An ailing plant is more susceptible to pests, just as we are more prone to disease when our immune system is weakened. There’s no need to resort to pesticides, however, as plenty of natural remedies effectively boost plant health and prevent infestations. “No matter what problem you have in the garden, you don’t need pesticides,” says Corinna Hölzel, an expert on pesticides with the German Federation for the Environment and Nature Conservation (BUND). She recommends to boost plant health using liquid manure, which you can make yourself from nettles or horsetail, for example. Put the shoots in …