After mildew, how to sanitize the soil?

In 2021, all gardeners (or almost) have complained about the damage caused by mildew in their vegetable garden, and more particularly, on their tomatoes whose mediocre harvests will undoubtedly remain in the annals. The main culprit of this devastation is the rain with the consequences in terms of humidity that it induces: what to do when the weather seems to be relentless and the barometer seems to be stuck on precipitations? The means are limited and to protect oneself from mildew is above all to act in a preventive way, but at the end of the season, you legitimately wonder about the way to eradicate mildew from the soil.

Pulling out the plants contaminated by mildew

Generally early in the season, in August, you will have to pull out the tomato plants as the leaves are dried out, black and damaged, although the tomatoes are not all mature yet but already victims of the pathogenic fungus. You may hear or read the advice not to put this vegetable waste in the compost, because of the risk of contaminating it with the proliferation of the spores, recommending to burn the diseased plants to get rid of them. Know that the compost heats up enough (60-70°C) to destroy and eliminate the mildew spores it contains.
Before storing the tomato stakes until next year, disinfect them carefully, as well as any tools you may have used (pruning shears…).

Sanitizing soil contaminated by mildew

Once the bare ground has been cleared of tomato plants affected by mildew, if you were thinking of using a thermal weeder or treating the soil with a fungicide to eliminate the spores of the fungus, forget these bad ideas: in the first case, the heat released by the tool will not be long enough to destroy the spores, and in the second case, a fungicidal product will have too broad an effect, killing the mycorrhizae in the soil, without which humus does not exist.

The prophylaxis of the soil can be done by strong winter frosts if the soil is not mulched: the mildew spores, on the surface, will not resist it.

Then, green manures such as phacelia or mustard will present a double interest: on the one hand, these cultures will avoid leaving the soil bare, and on the other hand, they will participate in cleaning and sanitizing the soil.

Soil preparation for new plantings

Even with diligent efforts to remove any remaining blight, it is still strongly recommended not to replant tomatoes on areas affected by the disease the previous year. Do not plant potatoes, eggplants, peppers or grapevines, all of which are also susceptible to late blight. Respect a crop rotation of at least 3 years and, in the meantime, plant salads, zucchinis, bush beans, etc.

If your garden is too small to allow you to rotate your crops, diversify your tomato varieties to prevent them from being affected as much as possible, and choose varieties known for their better resistance to mildew (Rose de Berne, Merveille des marchés, Côtelée de Provence, Beefsteack, for the old tomato varieties or opt for “F1” hybrids).

Place a thick mulch (10cm) at the foot of the tomatoes which will prevent the spores from developing and emerging on the surface of the soil.

During the growth of the tomatoes, do not hesitate to water them with nettle purin, comfrey purin or a decoction of horsetail, all “potions” which will contribute to strengthen the plants and reinforce their natural defenses, in particular against mildew. Be careful to always water well at the foot without wetting the foliage, avoiding to do it in the evening to avoid that the humidity settles the whole night.

If possible, avoid pruning the tomato plants or remove the suckers as soon as they appear, when the sun is shining: this way, the scar will dry quickly and will not be an easy entry point for mildew.

If, despite all these precautions, mildew appears the following year, intervene as soon as the first signs appear with sprays on and under the leaves, with Bordeaux mixture – with all the reserves that it is advisable to put – or with bicarbonate of soda (1 tablespoon/liter of water) or even with essential oils effective against mildew. The solution of covering the plants to protect them from rain showers can finally save a minimum of your harvest…

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