In this article, I will show you how to make two homemade potato blight sprays. One is called Bordeaux formula– made with lime and Copper sulphate. The other is called Cornell formula– made with dishwashing liquid and oil.
Over 150 years ago, a fungal disease unknown to scientists invaded Ireland.
Potato blight destroyed crops throughout the country for more than 4 years, creating widespread famine, leading to mass emigration, and causing over a million deaths from starvation and disease.
How to grow potatoes
If you would like more information about growing your own potatoes, please read my article How to grow potatoes. This covers everything you need to know from planting, preparing seeds, growing, harvesting, and storage, and more about this great vegetable.
Potato blight is a fungus
Late potato blight is caused by the Phytophthora Infestans fungus which affects not only potatoes but also other nightshade plants such as tomatoes, peppers, and eggplant. If left untreated, it can decimate the plant population and contaminate the soil for future plantings.
A more common disease among nightshade plants is early blight. This is caused by the fungus Alternaria solani. Though this disease can be difficult to eradicate, it is not nearly as devastating as late blight.
When does potato blight occur?
Blight develops when the weather is warm and humid for extended periods. The spores travel through the air or by water.
How does potato blight spread?
The wind carries the fungal spores from plant to plant and water can wash the spores into the soil, infecting the young tubers.
How much damage will blight cause?
Once infected by late blight, an entire crop can be destroyed within a week. The fungus can even spread among harvested crops that are stored in the same bin or bag.
Cold weather will kill blight, although there are some fungal strains that are resistant to cold and can survive over the winter, especially in the soil and plant debris.
What are the symptoms of potato blight?
One of the first signs of blight affects the leaves; you will notice brown freckling or lesions on the leaves of the potato plant.
They may develop larger patches of brown with a yellowish border spreading from them. Under moist conditions, a white fungal growth can appear under the foliage.
In severe cases, the potato foliage can rot entirely and the tubers are shrunken with dark patches on the skin.
Brownish rot spreads down from the skin and into the potato which eventually turns into a soft, soggy, foul-smelling mess.
How to prevent potato blight
The best way to prevent potato blight is to choose a blight-resistant variety and use certified seed potatoes from a reputable source.
Ensuring that the hill of soil around the plants completely covers the base of the potato will also prevent the fungal spores that cause blight from reaching the tubers.
Treating the plants with fungicide can help prevent the development of potato blight.
Good airflow around the plants will help to keep them dry which will prevent fungal spores from forming and/or flourishing.
Planting in the direction of the prevailing wind and spacing the rows well with allow the air to circulate around and through the plants.
You should check the plants regularly and be ready to act quickly if blight is present.
How to treat potato blight
If only a few leaves are infected, you can remove them and dispose of them under the soil.
Remember to wipe your pruner blades with detergent between cuts to prevent the infection from spreading.
Remove all infected foliage
If a lot of the foliage – typically more than 10% – appears infected, it should be completely cut away and discarded.
This will prevent the tubers from growing any larger but removing the foliage will stop the disease from moving down into the tubers.
Avoid burning foliage
Horticulturists suggest composting or burning the foliage. This will spread the spores and should be avoided.
Cover tubers with soil
Check the potatoes are earthed up or hilled. This will stop the blight spores from washing into the potatoes
Earthing up will also prevent sun damage and keep scavenging animals from the tubers.
Leave the crop untouched for at least two to three weeks before harvesting to allow the blight spores at the soil surface to die and the potatoes to develop a thicker skin.
Inspect regularly for rot
After harvesting your crop, check the potatoes in your store regularly for signs of blight and discard any suspect tubers immediately.
Dry the remaining tubers (never wash them clean as the remaining moisture may cause fungal growth) before storing them in a cool, dark, and dry place.
Failsafe approach to prevent blight
As soon as your potato plants develop the signs of late blight, they will need to be acted upon.
Spray the plants with a blight spray to control and “dry up” the blight immediately. If not, the plants need to be pulled up and buried under the soil or disposed of in the bin.
After you remove the plant, use a large plastic bag to cover the foliage and stems before pulling the plant up, roots and all. This should prevent the spreading of the air bourne spores to the other plants.
