Fusarium verticillioides dissemination among maize ears of field-grown plants

Consequences of the fungus Fusarium verticillioides (Sacc.) Nirenberg (synonym F. moniliforme Sheldon) (teleomorph, Gibberella moniliformis), colonizing kernels of maize, Zea mays L., may be plant disease and/or mycotoxin production. Plant disease may reduce crop production and mycotoxins may cause harmful, and often fatal, effects on humans and animals. The objective of the current experiments was to track migration of a F. verticillioides isolate transformed with foreign genes from inoculated to non-inoculated maize ears to determine the population flow of this fungus among maize plants under field conditions. Distribution patterns of F. verticillioides following artificial infections have been described within growing maize plants, but not among maize plants. Maize ears were inoculated through either the shuck or the silk channel at one of two stages of silk development, green or brown. The inoculum was a genetically transformed isolate, designated F. verticillioides PATg, carrying two foreign genes, one coding for hygromycin resistance (hygr) and the other for β-glucuronidase (GUS). The average expression of hygr was 66% and of GUS was 96% for mycelia emerging from kernels of ears inoculated with F. verticillioides. Mycelia resistant to hygromycin appeared in cultures of kernels from non-inoculated and water-inoculated ears, but only at ⩽3%. However, none of the mycelia stained positive for GUS in kernels originating from non-inoculated ears and only 7% of those from water-inoculated ears. Thus, dissemination of F. verticillioides PATg was minimal from plant to plant under field conditions existing in Georgia during the growing seasons of 2001, 2002, and 2003. Negligible F. verticillioides PATg was detected in a few ears inoculated with water through the silk during the green silk stage of development. Perhaps the mechanical injury during inoculation provided an entry site for the spores of F. verticillioides PATg dispersed by abiotic factors such as rain or wind.