indexing, method of testing vines for virus diseases and phytoplasma diseases and their like. Sap from the plant to be tested is exposed to so-called indicator plants which show typical symptoms if the disease is present. Sap from the two plants can be mixed by abrading leaves or by grafting. Cabernet Franc, for example, is used as an indicator vine variety to test for leafroll virus. Indexing tests are now being replaced by serological methods; see elisa.
