Severe Maize Chlorotic Dwarf Disease Caused by Double Infection with Mild Virus Strains

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Abstract
Severe symptoms thought to be caused by maize chlorotic dwarf virus (MCDV) in the field have not been consistently reproduced by isolates of the virus in the greenhouse. To determine if severe symptoms could be caused by multiple infections of mild MCDV strains, six MCDV isolates causing mild symptoms were separated from naturally infected johnsongrass (Sorghum halepense), the overwintering host, and then coinoculated into corn (Zea mays) in various combinations. One of the isolates, MCDV-Ml, caused mild symptoms by itself, but severe symptoms in combination with any of the other mild isolates or the type strain. MCDV-M1 was serologically related to but distinct from the other isolates and the type strain, and two of the three capsid proteins from MCDV-MI were larger than those of the other isolates and type strain. The other isolates were all indistinguishable from the type strain. Virus isolated from plants with severe symptoms after double inoculation with MCDV-Ml and another of the isolates gave a capsid protein pattern expected for a combination of both viruses. We conclude that MCDV-Ml is a strain of MCDV and that severe symptoms resulted form a synergistic effect involving MCDV-Ml and one of the other isolates. In a field survey, seven of 11 severely diseased corn plants and seven of 10 johnsongrass plants were doubly infected with MCDV-Ml and the type strain.