There are actually a few reasons. Garlic and onions are both susceptible to diseases like Botrytis, pink root, powdery mildew, purple blotch, and, in particular, Fusarium basal rot. Planting them ...

Fertilising Spring onions should be top-dressed four weeks after transplanting. CAN fertiliser should be applied as advised. Pests and diseases Diseases include black mould, botrytis leaf blight ...