Scale insects are widespread in our region and ecosystem, even if we tend not to notice them. Some scale species are native in our area and others are not. You can learn about scale life cycles in our Introduction to Scale Insects web page, but essentially they are largely flightless insects that are tiny mite-sized organisms as juveniles, so they hitch a ride from plant to plant on wildlife (bird feet, squirrel fur, etc.) and wind. They do not infest soil. Scale can crawl, but only limited distances since they're so small, and likely don't move far (like from one plant to another, unless they are touching) on their own. Once they "settle" on a host plant, they either barely move or never move again, relying on a shell-like body covering to protect them from the elements and some predators and pesticides.

There are entire textbook-sized references published about plant pests and diseases, and even then, they are not exhaustive since some conditions afflicting plants are being diagnosed or new information gathered about them all the time. It's also difficult to publish resources for diagnosis since many different causes of plant damage can overlap in appearance, creating similar symptoms that can be hard to confirm without laboratory testing. For some common home garden plants (often veggies like tomatoes or flowering plants like dogwoods), we do have resources providing an overview of typical pests and diseases.

Insects travel about either with flight or crawling or by traveling on wind or other animals. Plant disease spores tend to be wind-blown, rain-splashed from the soil surface, or present in the soil itself. They are fairly ubiquitous, but most only threaten plant health when the plant becomes injured, stressed, or is being grown in a location not suitable for its needs (say, too wet). Many microbes are also beneficial, not unlike the gut microbiome that we humans have, helping us to extract nutrients from our food; beneficial fungi and bacteria living in or around plant roots help them absorb moisture and nutrients plus defend against pathogens. This is one reason why we seldom recommend the use of fungicides to treat plant diseases -- not only would applying them after symptoms appear usually not fix the problem (and they won't cure a disease), but they might have unintended impacts on those beneficial microbes (or even pollinators) that you don't want to kill.

Miri

You're welcome.