Ethylene has been observed to both inhibit and promote the symptoms of Vert
icillium wilt (caused by Verticillium dahliae) in tomato. To test the hypot
hesis that ethylene has different effects at different stages in the infect
ion process, ethylene levels were manipulated in V. dahliae-infected tomato
plants by the application of an ethylene synthesis inhibitor aminoethoxyvi
nylglycine (AVG) and/or ethylene's biosynthetic precursor 1-aminocyclopropa
ne-1-carboxylate (ACC) and the effects on disease severity were examined. S
tatistically significant reductions in disease severity were consistently o
btained for AVG-treated plants that had ACC added at the time of inoculatio
n. A model is therefore proposed in which post-infection ethylene enhances
Verticillium wilt development in tomato whereas its presence at the time of
infection inhibits disease development.