Downy mildew is a well-known cryptogamic disease and is rightfully feared by gardeners.
Rain, then a certain coolness and you have the unpleasant surprise that suspicious spots appear on your tomato or potato plants…
To the point that it has become a fatality for many.
Is this your case?
Already know that preventive measures, natural and effective, exist (even if none are foolproof).
We will therefore see later how to prevent and even respond to mildew.
But for now let’s try to better understand this disease.
What is Mildew?
Mildew is thus a cryptogamic disease, meaning it is caused by a microscopic fungus (Phytophthora infestans for the strain that affects tomato or potato, Peronospora parasitica for crucifers or plasmopara viticola for the vine… to name only the most common forms here).
This disease first appears as brownish spots, with a light green outline and an oily appearance on the top of the leaves. A whitish felting can be seen on the underside.
These spots quickly turn black and the leaf dries out completely within a few days.
But powdery mildew is a disease that doesn’t just affect tomato leaves…
Indeed, brownish concentric spots may appear on the stems and quickly turn brown from the terminal bouquets, which curl up and die.
Finally, especially for the tomato, the fruits will rot…
If the plant is completely affected, it will completely dry out and die within a few days.
The conditions for the development of the fungus responsible for the disease
The fungus responsible for this cryptogamic disease overwinters in the soil and the germs are transmitted through the air at high humidity and moderate temperatures (the disease stops developing when temperatures exceed 26-28°C).
Protect from rain
The most effective method of protection is to protect the seedlings from the rain.
Whether it is a simple transparent canopy, a special tomato cover or simply a greenhouse (plastic or glass tarpaulin), the simple fact of growing under cover will greatly reduce the risks, provided the greenhouse is sufficiently ventilated, otherwise the high humidity results from poor ventilation, slowing down the drying of the plants can also cause cryptogamic diseases).
To guarantee production, I now grow about fifteen tomato plants in greenhouses… the rest is outside, depending on the vagaries of the weather.
Select more resistant varieties
Hybrids have been created for better resistance to downy mildew.
There are also grafted plants for this purpose these days (but unless you graft yourself, it’s not cheap)…
But if, like me, only the old varieties are to your liking, then know that certain varieties resist better.
At home I could see that Saint-Pierre tomatoes, Rose de Berne or Golden Jubelee were less easily affected than, for example, Cœur de Bœuf or Noire de Crimee. I also noticed that some of them (especially the Golden Jubelee) continued to produce healthy fruit without any problems despite the presence of the disease.
I want to point out that a breed can withstand better under certain circumstances, but not necessarily under others.
The best possible approach, therefore, is to test different varieties over the years (a practice common to all in natural gardening) and draw your own conclusions…
Wounds caused by pruning are entry points for cryptogamic diseases.
Not pruning will therefore limit the risks. However, it is then essential to place the plants further apart to allow sufficient ventilation.
Mildew treatment will not cure the disease.
However, the following treatments are useful preventively, before the appearance of the fungus, but also when the disease is present and to prevent it from spreading to the entire plant.
Bordeaux mixture or other copper based products
Copper products have long been commonly used to prevent this disease.
I would make 3 comments:
- the most recent fungal strains seem more virulent and the Bordeaux mixture is no longer always effective (hence the marketing of increasingly toxic chemical treatment products);
- copper is phytotoxic and especially harms flowering (read the recommendations for use on the packaging of Bordeaux mixture);
- copper builds up in the soil with significant consequences for the life of the latter and in particular for the populations of earthworms.
For these reasons I have been refusing to use copper in the garden for a few years now.
Baking soda is an interesting alternative to the Bordeaux mixture to protect your crops from cryptogamic diseases:
- Pour 1 teaspoon of baking soda into 1 liter of water.
- Add 1 teaspoon of black soap (or Marseille).
- Spray the mixture on the entire plant immediately after planting.
- Repeat the operation once a week for two months and after each rain shower.
However, keep in mind that, like the Bordeaux mixture, baking soda is harmful to flowering. It is therefore advisable to treat outdoor blooms or not to pay attention to the flowers in full bloom during the flowering period (treat below the last bloom).
Repeated horsetail manure treatments (see here) have some effectiveness in preventing the disease…
Comfrey manure and nettle manure
Likewise, watering with comfrey fertilizer, or even nettle fertilizer, will strengthen the natural defenses of plants and thus make them more resistant to disease, and in particular to the one that interests us here.
Eliminate Corrupted Parts
Remove the foliage and branches affected by the disease (if not the whole plant, of course…). This prevents immediate spread.
You can compost the diseased leaves. The rise in temperature of the latter will destroy the germs of the fungus. Some believe that such a compost would even have protective properties…
Semi-skimmed or whole milk (but unpleasant odor with the latter) can also be used in anti-fungal treatments.
It is more commonly used against powdery mildew, but some attest to its effectiveness, including on the cryptogamic disease seen here.
Mix 1 part milk with 9 parts water and spray quickly all over the plant.
This treatment would also have a certain effectiveness in curative…
Garlic has antifungal properties.
It can therefore be used in the form of a decoction to protect our crops from cryptogamic diseases.
View the garlic decoction recipe here.
Do you not grow in the same place?
If the disease has plagued you, it is best not to grow tomatoes in the same place for at least 5 years… at least that is what is usually recommended…
But you should know that the fungus responsible for this cryptogamic disease does not survive the frost.
And unless you live in an area where it never freezes, you can grow tomatoes in the same spot again.
The winegrowers (well, those who do not systematically and abundantly treat preventively with Bordeaux mixture, or worse…) know this very well: it is not because the disease has been serious for a year that it will be the case the following year … However, the vines remain in the same place. It is therefore the climatic conditions of late spring and early summer that will cause the appearance of this cryptogamic disease, not the presence of germs of the fungus in the soil.
In the same vein, it is generally recommended not to put the remains of plants (or fruits) affected by mildew in the compost… But a sharp rise in temperature (this is the case in a well-managed compost…) also gets the upper hand from the fungal germs…
On the basis of this principle, do not hesitate to apply surface composting to the beds of crops affected by the disease … This should destroy the germs of the fungus.
As a result, my potatoes were severely affected at the end of June (2016)…
My first reaction (after the destruction) was therefore to remove the affected parts… and in this case I mowed everything down.
Then I immediately harvested the most affected plants (before the disease spread to the tubers), with a nice surprise: a very nice harvest! It must be said that the plants were very well developed, with the disadvantage that the skin is still very thin… these potatoes will not keep for long.
So I risk leaving some of it in the ground (the less affected plants) but after I cut the foliage as well…then I’m going to do some horsetail treatments on the ground… hoping the potatoes don’t get contaminated touch.
Keep in mind that if the tomato plants are not completely affected and the weather conditions allow it, it is possible to stop the spread of this cryptogamic disease.
In this case, the temperatures at home quickly rose above 30°C (which, as we saw above, halts the development of the disease).
After removing the affected parts (leaves and a few branches*), I quickly performed a treatment with baking soda and then a combined treatment of nettle manure/comfrey. I repeat these treatments every 5 days or so…
*If I don’t prune my tomato plants, I can find healthy offshoots on most plants…
We’ll see what happens…
And at your house? Everything is fine…or not?
has this disease affected your tomato or potato crops?
In addition to this article, I strongly recommend that you read “Tomatoes Without Diseases”, presented by Nicolas Larzillière, to put your chances of the success of your tomato crops on your side.