How do plant diseases spread?
Through a small number of routes, and knowing which one a given disease uses tells you exactly which intervention will work — most garden disease advice fails because it addresses the wrong route.
The main pathogens: fungi, by far the largest group in gardens; oomycetes, which look fungal and are not, including downy mildew and potato blight; bacteria; viruses; and nematodes.
How they travel:
Wind-blown spores, over metres or hundreds of kilometres — rusts, mildews and blight. Little can be done to exclude these; resistance and timing are the defences.
Rain splash, moving spores from soil onto lower leaves. This is why mulching and watering at the base rather than overhead matters so much — it breaks the splash route directly.
Soil, where many pathogens persist for years as resting structures. Clubroot and white rot survive decades, which is why rotation cannot fix them and why contaminated soil must not be moved.
Water, including recirculated irrigation and water butts, which carry oomycete spores that actively swim.
Insect vectors. Most plant viruses spread this way — aphids especially — which means controlling the insect is the only way to control the virus, since there is no treatment for an infected plant.
Tools, hands and boots, which move soil-borne and viral pathogens. Tobacco mosaic virus is exceptionally persistent and transfers on handling.
Infected plant material, including bought plants — the commonest way a new disease arrives in a garden, and the route behind several major landscape-scale outbreaks.
What follows practically: buy from reputable sources and inspect new plants; isolate new arrivals for a few weeks where practical; water at the base early in the day so foliage dries; space plants for airflow; clean tools between plants when pruning anything suspect; remove and destroy infected material rather than composting it; rotate crops for soil-borne problems with short persistence; and choose resistant varieties, which is the highest-return decision available.
Do not rely on fungicides — most garden products are protectant, so they must be applied before infection, which is why they so often appear not to work.