[GPI]

Potato blight, how to treat Phytophthora infestans.
by GPI

Potato blight (Phytophthora infestans) thrives and multiplies at an alarming rate in mild & damp climates; It’s no wonder then that this is the main problem experienced by Irish potato gardeners. The first signs of blight are brown spots on the leaf tips of the potato plants, often but not always accompanied by a fluffy white fungal growth.
Following on from this, the foliage will begin to rot, calling a premature halt to the growth of the potatoes. To make matters worse, fungal spores drop from the leaves onto the soil, where they quickly infect the potatoes rendering them inedible with dark sunken areas on the potato skin and a bronze rotting of the flesh.

Potato blight Scientific classification.
Domain: Eukaryota
Kingdom: Chromalveolata
Phylum: Heterokontophyta
Class: Oomycetes
Order: Peronosporales
Family: Pythiaceae
Genus: Phytophthora
Species: P. infestans
Binomial name
Phytophthora infestans

An example of blight affected potato leaves and tubers, photo / picture / image.

Management of the problem organically.
1. When digging your potatoes each year, you should be scrupulous in clearing your soil of all potato tubers, even those as small as pebble. Ensure that there are no potatoes on site from last years growing, as blight spreads by fungal spores from old potatoes to new crops in mid-summer. Placing potato peelings on your compost heap can also carry over blight if the heap is not hot enough, so I would also avoid this.

2. Plant only blight-free certified seed potatoes into soil that has not grown potatoes for at least three years. If you forget about planting maincrop potatoes altogether and instead opt for a first early or second early variety, this will allow you to harvest before blight infection is at its highest (mid –summer onwards). “Colleen” and “Orla” are both early potato varieties with good blight resistance.

If you are going to grow maincrop potatoes (harvest-August/September) then I suggest you select a common variety with strong blight resistance such as “Cara”.

Some of the more blight susceptible potatoes to avoid are.....
Maris Piper ,
Arran Comet,
Arran Pilot,
Desiree
Epicure,
Foremost,
Golden Wonder,
‘Home Guard’,
Kerr’s Pink,
Majestic,
Sharpe’s Express,
Ulster Chieftain.

Reassuringly, one of the greatest hopes in the search for blight free potatoes has been the recent introduction of the Hungarian Sarpo varieties, “Mira” and “Axona”. Promising four times more blight resistance than any variety already available, I have heard great reports about these maincrops including boasts of high yields and a floury texture.

3. Avoid overhead watering of your potato crop, as this tends to wash the fungal blight spores from the potato plants leaves down into the soil, to infect the tubers. Of course, the rain will do this naturally, but if you have to provide additional irrigation, then I suggest you water the soil not the foliage.

4. When the weather conditions are conducive to the spread of blight, radio stations and daily newspapers will issue “blight warnings”, so watch out for these. Upon hearing a blight warning you can apply an “organic” blight spray known as Burgundy mixture, this is not a totally organic spray, but is a deemed acceptable by most organic growers.

Burgundy mixture is created by mixing......
50 grammes of copper sulphate (bluestone)
60 grammes of sodium carbonate (washing soda) or sodium bicarbonate (baking soda)
All in five litres of water.

As a prevention before blight symptoms show, you can spray this mixture on to your potato foliage every seven days during blight warnings, ensuring that the undersides of the leaves are coated as well.

5. Herbal teas concocted from common weeds/herbs such as nettle, horsetail (Equisetum arvense ) and comfrey can be sprayed on potato foliage to help prevent the spread of blight disease.

To make the horsetail tea you should ideally in early summer pick the green part of the plant, leaves and stem, excluding the root. You will need one kilogram (2 ¼ pounds) of equisetum to submerge in drum filled with 20 litres of rain water, stirring it daily over three or four weeks.

When the time comes, mix it in your watering can at a rate of ten parts water to one part horsetail tea, then water liberally onto your plants. Plants that show any symptoms of bight or other fungal type disease should be sprayed with this mix once every four days.

If in a rush to create a batch of horsetail tea, you can make up some by simmering 1/2 kilogram of the weed in a pot with 10 litres of rain water for about an hour. Once cooled it should like before be mixed it in your watering can at a rate of ten parts water to one part horsetail tea.

6. Be blight aware by checking the leaves of your crop daily, especially when the risk is high during July and August, at these times the disease can appear literally overnight. If you find blight on the foliage and you anticipate your potato yield is acceptable, then I would “bite the bullet” and cut away the stalks. Wait for about two weeks, then using a garden fork, gently loosen the potatoes from the ground discarding the few that the blight may have reached.

Management of the problem chemically.
All the measures mentioned above still stand, but instead of the Burgundy mixture many gardeners prefer to use stronger preventative blight sprays such as Bio Dithane945 or other fungicides containing the active ingredient Mancozeb.

So, here’s hoping for a dry summer and little blight.

Associated content.....
Nettle fertiliser, How to Brew your own.

Grow your own potatoes.

Increasing your stock of seed pototoes, How-to Video.

How to chit seed potatoes for earlier and heavier cropping.

How to grow new potatoes for Christmas.

How to manage the top two potato pests, Wireworm and Slugs.

Common potato Scab, Management of the problem.

Planning a Vegetable Garden? How to Make a Vegetable Garden.

(DISCLAIMER: The control methods are suggested here as a matter of general information. Under Irish and EU law it is illegal to use any preparation as a pesticide/fugicide/herbicide that is not approved for such use. The author and the website accepts no responsibility for how a user may mix, use, store, or any effects the mixture or its elements may have on people, plants or the environment. The information here is for reference only and does not imply a recommendation for use. If you disregard this warning and make any of the preparations you do so entirely at your own risk.

Any queries or comments on common potato blight, please post below.
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