Maize Dwarf Mosaic Disease

​Maize Dwarf Mosaic Virus (MDMV) is classified as closely related to sugarcane mosaic virus.  It is vectored by aphid and infects some grasses other than corn such as sorghum and Johnson grass (sorghum halepense).  Like other viruses it is constructed with a protein cover surrounding a nucleic acid, in this case being a RNA particle.   It codes for 5 or 6 proteins including the protein cover. At least 5 genetic strains of the virus have been identified.
 
Severe symptoms in the field is linked to vectors and other host species being present while the corn plants are young (less than V4 stage), although older leaves can be infected.  The virus is less likely to be transported within the plant and thus the plant may appear symptomless.
Resistance is linked to inhibition the virus being transported within the plant although the virus has been shown to replicate within cells where initially inoculated.
 
Although the virus may be damaging in susceptible varieties if the timing of infection is early, the damage is increased when other viruses are involved.  The damage to susceptible hybrids in Southern corn belt of USA is greatest when the infected by MDMV and Maize Chlorotic Dwarf Virus (MCDV). The latter is vectored by leafhoppers such as Graminella nigrifrons. This virus also infects Johnson Grass, allowing this alternate host to be an overwintering host which is also a source of food for aphids and leafhoppers.  The perfect storm is infected, perennial Johnson Grass, aphids and leafhoppers and late planted, susceptible corn.  Effective control comes with effecting with these three factors.
 
The MDM virus moves with the endoplasmic reticulum in the corn cell’s cytoplasm to infect the chloroplasts. The chloroplasts appear to be the main site of virus replication.  This must account for the major symptoms of initial yellow spots and the later mosaic appearance in leaves of susceptible varieties as the virus infects the parenchyma cells surrounding the vascular bundles.
Major virus replication occurring in chloroplasts and the fact that chloroplasts and their genetics originate in the female parent of a hybrid suggests that a source of strong resistance could come from the female parent.  On the other hand, the spread of the virus within the plant could be related to other plant structures, possibly inherited by both parents.
 
MDMV, and its related potyviruses appear to be worldwide with potential hosts and vectors.  Changing environments affecting vectors and alternate hosts affect the potential threat of these viruses.