Growing Potatoes – Pests and Diseases

Potatoes Pests and Diseases

See also Pests and Diseases of Seedlings and Young Plants


Blackleg (Erwinia carotovora)

Blackleg (Erwinia carotovora)

Typical symptoms: Leaves roll; individual stems grow poorly. Stems turn dark brown just above and below ground and rot in wet weather. Tubers may rot to a foul-smelling mess in the ground or may rot in store. Seed tubers are one of the main sources of infection. There is little spread between plants except in wet soils.


Prevention and treatment: Grow less susceptible varieties such as ‘Wilja’, ‘Marfona’, ‘Pentland Crown’ and ‘Romeo’ and avoid poorly drained land. Always harvest every tuber at the end of the season and compost potato foliage in a hot compost heap. If blackleg is a recurrent problem, do not mulch potatoes as this can exacerbate it. Check tubers in store regularly.


Common scab (Streptomyces scabies)

Typical symptoms: Tubers have rounded, corky, scabby patches on the skin, but the crop is otherwise usable.

Prevention and treatment: Scab is most common on light, dry and alkaline soils. Add organic matter to improve the soil and check the pH. Grass mowings in the potato trench will make conditions more acid. Spread a layer 2.5-5cm (1-2in) thick along the bottom of the trench, cover with 2.5cm (1 in) of soil then plant as usual. Watering the crop as the plants meet between the rows can also reduce the incidence of scab. Varieties such as ‘Wilja’ and ‘Pentland Javelin’ are less susceptible to the disease.


Frost

Typical symptoms: Foliage of plants turns brown, wilted and shrivelled overnight; severe frosts may also cause the scorching of shoots. Symptoms do not spread (unless there happens to be another frost).

Prevention and treatment: See Plant Disorders – Understanding the Problem


Magnesium deficiency

Typical symptoms: Leaves turn yellow between veins on older leaves first giving a marbled effect. Magnesium deficiency is common on light, acid, sandy soils especially in when the weather is wet.

Prevention and treatment: See Plant Mineral Deficiencies


Potash deficiency

Typical symptoms: The oldest leaves are affected first; the leaf margins turn brown, with a scorched look and may curl at the edges. Plants generally unhealthy.

Prevention and treatment: See Plant Disorders – Understanding the Problem


Potato blight (Phytophthora infestans)

Typical symptoms: Brown blotches appear on top of leaves in damp weather; white mould may develop underneath. The disease spreads very rapidly in warm, humid conditions, destroying all foliage. Tubers may rot in store.

Prevention and treatment: Where blight is common, grow less susceptible varieties such as ‘Wilja’ and ‘Cara’. Early varieties are more likely to produce a reasonable crop before blight appears. Earthing up or mulching plants can reduce damage to tubers, which results from blight spores being washed into the soil from infected leaves. Check regularly for blight symptoms from mid-summer. If it is seen to be spreading, cut off all foliage to ground level and compost it in a hot heap. Blight spores can survive for around three weeks in the soil; delay harvesting for this period to avoid infecting the tubers as they are lifted. Always check stored tubers regularly. Bordeaux mixture may slow down the spread of the disease if it is applied early enough, but it is of little use if the weather is wet.


Potato cyst eelworm (Globodera spp.)

Typical symptoms: Affected plants grow poorly and die down early; the leaves yellow and the crop is small. With the aid of a magnifying lens, pinhead-sized cysts may be seen on roots in mid-summer. There are two types of cyst eelworm: one has yellow cysts, the other brown or white.

Prevention and treatment: Cyst eelworm can survive in the soil for 15 years or more, so it is worth taking measures to avoid introducing it into the garden. Always use certified seed potatoes. Crop rotation can help to prevent pest build up in the soil. If the soil is infected, grow resistant varieties such as ‘Cara’, ‘Penta’, ‘Pentland Javelin’ and ‘Sante’. Most resistance is to the more common yellow cyst eelworm; a few, such as ‘Sante’, are also resistant to the brown or white cyst type. Increasing organic matter in the soil, growing early varieties and using the no-dig method will also help to improve the crop (see No-Dig Gardening).


Powdery scab (Spongospora subterranea)

Typical symptoms: Potato tubers develop irregular brown crater-like depressions and occasionally canker-like growths.

Prevention and treatment: Use a long crop rotation and avoid poorly drained soils. Do not grow ‘Pentland Crown’, which is particularly susceptible to this disease.


Slugs

Typical symptoms: Holes or extensive galleries are evident in tubers.

Prevention and treatment: Do not grow varieties that are prone to slug damage. Examples of these include ‘Maris Piper’, ‘Desiree’ and ‘King Edward’. Harvest all tubers by the end of the summer because it is during the early autumn that the most severe slug damage is caused. See Slugs and Snails – Garden Pest Control


Wireworm (Agriotes lineatus)

Typical symptoms: Often confused with eel-worm, this much larger pest makes knitting needle-sized holes in potato tubers.

Prevention and treatment: See General Garden Pests

02. February 2011 by Dave Pinkney
Categories: Kitchen Garden, Organic Gardening, Pests and Diseases, Root Vegetables | Tags: , | Comments Off on Growing Potatoes – Pests and Diseases

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