Mutations are the results of causes, not the causes of results. ~ Grantly Dick Read
The word ‘mutations’ in the quote above could appropriately be replaced with ‘disease’ or ‘pests’ or ‘pathogens’.
Any organism we assign one of these labels produces infections because of other causes, not only because the organism was present.
Other causes are often environmental in nature. Differences in the presence of beneficial microbial populations and soil nutrient levels determine whether a plant develops an infection more reliably than only considering the presence of an organism.