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

Authors: Smith, Barbara; Miller Butler, Melinda

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2016
Publication Date: 11/1/2017
Citation: Smith, B.J., Miller Butler, M.A. 2017. Effect of nitrogen fertilization and fungicides on Botryosphaeria stem blight lesion development on detached stems. Acta Horticulturae. 1180/61-70.

Interpretive Summary: Interpretive Summary Control of blueberry stem blight in the field using fungicides has been unsuccessful and growers have relied on cultural practices, such as avoiding wounding plants, pruning out diseased canes, avoiding the use of high levels of nitrogen fertilizer, and establishing blueberry fields using tissue cultured plants, to manage this disease. This research focused on the effect of various rates of nitrogen fertilizer and fungicides on the development of stem blight lesions on detached blueberry stems. No significant increases in stem blight severity was correlated with increasing rates of fertilizer, however, growers should avoid late summer fertilizer applications to reduce the occurrence of winter injury. The shortest lesions 21 days after inoculation of stems wounded before being treated with fungicides were from plants treated with pyraclostrobin, fosetyl aluminum, or pyraclostrobin + boscalid; however, lesions did develop on these stems which indicates that infection did occur. This suggests that these fungicides may have slowed the infection process, but did not prevent lesion development. In the in vitro assays, cyprodinil + fludioxonil, pyraclostrobin + boscalid, tebuconazole, pyraclostrobin, propiconazole, fludioxinil, and cyprodinil inhibited the growth of the B. dothidea and/or N. parvum isolates. These fungicides, except tebuconazole, fludioxinil, and cyprodinil, are recommended for control of one or more post-harvest foliar blueberry diseases and might also offer some reduction in stem blight especially if applied immediately following operations, such as mechanical harvesting or pruning that wound plants. These findings will be used by blueberry growers and extension agents to determine the best disease management practices for control of Botryosphaeria Stem Blight of blueberries

Technical Abstract: : Botryosphaeria stem blight is a destructive disease of blueberries that has not been well managed with fungicides. Field observations showed that stem blight is 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 the rabbiteye cv Tifblue. Partially-hardened stems were wounded, inoculated with mycelial blocks of the pathogens, and incubated for 10 to 21 days 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 four rates of two 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; however, lesions were longer on stems from plants receiving either fertilizer at any level compared to those from plants that were not fertilized. In the fungicide study, nine fungicides or water were applied as a foliar spray to ‘Tifblue’ plants. Stems on these plants were wounded either immediately before or 4 hours after fungicide application, collected 4 hours after fungicide application, and inoculated in the laboratory within 4 hours. Twenty-one days after inoculation among stems wounded before fungicide application, those treated with pyraclostrobin, fosetyl aluminum, or pyraclostrobin + boscalid had the shortest lesions; among stems wounded after fungicide application, there were no differences in lesion length between those sprayed with water (untreated control) and any fungicide treatment. Two isolates each of two Botryosphaeriaceae species were grown for 4 days on agar amended with 11 fungicides at a range of concentrations. After 4 days, 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 a.i./L.