Two promising alternative treatments for fighting powdery mildew

As the days grow shorter, the white flour-like mold called powdery mildew dreaded by nursery gardeners, horticulturists and gardeners reappears. To fight this very common mushroom, ASTREDHOR, the French plant professionals institute, undertook two research experiments reported on in 2021.

Read the full ASTREDHOR report

The sector needs to fight oidium

Powdery mildew starts as soon as the weather is wet with big temperature swings between day and night. The two most risky periods are mid-spring and early autumn. As soon as powdery mildew breaks out, unless treated it colonizes the whole plant, blocking photosynthesis. The plants become deformed, the leaves and flowers dry up and the fruit bursts.

This esthetic damage accompanied by a loss of earnings for the professionals is particularly tough as several fungicides have been withdrawn from the market in some countries and reduction of greenhouse heating increases the risks. This is why ASTREDHOR tested alternative solutions, enabling significant progress in powdery mildew treatment and reducing the need to use plant health products.

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Effectiveness of biocontrol products on pansies, dahlias and Indian lilac (Crepe Myrtle)

Although requiring more time spent in application than synthetic solutions due to their more frequent use and preventive objective, several products tested gave satisfactory results. Their specificity is to stimulate the defenses of pansies, dahlias and Indian lilacs to start a series of reactions. Although the experiments will run until 2023, Nicolas Guibert, research engineer at ASTREDHOR south-west and his partners have already noted the following facts:

• Armicarb® (potassiu
• SDPs were tested with a success rate of approximately 50%.

ASTREDHOR considers that some of these biocontrol products could be used as alternative solutions for treating powdery mildew, but we shall have to wait until the end of the study to define a framework to optimize implementation.

UV-C radiations to treat powdery mildew on rose bushes

Mainly used as a disinfectant, UV-C has a host of advantages for stimulating the defenses of the plants - preset doses, reduced need for PPE and no consumables…

This innovative use was closely examined in a study run from 2019 to 2021 by Jérôme Coutant, research engineer at ASTREDHOR Mediterranean, and his partners. Three varieties of rose bushes in greenhouses were irradiated with 800 J/sq.cm. of UV-C using a prototype. The results of the experiment were positive:

• average and severe symptoms of powdery mildew fell in GIARDINA by more than 60% with a reduction of 80% in the unmarketable stems;
• although less obvious there was significant reduction in the attacks of powdery mildew in INVITATION;
• the symptoms and intermediaries in the sanitary quality of the LONDON EYE harvest were variable.

This alternative makes it possible to significantly reduce the use of plant health products, without, however, eliminating them completely, while improving the marketable value of the rose bushes. Now we need to develop this technology for the sector as UV-C radiations are currently only available for vines.

Through these studies, the sector will be able to fight powdery mildew with a wider range of alternative solutions and further reduce use of the plant health products, already forbidden in public areas but still used in greenhouses.

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