Evidence for Infectivity of Maize Chlorotic Dwarf Virus and for a Helper Component in Its Leafhopper Transmission

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Abstract
The infectivity of maize chlorotic dwarf virus (MCDV) was demonstrated indirectly by in vivo neutralization of infectivity in its leafhopper vector, Graminella nigrifrons, and directly by leafhopper transmission of purified virus acquired through parafilm membranes. In neutralization of infectivity tests, transmission of MCDV by viruliferous leafhoppers was reduced (more than eightfold) if they were allowed to feed through parafilm membranes on a suspension containing MCDV antiserum or its IgG fraction compared to control leafhoppers fed on preimmune serum. Leafhoppers given access to membranes containing suspensions of crude, clarified, or concentrated extracts of MCDV or purified virus did not transmit virus to healthy maize plants. However, transmission of a purifed isolate (MCDV-white stripe) acquired by membrane feeding occurred if leafhoppers had previously fed on plants infected with another isolate (MCDV-mild), but not if they had first fed on healthy plants or plants infected with maize dwarf mosaic virus. Reversing the feeding sequence, when leafhoppers first fed on membranes containing MCDV-white stripe and then on plants infected with MCDV-mild, they did not transmit MCDV-white stripe. These experiments strongly suggest that a helper component produced in MCDV-infected plants is necessary for the transmission of MCDV by its leafhopper vectors. Use of the term "transitory" by some authors to describe the semipersistent transmission of MCDV and the related rice tungro viruses is discussed.