Maize rough dwarf - Maize rough dwarf fijivirus

Maize rough dwarfvirus (MRDV) was discovered in Europe when corn hybrids were introduced from the United States after the second world war. In 1949, in Italy, a severe outbreak of a disease of unknown nature threatened the maize cultivation lowering the yield by some 40%. Originally named "nanismo ruvido del mais" it was later renamed "Maize rough dwarf virus". MRDV and similar or related diseases have since been reported in several other European countries (France, the former Czechoslovakia, Israel, Spain, Switzerland). MRDV virions are isometric with double capsid and double-stranded RNA. The diameter of viral particles is 65 nm to 75 nm according to the contrast staining used. The MRDV genome is composed of 10 RNA segments named S 1 - 10. Only the size of the segments S6, S7, S8 and S 10 has been determined. MRDV is closely related to Rice black streaked dwarf fijivirus (RBSDV) and Mal de Rio Cuarto fijivirus and more distantly related to Pungola stunt fijivirus. MRDV and RBSDV are members of the genus Fijivirus group 2. MRDV has a fairly wide host range Poaceae, among which maize is the only one of economic importance. It was found naturally infecting oats, wheats, several grasses and its vector, the planthopper Laodelphax striatellus (Fallén). The virus can be detected in host plants and vector by serological methods such as DAS-ELISA using strongly antigenic antisera prepared against SVPs. In vitro transcripts can also be used to detect MRDV-RNA in plants and vectors. L. striatellus is the only known winter host of MRDV since no perennial natural host plants have been detected. In 1991, Conti and Milne, however, reported oats as the first overwintering plant in the conditions of Northern Italy. The hoppers overwintering as nymphs in diapause have long been known to cause serious epidemia of MRDV on spring sown maize. Early sowing is an important factor determining increasing MRDV incidences. The relative cool weather in April is favourable to MRDV multiplication in plants. The incidence of MRDV in maize varied cyclically during the years. There are no true resistant maize varieties known, so the control of MRDV epidemics is based on vector control.