rot. A dry rot appears on the tubers during storage.

Prevention

• Implement crop rotation.
• Avoid planting too densely.
• Water without excess.

Natural Treatments

Bordeaux mixture can be sprayed as soon as the disease appears.