Chemical management of Volutella leaf and stem blight of boxwood

Abstract

Volutella blight of boxwood, caused by the fungus Pseudonectria buxi, can cause extensive losses in commercial nurseries. To assess chemical methods for management, several fungicides commonly used to control diseases on ornamental plants were used in inhibition tests of fungal growth on amended media and on whole plants. Among 106 isolates from Ontario and 26 isolates from British Columbia, discriminatory threshold concentrations of propiconazole (1 μg/ml) and benomyl (10 μg/ml) fully inhibited growth, while iprodione (100 μg/ml) allowed minor hyphal growth of 4–11 % of the non-fungicidal control in agar tests. In pre-infection tests on whole plants, fungicides were applied and then 7 days later, leaf tips were cut, and the plants inoculated with a spore suspension of P. buxi. All treatments at 7 days after inoculation showed statistically significant disease control, all giving less than 13 % disease compared to the inoculated control at 91 % disease. Four of the treatments (chlorothalonil, copper-myclobutanil, thiophanate methyl, and propiconazole) were not significantly different from the non-inoculated control which showed no disease. In post-infection applications on whole plants where leaf tips were injured and immediately inoculated, followed 7 days later by fungicide application, all six fungicides (including mancozeb and iprodione) were found to significantly decrease sporodochial production by 57–89 % compared to the inoculated control, although diseased leaves did not recover. Further work is needed on the timing of applications.