By James Chatfield, Joseph Boggs, and Erik Draper
Plant pathogens—such as certain fungi, bacteria, phytoplasmas, nematodes, and viruses—by definition are parasites, infecting living plant cells. However, there is quite a range from obligate parasites (which only survive on living cells) to opportunistic pathogens (which mostly live on already dead cells as saprophytes, but also make a living as parasites on dying or stressed plant cells).
A good example of a pathogen thriving both as saprophyte and parasite is the Botrytis grey mould fungus, which infects many plants from herbaceous geraniums to woody plants in propagation situations. Imagine a world where a perfectly healthy geranium plant has a part that is dying, such as flower blossoms that senesce and die with age. As this living blossom tissue dies, Botrytis colonizes it by feeding on these dead and dying cells. The pathogen rapidly multiplies, especially in cool and moist conditions, and then those infested geranium florets fall on healthy leaf tissue in great numbers with moisture, blocking the sun where they fall. Those healthy leaves then develop Botrytis blight.
Read the full article: Managing Difficult Plant Diseases