Complete Guide on Potato diseases & common pests: identification, prevention & control

There are lots of different potato diseases and pests to watch out for. Very often, however, problems can be avoided with a varied crop rotation.

The potato (Solanum tuberosum) can be attacked by many different pathogens. Replanting the nutritious tubers in the same area often leads to infestation and the spread of pathogens through the soil or tubers left in the bed. So, before and after planting potatoes, take a cultivation break of 4 to 5 years in each bed. In this article we give an overview of the most common potato diseases and pests, as well as how to prevent and control them.

Potato diseases: overview

In the following article you will find the most common potato diseases, their pathogens, symptoms, and control measures. We only give a brief overview of rarely occurring diseases, as they are of less concern for hobby gardeners.

Bacterial potato diseases

Bacteria mainly cause soft, rotten spots on potatoes which often make the tubers completely inedible. The main carrier of bacterial diseases is an infected tuber, so the best prevention strategy is to buy certified, healthy seed potatoes.

• Stem and tuber soft rot (Pectobacterium carotovorum and Dickeya): Stem and tuber soft rot is also known as blackleg. From mid-June, infected potato plants wilt and curl their leaves. The bacteria block the water flow and cause soft rot on the blackened stem, making it easy to pull the shoots out of the ground. The inside of the stem is mushy and slimy. Tubers damaged during harvest develop soft rot in storage. The damaged area becomes mushy and soft and is separated from the healthy tissue by a black line. There is no way to treat the plants at this point. As a preventive measure, use only designated seed potatoes, do not plant the tubers too early or too late, and remove infected plants along with the tubers. Allow the tubers to dry after careful harvesting and check regularly in storage.

Other bacterial potato diseases:

• Ring rot (Clavibacter michiganensis subsp. sepedonicus): General symptoms develop on the potato stem, such as wilting, chlorosis (leaf yellowing) or curling of leaves. Bacterial ring rot can only be identified by cutting open the tubers. Infected tubers will display a brown discolouration a few millimetres below the skin and are often attacked by other pathogens and rot. Ring rot is listed as a notifiable quarantine disease!
• Bacterial wilt (Ralstonia solanacearum): Here, too, general disease symptoms appear on the plant, but inside the tuber a whitish bacterial ooze is secreted from the eyes and the brown tissue. Bacterial wilt is listed as a notifiable quarantine disease!

Fungal diseases on potatoes

Fungi cause a number of different diseases on potatoes. Here, prevention really is the best means of defence. For some diseases, however, there is no way to save the plants once the symptoms appear.

How to prevent fungal diseases on potatoes:

• Choose varieties with low susceptibility
• Remove potatoes that have grown from the previous year as early as possible
• Chit (pre-sprout) the tubers for earlier harvesting before infestation
• Keep wide row and planting distances to allow plants to dry
• Apply a well-balanced fertiliser
• Only harvest ripened tubers with firm skin
• Use crop rotation and healthy, certified seed potatoes to prevent soil-borne fungi
• In case of acute infestation, dispose of infected plants in household waste and use approved spray fungicides

Fungal diseases that hobby gardeners should look out for:

• Black dot (Colletotrichum coccodes): This soil-borne fungus causes entire shoots to die, especially in hot and dry years. The leaves wither, soon turn brown, and dry out on the stem, which often remains green. Small black spots appear at the base of the stem and the roots are brittle and rotten. The tubers may also be affected.
• Common scab (Streptomyces scabies): This fungus occurs naturally in the soil and infects the potato tuber from June to July; usually in dry, chalky soils. When infected, the tubers form corky lesions on the surface with sometimes web-like cracks, but there is no formation of spores. Potato scab is only an aesthetic problem, as it affects neither taste nor yield. Potato scab can be easily prevented by planting resistant varieties. In addition, healthy soil life acts as a competitor to the pathogen. Avoid adding lime to the soil before planting the potatoes.

• Late blight (Phytophthora infestans): Also known as potato blight, this disease is transmitted by oomycete fungi that overwinter in infected tubers left in the ground. The first symptoms appear from the end of June, depending on the weather. Yellowish spots form and soon darken on the potato plant leaves, and a greyish-white fungal layer appears on the underside of the leaves. Over time, the entire plant becomes infected and dies. The tubers develop grey-blue, hard, indented spots; under the skin, the tissue is hard and discoloured dark brown.

 

 

 

• Powdery scab (Spongospora subterranea): This is a fungal disease found mainly in damp and cool weather and at higher altitudes. Dark, wart-like bumps appear on the tubers, which later crack and release black spores. In the home garden, powdery scab spreads through potato peelings in compost or tubers left in the soil from the previous year. So, dispose of infected tubers and their remains in household waste. Using crop rotation, less susceptible varieties and healthy seed potatoes are the best control methods.
• Dry rot (Fusarium): This disease is developed in storage in which white fungal mycelium forms on the surface of the tubers and deep, dry rot underneath. The pathogens penetrate the potatoes through injuries during harvesting or from soil residues stuck to the skin.

