Potato blight
Potato late blight is caused by the fungus Phytophthora infestans. This fungus infects leaves, stems and tubers. It can cause devastating crop losses. This fungus also causes tomato blight (see factsheet DC20 Tomato blight).
Typical symptoms
Symptoms of blight
- Leaves and stems: Dark brown/ blackish round patches, often surrounded by a pale yellow halo. In warm, damp weather the patches quickly spread to rot the whole leaf. Stems can also be infected. The underside of the infected leaf develops a downy white coating of spores in moist conditions, particularly at night. If weather remains warm and damp, the disease spread rapidly, reducing the foliage to a rotting mass within a few days.
- Tubers: Dark, sunken areas, which may extend to cover the whole potato, giving a dry firm rot. Cutting the potato in half will show patches of chestnut-coloured rot starting just under the skin. Other fungi and bacteria may invade the tuber to produce a wet, foul smelling soft rot. Seemingly healthy tubers may rot later when in store.
Another blight
Potato early blight (Target spot) symptoms are often mistaken for late blight. Caused by Alternaria solani, it generally occurs earlier in the season (July). The distinctive dark brown spots, somewhat angular with concentric rings, are bounded by the leaf veins. Early blight rarely causes significant loss of yield and no treatment is necessary.
Life cycle
Potato late blight survives the winter in infected potato tubers. Infected ‘volunteers’ left in the soil, or discarded, will grow in the spring and develop blight, acting as a source of infection for new crops. Home saved tubers from an infected crop may also develop the disease when planted.
The initial infection may come from a local source, such as a neighbouring garden, but the spores may be blown in from many miles away.
Spores can only infect the plant if they land on wet foliage. Spread is very rapid throughout the crop when temperatures are above 10°C and humidity is over 75% for two days or more. Rain washes spores from the leaves down into the soil where they infect the tubers.
Until the 1970s, there was only one type of blight in the UK, and this was unable to produce resistant spores that could survive the winter. There are now two types and when both of these are present, reproduction can take place with the potential for new blight strains to develop.
Prevention and control
- A healthy start: Plant good quality seed tubers from a reliable source. Don’t bring seed tubers from other countries, or save your own if there is any chance that the crop was infected.
- Variety choice: Potato varieties vary in the susceptibility of their foliage and tubers to blight. Resistant foliage does not necessarily mean resistant tubers, and vice versa. Most early varieties are very susceptible, but can often be harvested before blight arrives. Similarly, early maincrop varieties are more likely to produce a reasonable crop before blight appears.
- Resistant varieties: Maincrop varieties that show some resistance and are slow to develop blight include Cara, Red Cara and Avondale. ‘Verity’ and ‘Sante’ have good tuber blight resistance. Many of the Hungarian ‘Sárpo’ varieties, such as Axona and Mira are extremely resistant in both foliage and tubers.
Research has shown that growing alternate rows of different resistant varieties can improve crop yield by a small amount.
The development of new, more virulent, strains of blight mean that some varieties that were once resistant to foliage blight are no longer so. These include Lady Balfour, Orla and Sante. Check up-to-date information before you buy on www.varieties.potato.org.uk - Good hygiene: Remove potential sources of infection. Try to harvest all tubers, even the tiniest, so that there are none to regrow next season. Remove all volunteer plants that come up – don’t be tempted to go for a harvest of volunteer potatoes. Never abandon old tubers around the garden or allotment, or try to compost them – they are best put in the dustbin, or buried deeply (over 60cm/2ft). Infected potato haulms (foliage) may be composted in a good active heap. The likelihood of resistant spores being present is very slim. Remove blighted tubers before putting potatoes in store and check every few weeks for rotting tubers.
- Spacing and watering: Avoid planting in sheltered sites. Plant rows into the prevailing wind. If watering is required, apply water to the base of plants and early in the morning to reduce long periods of moisture on foliage. Soil improvement and mulching will reduce the amount of watering needed.
- Ridging and mulching: Blight spores on foliage are washed down through the soil to infect tubers. Earthing-up potatoes, or mulching the soil with a thick layer of hay or straw or other organic material, can reduce the levels of tuber infection.
- Blight forecasting: There is a national blight forecasting system. When on at least two consecutive days the minimum temperature is 10ºC or above and on each day relative humidity is greater than 90% for at least 11 hours, blight is very likely to develop. This is known as a ‘Smith’ period. Go to www.potatocrop.com to sign up for a blight alert in your area.
- If blight strikes: Initial blight symptoms may not spell disaster - the disease may not spread through the crop if the weather turns dry.
If the crop is badly infected (10% or more foliage killed), it is advisable to cut off and remove all potato foliage to help prevent spread. This obviously stops any further growth, highlighting the benefit of growing early maturing varieties, and early planting of maincrop varieties to ensure maximum growth before blight strikes.
Don't harvest the crop for two to three weeks after defoliation. By then, tubers will have thicker skins and blight spores in the soil, that could infect tubers when lifted, will have died. - Composting: Blight infected foliage can be composted. The blight fungus requires a living host to survive. When put in the compost bin potato foliage will die quickly and with it the fungus. Infected tubers should not be composted as they may re-grow. Potato peelings tend to break down quickly and rarely re-sprout and so are considered fine to compost, but not if they are particularly thickly cut.
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