potato plants grown in the garden

Copper Sulphate As A Potato Blight Spray: what is it and is it good?

Blight is a common fungal disease that affects potatoes. It can destroy the entire crop in less than a week if left unattended. Unfortunately, there is little you can do to save your potatoes once they have been infected with blight. This makes prevention key to raising healthy potatoes. While the best prevention for potato blight is growing blight-resistant varieties, spraying your potatoes with copper sulphate is also an effective way to prevent blight in your potatoes.

What Is copper sulphate spray for potato blight?

Copper sulphate is a combination of copper and sulphuric acid. This compound combines copper and sulfur to make a solution that can be used to kill fungicides, like Alternaria solani responsible for early blight and Phytophthora infestans responsible for late blight. While the compound is inorganic, copper occurs naturally in the soil and does not pose a health risk (unless inhaled or ingested) at the rates applied to control potato blight. Copper sulphate has been approved for organic gardening.

Does copper sulphate prevent or cure potato blight?

There is no cure for potato blight once it infects your potatoes, but you can use copper sulphate to prevent an infection or to prevent the infection from spreading to uninfected potato plants. Copper sulphate is best applied before you see any signs of blight. It can be applied every 7 to 10 days until harvest following the manufacture’s application rate. The copper ions in copper sulphate formulas bind with the proteins in fungi causing cell damage. This kills the fungi and prevents them from spreading.

Is copper sulphate a natural compound or synthetic chemical?

Copper sulphate is a synthetic inorganic compound. However, copper occurs naturally in soil. The National Organic Program (NOP) lists copper sulphate as allowed for organic farming, says the USDA. When applied according to recommended doses, copper sulphate poses little risk to humans or the environment. When applied at higher doses it can damage crops and contaminate the soil.

Can copper sulphate be used in organic potatoes?

Yes. Copper sulphate can be used in growing organic potatoes. It is on the NOP’s list of synthetic compounds approved for organic growing. 

How to Make Copper Sulphate Blight Spray at Home

Making your own copper sulphate at home isn’t difficult. The University of California Pest Management Dept. recommends a Bordeaux mix as a homemade copper sulphate spray. You will need copper sulphate, dry hydrated lime, water, and a gallon container. You can purchase copper sulphate and hydrated lime at your local hardware or home improvement centre.

  1. Fill a clean gallon container with water allowing an inch or two for mixing.
  2. Add 3.3 tablespoons of copper sulphate.
  3. Mix in 10 tablespoons of dry hydrated lime.
  4. Stir the mixture to dissolve the lime and copper sulphate. Alternately, cap the gallon container and shake it to mix all the ingredients.
  5. Label the container and store it out of the reach of children. Although the mixture is approved for organic gardening accidental ingestion or inhalation can cause health risks. 

Why is Copper Sulphate Spray Called Bordeaux Mix?

Bordeaux mix gets its unusual name from its origin of use. This formula was first used in the Bordeaux region of France in the late 1800s to combat potato blight.  You can buy premade Bordeaux mix at your local hardware or home improvement center. There are several variations in the formula.

How do you spray your Bordeaux mix?

Spraying your potatoes with copper sulphate or Bordeaux mix is simple, but it does require a sprayer.  Here’s what you need to do.

  1. Wear protective clothing, such as long pants and long sleeves, to prevent contact with the skin. Gardening gloves and a face mask are also recommended.
  2. Fill your sprayer with your homemade Bordeaux mix or other copper sulphate formula. Always follow the directions for mixing commercially prepared copper sulphate mix.
  3. Choose a day when your potatoes are dry and there is little to no wind. It is best if the weather is expected to remain dry for the next day or two.
  4. Spray the foliage and stems of your potato plants. It is important to wet all sections of the plant, including the undersides of leaves.
  5. Repeat the procedure every 7 to 10 days to prevent blight on your potatoes.

Avoid getting copper sulphate on your skin or inhaling the spray. Wash your hands thoroughly before eating or smoking to avoid accidental ingestion of copper sulphate.

Does copper sulphate harm bees and beneficial insects?

There is no evidence that copper sulphate poses a risk to bees and other pollinators. However, it is best to avoid spraying it directly on bees and other insects.  If your potato plants are in bloom, avoid spraying them in the morning when bees and pollinators are most active. Spray your potatoes late in the day when insect activity is done for the day.

Does copper sulphate harm birds?

Many pesticides pose a risk to birds and wildlife, but copper sulphate poses very little risk.

Is copper sulphate safe for kids and pets?

When applied properly copper sulphate does not pose a health risk for children and pets. However, it should never be applied when young children and pets are present as contact with the skin and mucus membranes can cause irritation.  Inhalation of the mist can cause breathing issues and irritation. Accidental ingestion can cause nausea and vomiting and may damage body tissues, cause shock, or even death if it is ingested in large amounts.

It is vital to store unused copper sulphate spray out of the reach of children and pets. Always label the container and treat it like you would any other pesticide.

Copper sulphate is a safe and effective way to safeguard your potato plants against blight, but it is not foolproof. To protect your potatoes from blight you should also practice good gardening techniques like avoiding overcrowding your plants to allow good air circulation, keeping your garden tools clean, and sterilizing clippers or other tools that may be contaminated.