Fusarium circinatum Nirenberg and O’Donnell is an important pathogen of pine seedlings and cuttings in South Africa. The fungus causes plant death in nurseries, as well as during establishment of pine plantations. The aim of this study was to consider the effects of wound type, spore concentration, and environmental stress on infection incidence. Pine seedlings were inoculated using three wounding methods and five spore concentrations. Inoculated seedlings were incubated under optimal environmental conditions, suboptimal conditions, and suboptimal conditions combined with a fungicide treatment. Results showed that the mean percentage infection caused by increasing spore concentrations can be
described by the Michaelis–Menten function. The gradient of the function, as well as the asymptotic maximum level of infection, was dependant on environmental stress and the physiological state of the host, as well as the wounding method. Spore concentration had the highest influence on infection incidence in physiologically stressed seedlings. Fungicide treatment did not influence the rate of infection incidence in comparison with the treatments conducted under optimal environmental conditions, but significantly lowered the asymptotic maximum level of infection incidence. Seedlings wounded on
the stems had the highest infection incidence, when compared with other wounding methods.