Maize dwarf mosaic virus and maize chlorotic dwarf virus are the most common viruses associated with the complex in Kentucky, causing mosaic patterns of the foliage, stunting and red-purple discolorations of the
terminal leaves. Mycoplasma-like organisms may also be involved. The diseases are characterized by symptoms common to several other diseases. This has led to confusion in disease identification. Symptoms are helpful in the identification of the virus-like diseases; however, positive identification requires additional procedures conducted by virus-identification specialists. Maize dwarf mosaic (MDM) symptoms are distinctive on plants in the pretasseling stage. Affected young plants
have a fine stippling of dark green streaks on light-colored young leaves. Upper internodes are shortened. Older plants have yellowish leaves and are stunted; they produce excessive tillers and multiple-ear shoots with poor earfill. MDM is caused by a virus that overwinters in grasses. Several virus strains exist. Johnsongrass may be a major overwintering host for most strains; strain B does not overwinter in this host. At least 12 kinds of aphids transmit the virus from infected grasses to corn. Maize chlorotic dwarf (MCD) symptoms appear first as a yellowing in the whorl. Infected plants then become stunted, usually with reddish leaves. MCD is caused by a virus that overwinters in johnsongrass. The virus is spread by a leafhopper. Corn stunt is thought to be caused by a mycoplasma. Early symptoms are small circular to elongated yellowish spots at the base of leaves in young plants. These spots often coalesce and become elongated stripes that may be discrete or diffuse. As the plant develops, general yellowing, leaf reddening, ear shoot and sucker proliferation and relatively short internodes become distinctive features of the disease. Corn stunt is widely distributed in the southern areas of the United States. The mycoplasma is transmitted by at least five kinds of leafhoppers; it cannot be spread mechanically and is not seed-borne. Late plantings have greater risks than early plantings, especially those nearest older corn or johnsongrass. No highly resistant sweet corn cultivars have been identified in trials, in part because a mix of viruses are involved; however, some tolerate MDM better than others.
Cultural: Control Johnson grass within and adjacent to the field. Plant cultivars known to be more tolerant of MDM, such as Bi-Guard, Enforcer, Sundance and Silverette. Silver Queen and Merit have shown tolerance in
some trials. Send suspect samples to the extension for analysis.
Chemical: None available.