My rhodies have black spot, rust. Is there a plant medicine I can put in the soil so it will get absorbed by the entire plant rather than spraying every other leaf.
I am sorry to hear about your sick Rhododendrons. You should take a take a leaf sample into a Master Gardener clinic for (free) diagnosis. I have linked a list of clinics in Snohomish County below. Their volunteers are trained in identifying plant diseases and suggesting solutions.
If you cannot get into a clinic try the HortSense webpage from Washington State University Cooperative Extension, and search the common ailments affecting rhododendrons (in the Ornamental Shrubs section).
The reason why it is vital to get an accurate diagnosis is because some fungal diseases do not have treatments that really work, such as rust, while other “leaf spot problems” are not caused by fungus at all, therefore spraying with fungicides or applying a systemic to the soil would only be a waste of time and money!
Try contacting the Snohomish County Master Gardener Clinics to see if you can bring in samples.