Blight is the worst disease for potatoes, with the fungal spores being spread in the wind. Late blight (Phytophthora infestans) had been present in the UK for some 200 years as a single strain of blight. In the 1970’s, new strains of blight were accidentally introduced to Europe from Mexico with disastrous results. These new strains bred with the existing strain to produce more virulent strains, especially “Blue 13” which appeared in 2005. This strain infects previously resistant potato cultivars such as Cara, Valor, Remarka, Setanta, and Lady Balfour.
Blight usually strikes every year when there is high temperature and humidity, sometimes in July, then August and September. As it can strike so quickly, within a couple of days, keep your eyes skinned for the first signs of Blight on your neighbours’ potatoes, so that you can take immediate action to protect your own potatoes. There is a Blight Internet warning service during the blight season, showing the location of reported outbreaks of Blight on farms at www.potato.org.uk/blight This gives a general warning for blight attacks in your area.
To get Blight warnings sent directly to your e-mail address or SMS alerts sent to your mobile phone, sign up at http://blight.potato.org.uk/signup.html .
The weather conditions that are likely to lead to an outbreak of Blight, are known as “Smith Days”. This is when there are two consecutive days where the minimum temperature is 10 C or above. In addition, for at least 11 hours each day the relative humidity is greater than 90%. In other words, two days of particularly hot and sweaty weather!
The first line of defence is to exclude sources of infection such as infected seed tubers, infected ground keepers, dumps of discarded tubers and clamp sites.
Then choose potatoes that are reputed to have some resistance to blight. The new Sarpo Hungarian varieties have better Blight resistance, as vividly demonstrated in the adjacent photo. Try Axona or Mira for almost complete blight resistance, though can be affected by early Blight but will usually recover and continue to grow to maturity. Sadly, new strains of blight are evolving and now attack varieties that used to show some resistance.
Some of the early potatoes and salad potatoes are particularly susceptible, but luckily, they will probably be lifted by the end of July before the main Blight season in August.
The first sign of a late Blight attack is when the potato leaves suddenly have large irregular or rounded brown and blackish patches on the leaves and stems, often surrounded with a yellowish halo. On the underside of the leaf, you may be able to see a ring of fungus which spreads daily if the “Smith Days” continue. This is followed by the collapse of the leaves and then the potato stems. Often, there is also a distinctive smell of rotting vegetation in a severe blight attack.
At the first sign of late Blight, cut off the affected foliage and destroy. Do not put it on the compost heap as it will spread the disease. If the potatoes are almost mature, it would be better to lift the potatoes as soon as possible, as the blight spores can be washed into the soil by rain and the potatoes will start to rot as well. See the adjacent photo.
If the potatoes are maincrop varieties and still have a lot of growing to do, consider spraying against blight.
Prevention of Blight is possible by the prior spraying of potatoes with a fungicide such as “Fruit and Vegetable Disease Control” (Dithane is now withdrawn), but this is not an organic method. The only organic method allowed is to spray with a copper fungicide.
So called Early Blight or target spot (Alternaria solani), sometimes seen in June or July, is often mistaken for true blight (Phytophthora infestans). Early blight has dark brown angular spots in between the leaf veins. The spots are concentrically zoned. Remove the individual diseased leaves by hand and destroy to prevent it spreading. It can also be controlled by spraying with a fungicide such as “Fruit and Vegetable Disease Control”.
Slugs cause an immense amount of damage to potatoes, especially in wet seasons. The worst damage is caused by Keeled slugs, which produce two generations of eggs, which hatch in the spring and Autumn. As slug damage increases the longer you leave the potatoes in the ground, it is advisable to lift the potatoes as soon as they die down. Again as some varieties are more susceptible to attack, try and choose ones that are known to have some resistance to slug attack with their thicker skin. Potatoes with coloured skin generally have thicker skins giving better slug resistance. The colour is contained in an extra layer of skin. Axona is also proving to be slug resistant. For a list of suggested slug resistant potato varieties, go to this page, click on JBA potatoes and go to their list of slug resistant potatoes.
It also helps to apply a suitable slug treatment at planting time and again just before the potato haulms meet between the rows, and in August, to control the number of slugs in the area. Make the habitat unfriendly for them by trying to keep the weeds well under control.
Wireworms are a great pest of potatoes causing many of the smaller 3 mm holes in the tubers. They are the larval stage of a beetle, Agriotes lineatus, and are particularly common in ground that has been very grassy or weedy for several years. Sadly, the larvae can live in the ground for up to 4 years eating roots, before they pupate and emerge as click beetles in the summer.
Destroy them by crushing between your fingers whenever you see them on site while digging over your soil and when earthing up your potatoes. Lift your potato crops as soon as they are full grown to prevent further damage.
Potato root eelworm is the common name for the potato cyst nematode, whose presence may be indicated by the early yellowing and die back of the potato haulms, together with a poor crop of small potatoes. It can also affect tomatoes.
As the eelworm is only visible with a microscope, the only way for the amateur gardener to check for the presence of eelworm is by inspecting the roots of a suspect plant at harvest time with a magnifying glass, for the presence of the tiny egg bearing cysts.
Unfortunately, these cysts about 1 mm dia, can stay viable in the ground for 7 years or more and spring into life when they detect the roots of a growing potato.
For a detailed photo and further advice on eelworm resistant varieties of potatoes, go to this page.
There is a new organic control for Eelworm, but it requires the ground to be taken out of productive use for a growing season. This could prove useful where an allotment has been badly managed for several years and the eelworm eggs have become established and are lying dormant in the soil. The organic control is a selection of Solanum sisymbriifolium, a thorny, inedible relative of potatoes which strongly stimulates potato eelworm eggs in the surrounding soil to hatch, but there is nothing for them to feed on. According to Alan Romans, in trials 50-90% hatch rates were achieved, which is a stronger reaction than that produced by potatoes themselves. The plants can be chopped down to 15 cm in summer for composting and the re-growth will produce more beneficial root growth as well as more composting material. Assume all parts of Solanum sisymbriifolium are poisonous.
Scab, a wart like skin condition, does not affect the eating quality, and is associated with sandy soils.
Rust spot is also associated with sandy soils. Both Scab and Rust Spot are controlled by adding humus and a general fertiliser.
Spraig, a brown, corky discolouration is caused by a virus, and is again controlled by strict rotation.
For a full description and photos of potato diseases, go to this page.
Further information on British potato variety database characteristics, giving descriptions of the varieties, and including pest and disease resistance such as blight, and yield for common potato varieties, is available under “variety tolerance tables” at this page
POTATO DAMAGE CAUSED BY MANURE CONTAMINATED WITH AMINOPYRALID