Preventing & Controlling Potato Blight

Potato blight (Phytophthera infestans) requires a mild & damp climate in order to infect potato foliage. In may parts of the UK & Ireland this is the norm during the potato growing season.

Whilst I would personally rather not use them, over 20 years of growing potatoes leads me to the conclusion that it may not be entirely possible to avoid using fungicides on organic potato crops. The advice given below is based on the standards followed by commercial organic potato producers in the UK.

Blight prevention

Potato blight cannot be cured and, particularly in an organic situation, avoidance is definitely the best policy. Under commercial UK organic standards certain protectant fungicides are currently permitted, but not on a routine basis. However, it is highly likely that some of them (copper-based) will be withdrawn at the end of March 2001.

Hygiene is very important

• make sure there are no tubers left-over from last year growing and creating a possible source of blight infection
• fortunately, blight does not normally survive in soil
• a compost heap which heats up properly will kill any blight spores on foliage and tubers

Avoiding blight

Blight is not generally a problem with early harvested, early varieties.

• plant early varieties if suitable/possible
• plant healthy, blight-free seed
• select varieties with high blight resistance
• listen for, and pay attention to, blight warnings on the radio or in the press
• if the blight pressure is high apply a permitted fungicide, particularly to very susceptible varieties

If blight is found on foliage and tuber yield is acceptable

• remove haulms (tops) immediately and compost them
• apply a permitted fungicide to remaining stalks
• once skins have set, harvest and use the crop promptly
• carefully remove all blighted tubers during lifting and storing
• do not consider long-term storage for potatoes which have been infected with blight

If blight is found on foliage and tuber yield is still very low

In this situation the likelihood of having a low yield of blighted potatoes is considerable. This is one of the main risks of growing organic potatoes.

• apply a permitted fungicide as soon as blight is diagnosed
• monitor blight development on a daily basis
• if blight continues to develop rapidly either remove and compost the haulms, or leave them to die naturally
• a further application of fungicide should be made once the approved time interval has elapsed since the last application
• once skins have set, on the tubers harvest and use the crop promptly
• carefully remove all blighted tubers during lifting and storing
• do not consider long-term storage at all

In reality organic potato producers frequently have to remove potato haulms early because of foliage blight. This reduces yield but helps to avoid tuber blight

Fungicide selection

Only selected sulphur and copper-based protectant fungicides are currently permitted, under commercial UK organic regulations. The copper-based ones may be withdrawn at the end of March 2002. This will make preventing blight on organic potatoes very much more difficult and risky.

Only use proprietary products, as home-made formulations are not allowed under the UK's Food and Environment Protection Act 1985 (FEPA) and the Control of Pesticides Regulations 1986 (COPR). Similar legislation may also apply in other countries.

Fungicide application

• only apply fungicides when a real need exists - ie when the risk is very great
• do not apply fungicides on a routine basis
• apply fungicides at the rate, water volume and spray pressure given in the product labelling
• with tractor mounted sprayers pay particular attention to tank agitation and nozzle sizes as some products have a tendency to 'settle out' and/or block small nozzles
• observe minimum time intervals when giving subsequent applications

Beware of unexpected tuber blight !

Foliage (and subsequent tuber blight) blight control has been covered above in relation to an identified foliage blight infection.

However, blight spores can sometimes wash off leaves into the soil without infecting leaves, resulting in unexpected tuber blight.

To avoid taking blighted tubers into store

• at the time of haulm removal, and a week later, inspect a sample of tubers from across the field carefully
• if blight is found the crop should be left in the ground for several weeks until blighted tubers rot and 'melt' into the ground.

Once the crop in which blight has been found has been harvested

• inspect the crop frequently (weekly) for signs of further tuber blight
• use the crop quickly, ensuring all blighted tubers are removed

Tuber blight found during storage

The measures given above offer the best means of avoiding blight entering the potato store.

If blight is discovered during storage

• sort the crop rigorously to exclude blighted tubers
• use the crop as quickly as possible

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