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My quince has a lot of brown marks on the leaves, what can I do to prevent this?
Without seeing the actual damage, based on your question I would suspect that the quince leaves are infested with Diplocarpon mespili, also known as Diplocarpon maculatum. This leaf fungus is especially common in wet weather or wet microclimates (e.g. dense tree population or locations with very few thinned-out areas and a lack of ventilation) at the height of summer and in the second half of the year.
As well as aerating the location and plant (cutting/thinning), you can also use tackle the fungus with a fungicide, such as "Atempo Fungus-Free copper" from Neudorff. For stubborn infestations, alternating applications of two compounds with different active substances is recommended. Combating Diplocarpon mespili is similar to the method for dealing with Diplocarpon rosae in roses. The same precautionary measures apply, such as removing the infected leaves to curb the spread of fungal spores.