Plant diseases in the forest garden
There is no set recipe for how to avoid diseases on plants, it is particularly challenging when you grow susceptible species such as apples, pears and plums. There are, of course, some rules of thumb.
Resistant varieties
Fruit and berry developers do not only have large and richly producing crops in mind, several varieties have been developed to avoid susceptible diseases. In some cases, in certain areas it may be sufficient to have varieties that produce extra early or late, thus avoiding climatic conditions for the presence of a given pathogen.
Limited nitrogen fertilization
A lot of nitrogen leads to a lot of growth and vulnerable shoots that can be more easily penetrated by insects that create entrance wounds for diseases.
Raised beds and mineralization of the soil
More pathogens gain a foothold in waterlogged soil. Composting and the supply of organic material that can build up the humus levels is then necessary. While you wait for the soil organisms to magically bring the soil to life, raised beds can be a good solution. Living soil also means mineral-rich soil. If the soil runs out of an element, it often presents problems.
Avoid agents that kill fungi
Fungicides almost always kill the fungi that work for the plants, especially endophytic fungi that are very important to the plant's defense system against pathogenic attacks. Most agents that aim to remove specific pathogens end up with an ecosystem out of balance that can take a long time to correct.
Fresh plant material
You have a very bad starting point if you start with plants that already carry serious diseases even before you have them in the soil. Many plant imports unfortunately carry pathogenic organisms every year. Avoid contributing to exactly that. If you as a private person import the wrong plant, you may risk making many farmers unemployed in the future. Avoid planting out shrubs and trees with root wounds that are susceptible to honey fungus. The fungus is almost ubiquitous and creates a lot of trouble in material with root wounds.
Pruning and removal of infected material
Many fungal diseases do not gain a foothold if there is enough air flow. In that case, it may be sufficient to thin out the branches and foliage to provide better ventilation, and possibly remove and prune infected material if it is not too widespread. Feel free to also prune wounded branches with clean cuts. Clean cuts make it harder for insects and pathogens to penetrate the plant. Avoid pruning during periods that are unfavorable for the species.
Remove problematic species
If you have problems with, for example, rust, this fungus almost always changes growth with another species. Removing this species will then remove the lifeline for the fungus.