Crown rot

Suspected Crown Rot with my Rheum Palmatum

Question from Stephanie Hill

Hi,
leaves on my Rheum Palmatum are turning yellow and Redon patches. new leaves are yellowed and growth has slowed overall. should I lift it, add grit to the soil & replant or it it too late? Also unidentified insect (see photo) has appeared on underside of leaves. I have picked these off - are they bad for the plant?
Thanks