Effect of nitrogen fertilization and fungicides on Botryosphaeria stem blight lesion development on detached blueberry stems

Botryosphaeria stem blight is a destructive disease of blueberries that is not well managed with fungicides. Stem blight has been reported to be more severe on vigorously growing plants than on slower growing plants. Detached stem assays were used to compare the effect of fertilizers and fungicides on lesion development following inoculation of greenhouse-grown plants of rabbiteye 'Tifblue'. Partially-hardened stems were collected from plants, wounded, inoculated with mycelial blocks of the pathogens, and incubated in a growth chamber. Plants in the fertilizer study were inoculated with a virulent isolate of Botryosphaeria dothidea and those in the fungicide study were inoculated with two isolates each of B. dothidea and Neofusicoccum parvum. The fertilizer study compared 4 rates of 2 fertilizer types, and resulted in no significant differences in lesion length 10 days after inoculation on stems from plants receiving 1, 2, 4, and 8 times the recommended levels of either a liquid or a slow release fertilizer. Lesions were longer on stems from plants receiving either fertilizer at any level than on plants that were not fertilized. In the fungicide study, 9 fungicides or water were applied as a foliar spray to 'Tifblue' plants whose stems were wounded either immediately before or 4 h after fungicide application, then collected, and inoculated in the laboratory within 4 h. Among stems wounded before fungicide application, those treated with pyraclostrobin, fosetyl aluminum, or pyraclostrobin + boscalid developed the shortest lesions; among stems wounded after fungicide application, there were no differences in lesion length between the untreated control and any fungicide treatment. In vitro trials showed the growth of each isolate was significantly reduced on agars amended with azoxystrobin, cyprodinil + fludioxonil, fludioxonil, propiconazole, pyraclostrobin, pyraclostrobin + boscalid or tebuconazole at concentrations above 0.1 mg L^-1 active ingredient.