Weather and corn virus diseases

​Apparent changes in weather patterns during recent years affects corn and pathogen biology.  Diseases may be more prevalent in areas that they were virtually absent and nearly absent in areas in which they were frequently damaging because of timing of particular weather.
 
Diseases caused by viruses such as Maize Dwarf Mosaic (MDMV), Maize Chlorotic Dwarf (MCDV, Maize Chlorotic Mottle (MCMV) are transmitted by insect vectors which are also affected by weather.  Virus generally must reach the growing point of the corn plant to damage the plant or even show symptoms.  Consequently, the infection must occur before the V4 seedling stage before the apical meristem is pushed upwards by cell elongation.  Wet weather can delay planting, allowing increasing populations of the vectors feeding on alternative virus host plants such as Johnson grass (Sorghum halepense). As result of increase of aphids feeding on MDMV-infected plants and delayed corn planting, transmission of the virus into corn seedlings allows the virus to become systemic after it reaches the apical meristem.  This becomes especially damaging if the leafhopper (Graminella nigrifrons) picks up the MCDV virus from grass hosts and then feeds on the same young corn plant.  Synergistic effect of concurrent infection by these two viruses can cause extreme damage to susceptible corn genotypes.
 
This synergism between two viruses has even be more damaging when a MDMV or Wheat Streak Mosaic Virus (WSMV) infect the same plant infected with MCMV.  The result is Corn Lethal Necrosis disease.  WSMV is transmitted by wheat leaf curl mite (Aceria tosichella), that frequently picks up the virus from infected wheat.  Maize Chlorotic Mottle Virus is transmitted by beetles, primarily Diabrotica species in the USA and by thrips (Frankliniella williamsi) in Africa.  Transmission can be done by larvae and adults.  Corn plants infected as seedlings with MCMV vectored by infected rootworms (Diabrotica species) and MDMV or WSMV as vectored by aphids or wheat leaf curl mites will be severely damaged.  Weather affecting timing of planting of corn, growth or control of alternate hosts such as Johnson Grass, harvest of wheat all interact in determining the damage from these viruses.
 
Several other viruses, each with unique dynamics of vector biology, alternate hosts and corn development cause significant damage in specific environments. Weather is significant in each development of each of the diseases. Many perennial grasses are infected with viruses and are vectored by insects that feed on both corn and the grasses.  Weather affects the plant and insect biology in relation to timing and intensity of virus infection in corn.  Infection by one virus species may go unnoticed but dual infections because of the coincidence of many variables can result in considerable damage to corn.