@KoiwiGal - Getting the right variety is only the start of it. It's quite easy to keep any plant disease free as long as you do the prep for it, and know what you're doing.
For example, you can't continue to plant the same variety of plant in the same place multiple years in a row. No matter how well your tomatoes did in that bed last year, they probably put some kind of pathogen into the soil. That will only get worse if you don't swap them with something radically different, like a bean.
Other kinds of plants, like strawberries, are only meant to be used for a couple of years. Strawberries are all very inbred and prone to disease. Once they've fruited once or twice the berries are going to get smaller and less abundant because the diseases will increase until the plant is basically useless.
It's annoying, but that's the way it goes.