Effect of fungicide mobility and application timing on the management of grape powdery mildew

Authors: WARNEKE, BRENT - Oregon State University; THIESSEN, LINDSEY - North Carolina State University; Mahaffee, Walter - Walt

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2019
Publication Date: 2/13/2020
Citation: Warneke, B., Thiessen, L., Mahaffee, W.F. 2020. Effect of fungicide mobility and application timing on the management of grape powdery mildew. Plant Disease. 104(3):1167-1174. https://doi.org/10.1094/PDIS-06-19-1285-RE.
DOI: https://doi.org/10.1094/PDIS-06-19-1285-RE

Interpretive Summary: In order to improve management of grape powdery mildew while reducing reliance on fungicides prone to developing resistance, fungicides were examined for their ability to redistribute and whether the redistributing fungicides where effective when only applied during the critical bloom and fruit set phase of grape berry development on the management of grape powdery mildew. Results indicated that all fungicides examined redistributed through at least one mechanism and several fungicides were able to redistribute on grape leaves through multiple mechanisms. In addition, those showing movement through leaf tissue also moved through flower caps to the underlying flower parts. Field trials demonstrated that integrating two applications of redistributing fungicides initiated at end-of-bloom into a fungicide program appear to be an effective strategy for wine grape growers in western Oregon to produce fruit with low grape powdery mildew infection.

Technical Abstract: Grape powdery mildew (GPM) fungicide programs consist of 5-15 applications in an attempt to achieve the high fruit quality standards demanded by the market. Understanding how fungicides redistribute and targeting redistributing fungicide to critical crop phenological stages could improve fungicide protection of grape clusters. This study evaluated fungicide redistribution in grapevines from major FRAC groups labeled for GPM control. Translaminar and xylem redistribution was examined by placing fungicide-soaked filter disks on the adaxial or abaxial leaf surface of detached leaves for 10 min, incubated for 48 before inoculating the abaxial surface with conidia. Vapor redistribution used Teflon disks sprayed with fungicides and placed on the abaxial leaf surface of detached leaves 48h before inoculation. Disease development was rated 10 days later. Translaminar movement through calyptra was tested using flowering potted vines. All fungicides tested redistributed through at least one mechanism. Fungicide timing at critical phenological stages (early, mid, and late bloom) was assessed in small plots of Pinot noir vines. The application of either trifloxystrobin, quinoxyfen, or fluopyram at different bloom stages showed that applications initiated at end-of-bloom resulted in the lowest respective berry infection probabilities of 0.073, 0.097, and 0.020. The results of this study suggest that integrating two carefully timed applications of redistributing fungicides initiated at end-of-bloom into a fungicide program may be an effective strategy for wine grape growers in western Oregon to produce fruit with low GPM infection.