AN ASSESSMENT OF METHODS FOR QUANTIFICATION OF GENETIC RESISTANCE TO CORN LETHAL NECROSIS (CLN) IN MAIZE (ZEA MAYS L.)

CLN is a severe disease of maize (Zea mays L.) localized in southcentral Nebraska and northcentral Kansas, and caused by the interaction of maize chlorotic mottle virus (MCMV) with maize dwarf mosaic virus (MDMV) or wheat streak mosaic virus (WSMV). In a greenhouse study seven inbred lines were evaluated as seedlings for their resistance to CLN using four different measurements: a sympton-severity rating, decrease in dry weight, decrease in dry weight as a per cent of control weight, and a disease index based upon rapidity of symptom development. The methods were assessed for their ability to differentiate among the lines by use of the Friedman\u27s statistical test. The per cent reduction in dry weight and the sympton severity rating were the most effective in differentiating the virus resistance on the lines. A procedure was developed to measure the concentration of MCMV and MDMV-B in CLN-infected leaves to quantify host plant resistance. Viral concentration was determined by measuring the densities of coat protein bands after SDS gel electrophoresis of high-speed pellets, which include both aggregated and unaggregated virus. This procedure should be applicable to many other viruses with the exception of those that are insoluable in, or destroyed by, Triton X-100. With four inbred corn lines, good agreement was seen between field symptom severity and the concentration of both viruses nine days post-inoculation. The concentration of MDMV-B differed significantly (P = .01) between the lines, whereas the amount of MCMV from line to line was more uniform (P = .05). Measurements over a 20-day post-inoculation period revealed that the concentration of MDMV-B peaked at ten days after inoculation whereas MCMV did not reach a peak in two of three genotypes within this time period