Bordeaux mixture against mildew: why not use it in the vegetable garden?

If you've ever done a bit of gardening, you may have heard of Bordeaux mixture. This mixture of water, lime and copper sulphate is a fungicide used to prevent certain diseases that can infect plants in your garden. However, although its use is very common, it's better to know how to do without it when possible; find out why in the lines that follow!

Bordelaise sludge, a miracle but destructive solution

The bluish colour of Bordeaux mixture is quite distinctive. This substance is generally used to treat infections such as mildew, responsible for the appearance of brown spots on your tomatoes, potatoes or vines, for example. Left untreated, mildew often leads to partial or total failure of the crop... quite frustrating for us gardeners. That's why this product, readily available from garden centres, is so widely used in our vegetable gardens. It's radical and will often eliminate this fungal disease in just a few days.

A miraculous solution, perhaps, but numerous studies today demonstrate the harmful effects of Bordeaux mixture on both humans and the soil, despite the fact that it is tolerated in organic farming. This is because it is mainly made up of copper, or copper sulphate to be precise. This compound is said to cause a number of undesirable effects:

• Toxic to soils: copper destroys mildew, but it also destroys all the fungi that are beneficial to your soil, those that promote fertility and the development of humus, for example. Once applied, and even more so if it's recurrent, your soil will become impoverished and in permaculture, isn't it precisely growing on living soil that interests us most?
• Toxic to humans: some studies try to show that it is indeed toxic and this aspect is very often ignored or misunderstood by the amateur gardener. In fact, it is said to cause skin and eye irritation, and to prove particularly toxic if ingested.
• Unfavourable to the plant grown, the breathing orifices will tend to be blocked, resulting in the plant breathing less well to lead to a drop in quality and productivity.

Another problem: its dosage by amateur gardeners since most preparations are homemade. Before application, you should therefore check the instructions carefully. In general, 10 to 25g of product is applied per litre of water, depending on the vegetable to be treated. It's also possible to find products sold in shops already prepared.

How to prevent downy mildew "naturally"

To be truly effective, Bordeaux mixture should ideally be applied several times and recurrently. A large amount of rain tends to leach the copper and therefore render the product ineffective. To prevent mildew, this spray should be used after each spell of bad weather. There's no need for this chore, I assure you!

In fact, if you want a good crop, you don't have to use Bordeaux mixture - it's possible to grow good tomatoes without it. Good to know: prevention is better than cure, and here are a few solutions to prevent this disease from appearing on your plants:

• The paillage is a way of limiting the appearance of mildew, which very often appears after heavy rain. In fact, rain droplets that crash onto bare earth can project spores onto the plant. If the conditions for its development are right (damp weather and lack of sunlight to eliminate stagnant water, followed by heat), it will quickly colonise your unfortunate plants. Find out here how to make a good mulch easily!
• The planting of mustard in autumn on the bed where you will install solanaceous plants (tomatoes, aubergines, potatoes) the following year will "purify" your soil. This green manure is a natural fungicide. Note that mildew can also attack other crops such as onions, melons, spinach, leeks, lamb's lettuce, etc, but this is much less common.
• Spacing out your plants sufficiently, or scattering them around your vegetable garden is also a way of preventing this disease, which is likely to spread from plant to plant if you have planted too tightly. Also, avoid putting tomatoes near potatoes or aubergines.
• Favour rotation of crops by not planting the same families of vegetables in the same place every year.
• Avoid watering the foliage of plants in the Solanaceae family (and cucurbits too, for that matter). Prefer watering at the foot, as stagnant water on the foliage encourages the development of mildew.
• Pulverise a mixture of nettle manure as a preventive measure when weather conditions may be at risk (as said above, rain and lack of sun, then heat).
• Prefer growing in a greenhouse tomato or aubergine plants or under protective plastic. This will protect them from the rain and speed up their growth while protecting them from certain pests.

How to treat downy mildew naturally

If the damage has already been done, you'll have no choice but to treat your diseased plants, which you'll easily recognise from the brown spots on the leaves or stems. There are solutions other than Bordeaux mixture for this too:

• Cut off the parts affected by mildew and ESPECIALLY disinfect your secateurs before moving on to the next plant. To do this, you can run the blades under the flame of a lighter or brush them with pure alcohol between each cut on another plant.
• Some people also spray a infusion of tansy or oregano essential oil. Although permacultured, this solution has never worked in my home to treat mildew but feel free to tell me in the comments if it has had a better effect in your home. I prefer, for a natural treatment, to use baking soda at a rate of 5g per litre. I also add a littleblack soap to it so that it binds more easily to the leaves.
• As a last resort, use bouillie bordelaise sparingly, but don't forget to implement the prevention solutions described above, which is proving to be frighteningly effective. In my vegetable garden, which I've been growing since 2017, I've never had mildew before October (which turns out to be normal at this time of year given the weather conditions in northern France or Belgium).