Experiments were done to determine the effects of temperature, light and humidity on infection of carnation [Dianthus caryophyllus] by G. zeae, causal agent of fusarium stub dieback. The incidence of infection was the same at 13 or 18.degree. C, but was significantly higher at 24.degree. C. Severity of infection also was greater at 24.degree. C than 18.degree. C and was less at 13.degree. C than 18.degree. C. Incidence of infection was significantly lower at 50% relative humidity than at higher humidities. Intermittent wetting or presence of a saturated atmosphere for a short period following inoculation increased infection frequency. The combination of high temperature (24.degree. C) and intermittent high humidity resulted in the greatest incidence and severity of infection. Growing plants at high light intensity prior to inoculation resulted in more disease than in plants grown at lower intensity. These results explain the common observation that fusarium stem rot and stub dieback is frequently a problem in early summer in the eastern USA where conditions of temperature, humidity and light are most favorable for infection and pathogenesis of carnation by G. zeae.