Leave the plastic-wrapped plants in the sun for at least a day to ‘cook’ which may help to kill off the spores, before burying in the soil.
Natural methods for blight treatment
There are several ways to prevent and treat cases of potato blight without resorting to harsh chemical compounds.
The first is crop rotation. As this disease infects only nightshade plants, moving them to another area of the garden and planting a different crop in their place can be an effective means of reducing the reoccurrence of potato blight.
Proper garden hygiene is paramount. Blight is a pathogen that is easily spread by dirty hands, boots, or tools. Remember to sterilize your gear after coming into contact with infected plants.
Apply organic fungicides
Organic fungicides such as Copper Sulphate sprays can be effective on early and late blight.
The disease-preventing agent Bacilus subtilis, is a naturally occurring organism. This is the active ingredient in the following sprays- Serenade or Plant Guardian Biofungicide.
Homemade potato blight spray
There are two solutions you can make at home to help fight blight in your potatoes. Follow the instructions below to learn how to make your own homemade potato blight spray.
The first and most effective of the two is Bordeaux formula. This is a mixture of lime, water, and copper sulfate.
The second solution is Cornell formula which is also a good preventative spray against potato blight.
To make a solution of Bordeaux formula you should:
- Mix 1 pound of slaked / hydrated lime in 1 gallon of water
- Mix 1 pound of Copper Sulphate crystals/powder in 1 gallon of water
- Fill a container with 2 gallons of water and add 1 quart (950mls) of copper sulphate solution and 1 quart of lime solution.
- The Bordeaux formula is now ready to use.
- 1 gallon of water with 1 tablespoon of baking soda,
- 1 tablespoon of oil (although vegetable oil will work, horticultural oil is best)
- 2 drops of dishwashing liquid or insecticidal soap.
- Mix it thoroughly and repeatedly shake it during use.
In the morning, remove infected leaves and apply the spray. Leaf removal may not be an option if you have too many potatoes to go through.
Ensure to get the spray on the undersides of the leaves as well as on top of the leaves with your chosen fungicide.
Try to get this applied during dry weather and allow it to dry. Many of the solutions are rain-fast when dry.
If you’d like to expand your knowledge on potato growing take a look at the short course in the education section of the menu. There is a section in module 3 all about pests and diseases in potato crops. Good luck!
11 thoughts on “Homemade Potato Blight Spray: make your own”
This is the first time that I’ve sown potatoes, I have planted them in a container, the leaves have just grown above the soil , I have kept the soil moist and I keep the container in a sunny place, I just heard that there is blight weather expected, what can I do to save the plants.
Hello Agatha, I would check the plants daily for black blight spots. You can then remove the infected leaves and mix up a blight spray as per the article and apply it immediately. Alternatively, you can apply some now before the blight hits. The most important thing is to them check daily. Richard.
I am very interested in this formula,( bordeaux Mixture) no signs of blight yet but will try it on my potato plants tomorrow and will let you know the results blight has always been a problem for me.
Interesting this bordeaux mixture, blight has always been a problem, will try it on my plants soon and let you know the results.
Hello Geoffrey, Bordeaux formula is a great homemade alternative for blight treatment. Inspect your crop daily for signs of blight. Thanks for your reply, Richard.
How effective is the baking soda/oil formula? I made some last week from a different recipe. Doesn’t seem to have done any harm but also not sure how effective it has been. Did you have good results with it?
Hello Diarmuid, the spray is effective, keep applying it once every week and it should keep the blight away.
How much of the Bordeaux formula should I use per square metre/yard?
Hello Blathnaid, great question.. Use enough so that all the leaves of each potato plant get coated with a light mist of spray. Thank you, Richard.
Are the 2 formulae complementary and give better protection? Would you recommend using both say a few days/week apart?
Hello Dougie, if you have spare mix left over you could use it the next time you spray. I would advise spraying for blight every 7 days or until you see blight returning. It is a good idea to change the blight spray every few weeks to stop the plant from becoming unresponsive to it. You don’t need to use two different sprays every time you spray. Richard.