Downy mildew is a cryptogamic disease caused by a pseudo-fungus of the Phytiaceae family:
Phytophthora infestans
. It is the main disease of potatoes, which can cause irreversible damage to crops. It is also responsible for the great famine which affected Northern Europe in the middle of the 19th century. It is therefore a formidable disease, which must be fought quickly so as not to lose all of its crops.
How to recognize potato late blight?
Late blight is a disease that mainly affects nightshades: potatoes, tomatoes, eggplants, etc. It is mainly detected by the
brown, oily spots
that form on the leaves, which eventually dry out. In a humid environment, a whitish down characteristic of
Phytophthora infestans
may
appear over the leaf spots.
These spots then spread to the petioles and stems of the plant, before reaching the tubers. Progression can be rapid and
reach all crops
in just a few days. This is why it is recommended to never plant Solanaceae next to each other.
On the potatoes themselves, late blight manifests itself not only by the appearance of
brown spots on the surface,
but also deep down. They have a dry, marbled appearance when a tuber is cut in half.
How does potato late blight develop?
As a fungal disease, downy mildew requires two conditions to thrive: high humidity and moderate temperatures Copyright (c) 2024 La Huertina De Toni/Shutterstock. No use without permission.
As a fungal disease, downy mildew requires two conditions to grow:
high humidity and moderate temperatures
. The first signs of infestation of this disease generally appear after heavy rains, when the humidity exceeds 90%, followed by temperatures which oscillate between 10 and 25°C.
Thunderstorms, which are more common
from March to June
in mainland France, provide ideal conditions for the development of downy mildew. It is therefore in the middle of spring, where the nights are still cool and the days are moderately warm, that the disease is most likely to develop. Its progression stops as soon as temperatures exceed 25°C for several hours per day.
Note that the disease can spread
thanks to the wind
, which transports the zoospores (reproductive cells) of
Phytophthora infestans
, but also directly via the earth contaminated by its oospores (fertilized eggs), and this from one year to the next. other.
How to fight potato late blight?
Potato late blight is a formidable disease that spreads quickly and devastatingly. The best way to fight this disease is therefore
prevention
, by adopting simple actions to limit humidity:
Water in the morning, avoiding spraying the leaves as much as possible, so that the water has time to evaporate during the day;
Space the plants
far enough apart to allow air to circulate;
Avoid planting other nightshades near your potatoes;
Weed
regularly and remove plant debris to prevent mulch from forming;
Spray a horsetail decoction or a solution of baking soda and black soap
after periods of rain;
Respect
crop rotation
to avoid contamination by oospores at the start of cultivation;
If in doubt, do not replant your potatoes the following year: buy new certified tubers from a garden center, targeting
resistant varieties
: Amandine, Vitabella, Désirée, etc.
To prevent downy mildew, you must spray an effective fungicide on the plants which do not yet appear affected to save your harvest: the famous Bordeaux mixture. Copyright (c) 2023 HenadziPechan/Shutterstock. No use without permission.
If you suspect that your potato plants are being attacked by late blight, you must act quickly to prevent the spread of the disease:
Remove stained leaves and stems;
Pull out plants that seem too affected;
Destroy the removed parts: do not add them to your compost.
Once this work is done, you will need to spray an effective fungicide on the plants which do not yet appear affected to save your harvest: the famous Bordeaux mixture.
How to treat potatoes with Bordeaux mixture?
Bordeaux mixture has been
used for more than a century
against vine downy mildew: it is therefore a treatment with proven effectiveness against this disease. The copper it contains has the ability to block the production of
Phytophthora infestans
spores .
Sold in the form of a blue powder, the Bordeaux mixture must be diluted and sprayed on the affected plants. Against potato late blight, it is recommended to use
25 g of powder per liter of preparation
, to be sprayed every 15 days or after heavy rain.
If Bordeaux mixture is compatible with organic farming, it should not be abused either. The copper it contains can indeed have a harmful effect on the quality of the soil. We consider that
a dose of 4 g per year
should not be exceeded for 10 m² of vegetable garden
. Use a fine nozzle to control the dosage and distribution of the product.
Are potatoes with late blight edible?
It is possible to eat potatoes affected by late blight if they were harvested before being badly affected. Copyright (c) 2023 Tanita_St/Shutterstock. No use without permission.
It is possible to eat potatoes affected by late blight if they were harvested before being badly affected. However, they will not keep well, especially since the disease opens the door to other fungi and bacteria! Potatoes that appear healthy at the time of harvest can then rot in the days that follow. Don't store your infected potatoes next to other healthy tubers and
eat them quickly
.