The disease caused by Maize rough dwarf fijivirus (MRDV) is one of the most important diseases in corn production in North China.Severe dwarfing,distortion of the plants bearing the small malformed ears with few or no kernels,and enations on the underside of the leaves are the typical symptoms of the disease.Utilization of the disease-resistant cultivars is the most effective and economical way to control the disease.Yet there is no practical criterion reported for screening and evaluating the disease resistance in corn breeding and production.Studies were conducted from 2001 to 2004 with 93 inbred lines and 9 hybrids under the nature infection in the field.To examine the disease severity of the plants practically,a grading standard was set with a scale of 0 to 4:0 for no symptoms;1 for four fifth of healthy plant height;2 for one half of healthy plant height;3 for one third of healthy plant height and 4 for equal and less than one fourth of healthy plant height or dying back at early infection stage.A criterion for evaluating the disease resistance was established based on the disease index,in which disease index ranging 0-3.0 standed for higher resistant;3.1-10.0 for resistant;10.1-20.0 for medium resistant;20.1-40.0 for susceptible,40.1 and above for higher susceptible.An artificial inoculation test with vectors (Laodelphax striatellus Fallen) in a screenhouse by applying this criterion confirmed that this criterion was reliable and practical to screen the resistant lines and hybrids in corn breeding and production.