• Root rot (Rhizoctonia solani): A potato disease that causes dark, surface spots on the tubers and indented, brown spots on the stem. The plants grow stunted and produce only a few shoots. A white fungal coating can form at the base of the stem, and sometimes the leaves of the shoot tip curl up. The pathogen is soil-borne; only crop rotation prevents renewed infestation.

Other fungal potato diseases:

• Early blight (Alternaria solani): This is a fungal disease mainly found in late potato varieties. From June onwards, sharply defined, round, brown spots appear on the older leaves, which can break off. And sunken, brown spots with firm tissue develop on the tuber.
• Grey mould (Botrytis cinerea): This fungal disease causes grey spots on dead tissue of the leaves, especially after a dry period in cool and humid weather. It does not need to be treated in potatoes as it disappears on its own in drier weather.
• Potato wart disease or black scab (Synchytrium endobioticum): This is a notifiable quarantine disease that is caused by a soil-borne fungus. Bulbous, cauliflower-like growths appear on the tubers and partly on the stems, which later rot and decay.
• White mould or stem rot (Sclerotinia sclerotiorum): A disease in which the mycelium of the fungus covers parts of the stem. In addition, dark, round, persistent growths, called sclerotia, appear. The infected stems bend easily in wind or storms.

Viral potato diseases

Viruses on potatoes are mainly transmitted by pests, above all aphids (Aphidoidea) and some plant-damaging nematodes (Trichodorus and Paratrichodorus). They cause growth disorders, leaf symptoms, and yield losses between 10 and 80%. Using resistant varieties, certified seed potatoes and crop rotation are helpful measures for prevention. Viruses can also be transmitted through injuries to plants when earthing up, hoeing, etc. Remove infected plants immediately, while the tubers are still safe to eat.

• Potato leafroll virus (PLRV): This is a viral disease with typical upward curling of the leaves and yellowing. The plants are noticeably smaller and have upright leaves which are rigid and dry and leads to yield losses of up to 80%.
• Tobacco rattle virus (TRV): A disease that is transmitted by nematodes that have previously sucked on infected wild herbs such as chickweed (Stellaria media) or shepherd’s purse (Capsella bursa-pastoris). The shoots, which are often stunted, rarely show symptoms on the leaves and the yield is hardly affected either. However, black-grey spots and sunken rings are found in tubers when cut open.
• Potato virus Y (PVY): One of the most important potato viruses worldwide that causes dark brown, necrotic spots on the underside of the leaves, as well as a slight mosaic yellowing. The leaves may die completely and sometimes the tubers are affected, developing dark spots, which do not penetrate as deeply as tobacco rattle virus.

Soil-borne pathogens can make potato cultivation on one plot unviable for years. One alternative to beds is growing in pots, because most pests cannot access the soil there.

Common potato pests

Various pests cause damage to potatoes, sometimes only on the plant, but often also on the tasty tubers. Here are the most common potato pests and our tips on how to avoid infestations and control them.

• Aphids (Aphidoidea): These small, sucking insects can cause leaf deformation on potatoes. In nature, however, they are no match for beneficial insects such as ladybirds (Coccinellidae) and lacewing larvae (Chrysoperla carnea), among others, who serve to keep the aphid numbers under control.
• Wireworms (Agriotes): These dark yellow, several millimetre long beetle larvae eat deep tunnels into the nutritious tubers. Good soil preparation, an early harvest (using early variety and pre-sprouting seed potatoes), and a balanced crop rotation with a cultivation break of 4 to 5 years greatly reduces the risk of a wireworm infestation.

• Cutworms (Agrotis): The caterpillars of moths, known as cutworms, prefer to live in light, warm soils and can cause severe feeding damage to potato tubers. The grey-brown shiny cutworms, which measure up to 5cm long, curl up when touched. The moths also lay their eggs on weeds, so hoeing the potato rows is an important preventive measure. An acute infestation of cutworms on potatoes can be controlled with nematodes of the genus Steinernema carpocapsae, which are simply stirred into water and spread with a watering can.
• Colorado potato beetle (Leptinotarsa decemlineata): This beetle hibernates in the soil and lays its eggs on the underside of the leaf shortly after the first potato shoots appear. The potato beetle larvae eat entire potato plants within two to three weeks. Spraying oil of the neem tree (Azadirachta indica) combats the potato beetle larvae in the young larval stages L1 to L3. It is important to check the plants regularly to spot damage in the early stages. Colorado potato beetles and their larvae can be removed by hand when they are too large for neem treatment.

• Potato root or potato cyst nematodes (Globodera): If potatoes are planted without a long enough cultivation break (4-5 years), harmful nematodes can colonise and repeatedly infest the roots of the plants. A severe infestation is indicated by growth disturbances and yellowing of the leaves. Small, brownish cysts appear on the roots and can survive in the soil for up to 15 years. The potato plant yields drop by up to 50%. Good crop rotation, avoiding other host plants, and using resistant varieties prevent nematode infestation.
• Voles (Arvicolinae): Various species of Arvicolinae, especially the field mouse (Microtus arvalis), like to gnaw on potato tubers and can cause great damage. Above ground, individual shoots may wilt and die because their roots have been cut. Read our article on how you can deter voles to find out more about these rodents.

Article Credit: Plantura