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 dish washing 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.
- 1 How to grow potatoes
- 2 Potato blight is a fungus
- 3 When does potato blight occur
- 4 How does blight spread
- 5 How much damage will blight cause
- 6 What are the symptoms of potato blight?
- 7 How to prevent potato blight
- 8 How to treat potato blight
- 9 Failsafe approach to prevent blight
- 10 Natural methods for blight treatment
- 11 Homemade potato blight spray
- 12 Bordeaux formula
- 13 The Cornell formula
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 seed, 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, which 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 can develop when the weather is warm and humid for extended periods and can be spread by air or water.
How does blight spread
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
If infected by late blight, an entire crop can be destroyed overnight. The fungus can even spread among harvested crops that are stored in the same bin or bag.
Though potato blight is usually killed by cold weather, 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 prepared to act quickly if you should notice any signs of blight.
How to treat potato blight
If only a few leaves are affected, you can remove them and dispose of them safely.
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 may prevent the disease from moving down into the tubers, so long as they’re well covered with earth.
Avoid burning foliage
Though some horticulturists suggest composting or burning the foliage, others insist that may cause the disease to spread and should be avoided.
Cover tubers with soil
You should cover any exposed tubers to keep the spore from reaching them, which will also prevent sun damage and keep scavenging animals from the tubers.
Leave the crop untouched for at least two to three weeks to allow the blight spores at the soil surface to die and the potatoes to develop a thicker skin.
Inspect regulary 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. You will either have to spray the plants with a blight spray to control and “dry up” the blight or they must be pulled up and buried under the soil or disposed of in the bin.
The reason for the removal of the plant is because the spores will multiply and the plant cannot be saved. Should 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 spreading the air bourne spores to the other plants.
It’s also recommended that you 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.
Next, proper garden hygiene is paramount: blight is a pathogen that can easily be 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 – as is the disease-preventing agent Bacilus subtilis, a naturally occurring organism which may be sold under the brand names 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.
The first and more effective of the two is known as Bordeaux formula which is a mixture of lime, water and copper sulphate.
The second solution is known as the Cornell formula and 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.
The Cornell formula
- 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.
No matter which spray you choose, it should be applied in the mornings after removing any blighted leaves if possible. (this 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. I have linked to the products below. Good luck!