Rhubarb---Take out the worst looking clump, and dissect the roots, if their is a critter in there you'll find it,and if their was all you were doing was giving it a ride to a new location by transplanting it, if it is a critter a good dose of Seven should cure it, may have to repeat the dosage later on, if it isn't a critter I don't know, I'm not familliar with the plant in itself.
Potatoes---did they flower yet? I think your jumping the gun on them, take one plant and use it as yor standard for testing. CAREFULLY remove the soil from the plant on opposite sides of the row, and you can check the progress of the plant, replacing the soil CAREFULLY when you are finished........steg