Zoöp Observations: Hawthorn MildewZoöp Observations: Hawthorn Mildew

A guiding principle of the Nieuwe Instituut is that non-human interests are taken into account as much as possible. It applies to the interior space of the Nieuwe Instituut, as well as the outdoors, mainly The New Garden. The principle often results in compromises between what is interesting, fun or attractive for human visitors of the garden, and what would be beneficial for the plants, insects and other invertebrates living there, and the birds and small mammals that visit and forage in the garden. As a gardener, I unfortunately have to negotiate conflicting interests between various non-human species as well. Intervening in the growth of certain plants by weeding, pruning and mowing is often inevitable. The dimensions of the New Garden are finite after all, and at least some degree of biodiversity is desirable; plants that become too dominant have to be restrained. On principle, however, I intervene only when this is (absolutely) necessary. The same applies to another traditional aspect of gardening: combating pests and plant diseases.

The hawthorn hedge (Crataegus monogyna) that was planted along the Jongkindstraat in the spring, suffers severely from powdery mildew, a fungal disease that occurs in many different plants. Its first symptoms are powdery white spots on the leaves. As the disease progresses, these white spots cover the leaves completely – hence why powdery mildew is also called white disease.

There are hundreds of species of powdery mildew fungi. Most have a host range restricted to a relatively few related plant species. Hawthorn is usually beset with a species called hawthorn mildew (Podosphaera clandestina).

There are several fungicides for powdery mildew on the market. Chemical fungicides used in large-scale agriculture, such as myclobutanil and propiconazole, should not be used in urban areas, for good reason. The use of biological fungicides based on sulfur is equally unfeasible, as these sprays have a preventive rather than a curative effect. The thin layer of sulfur on the leaves ensures that mildew spores cannot germinate. The same applies to a popular home remedy against mildew, namely spraying leaves with a mixture of milk (40%) and water (60%).

Pruning infected hawthorn branches during the growing season is futile as well. It has been common knowledge since at least the middle of the last century that the buds that develope after pruning might become infected and serve as a place for mildew spores to hibernate, to almost certainly plague the hawthorn hedge again next year.

Weirdly enough, the best course of action is to do nothing at all. Hawthorn is a robust shrub that can not only survive serious drought and insect damage, but usually also mildew attacks.