Answer: I think a soil test is certainly in order. There are private labs which will test your soil for you or you can contact your local Cooperative Extension office for a soil test kit. The University of California will do a complete soil test for a nominal fee. While it's true that diseases can be spread through the use of contaminated pruning shears, it isn't likely that a pathogen would travel from one plant to another (most pathogens are host-specific) and wipe out your entire yard. If you see no symptoms of disease but your plants die suddenly, there may be some root rot problems - perhaps from poorly draining soils. I'd have a soil test done first and talk to some local Master Gardeners. They can guide you through the process and maybe even diagnose the problem before you lose more plants. Best wishes with your landscape!
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