Serotyping and strain identification of maize streak virus isolates

Four strains of maize streak virus, namely the Panicum maximum, Digitaria setigera and sugarcane strains, have been identified from 19 isolates by ELISA using polyclonal antisera cross-absorbed with particles of the maize strain. The results suggest there is an epitope of the maize strain which is not dependent on the capsid being intact and which is common to all the members of the group; other strain-specific epitopes are probably conformation-dependent. A specific epitope (probably internal) occurs on the coat protein of a maize strain isolate, D(R)D, grown in D. velutina, that is also present on the coat protein of the D. setigera (previously reported as D. sanguinalis) strain. Specific internal epitopes also occur in the coat proteins of sugarcane and P. maximum strains. The use of indirect ELISA was necessary for accurate serotyping. The serological reactivities of particles of all the members of each type were identical irrespective of the host from which they were extracted. Sap extracts proved to be more reliable sources of antigen than virus preparations, which could vary in their serological reactivity. Nevertheless, serological differentiation index values determined in tests using either type of antigen proved reliable and consistent.