Watch out for Oidium!

What is Oidium?

It is the common name given to fungal diseases of plants caused by a group of fungi in the family of Erysiphaceae whose symptoms are very often a white down and deformations of foliage or fruits.

Why can dry weather favor contamination?

Unlike other fungi that need high air humidity or even, like mildew, water in liquid form and high heat, Oidiums prefer fairly low temperatures and above all a hygrometry of 70 to 80 %. We can even say that the rain hinders the germination of the spores.

An enemy sometimes not very serious but which can be formidable:

On a flat maple for example or on a Japanese euonymus, we will generally only see the white down on the foliage, but on bunches of grapes this parasite will lead to the bursting and rotting of the grains in the worst case. , at best it will be poor taste quality. For other plans, growth will be completely blocked.

Beware of a common mistake

In some documents there is sometimes a description of a perforating powdery mildew of the cherry laurel, the hedge laurel. It is a mistake. Indeed there is confusion between powdery mildew, Podosphaera pannosa, which whitens and deforms the leaves and the perforation of the leaves which is due to another fungus coryneum, Coryneum beijerinckii.

How to fight:

When possible, cultural methods can greatly help to reduce contamination.
Avoid manure that is too rich in nitrogen or excessive irrigation of crops because too rapid leaf growth makes them more susceptible to this type of parasite.
You can also help the plant to defend itself by spraying in dry weather with natural defense stimulators such as Bacillus subtilis (strain QST 713, the only one authorized for sale to amateurs).

In arable farming, farmers have access to synthetic products or biocontrol products.

For amateurs, this is limited to sulfur and potassium bicarbonate (potassium hydrogen carbonate) or sodium.

· Sulfur is only effective in prevention because it blocks the germination of spores.
It can be used as a spray in the form of wettable sulfur or as a powder (in a closed site). In dusting, sulfur acts by sublimation, that is to say that it passes directly from the solid state (powder) to the gaseous state. This is very interesting because it allows it to penetrate through the stomata of the leaves.
Sulfur also has a repellent effect for mites, parasites and useful ones. In Integrated Biological Protection, especially in greenhouses, this limits its use.
PLEASE NOTE Sulfur in liquid spray must not be used above 25° C because it becomes phytotoxic. On the other hand in dusting there is not this problem on condition of the spreading with the freshness.

Sodium bicarbonate, food grade is a “basic substance” that is to say that it does not need MA while potassium bicarbonate, more effective, requires approval like all products phytosanitary.

· Different manures or plant extracts are reputed to be useful against powdery mildew, but there is a lack of scientific proof of their real effectiveness.

· In autumn, spraying with Bordeaux mixture can reduce the inoculum.

Ask all your questions during the gardening program on Sunday morning on France bleu Isère from 9 a.m. to 10 a.m. by calling 04 76 46 45 45.