Evaluation of selected fungicide application regimes and biotic agents for the management of basil downy mildew

Authors: Zhang, Guirong; BABADOOST, MOHAMMAD - University Of Illinois; DE YOUNG, ALAN - Van Drunen Farms; Johnson, Eric; Schisler, David

Submitted to: HortTechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2018
Publication Date: 12/1/2018
Citation: Zhang, G., Babadoost, M., De Young, A., Johnson, E.T., Schisler, D.A. 2018. Evaluation of selected fungicide application regimes and biotic agents for management of basil downy mildew. HortTechnology. 28:822-829.

Interpretive Summary: The pathogen Peronospora belbahrii causes basil downy mildew, a destructive disease of sweet basil in the United States and globally. Foliar fungicide treatments show promise in reducing the severity of symptoms on basil leaves but repeated use of a fungicide can result in the pathogen population becoming resistant to the fungicide, limiting its effectiveness. Biocontrol agents have potential for reducing downy mildew development but few have been evaluated. In this study, ARS researchers and collaborators at the University of Illinois found that weekly field applications of multiple fungicides, combinations thereof, application rates and sequences reduced downy mildew on basil by 80-95% when applied to basil foliage weekly over the course of a summer field season in Illinois. However, a previously untested fungicide product and two biocontrol agents discovered on potato tubers by ARS scientists in Peoria, Illinois were not effective in reducing downy mildew in tests on greenhouse grown plants. These results demonstrate the effectiveness of new fungicide treatment combinations and sequences that have promise in reducing downy mildew on basil for basil producers and consumers without rapidly selecting for fungicide tolerant variants of the pathogen.

Technical Abstract: Basil downy mildew (Peronospora belbahrii) is a destructive disease that occurs in sweet basil (Ocimum basilicum). Foliar fungicide treatments could reduce infection and the severity of foliar symptoms over the course of the growing season. Multiple fungicides in variable combinations, application rates, and sequences were applied to basil foliage weekly over the course of a field season in Illinois (14 July to 8 Sept. 2014), and the treatment effects on basil downy mildew were evaluated three times. The evaluated treatments included mixtures and different rotations of azoxystrobin, potassium phosphite, mandipropamid, cyazofamid, oxathiapiprolin, experimental compound A18269SE, dimethomorph, zoxamide D mancozeb, fluazinam, fluopicolide, mefenoxam D copper hydroxide, fenamidone, mancozeb, and ametoctradin D dimethomorph. Potassium phosphite, which is known to be effective against other downy mildew pathogens, was included in combination with other fungicides or in fungicide application sequences. Disease severity was rated in fungicide-treated plots (0% to 20%) and compared with the control (73% to 80%) at each evaluation time. All fungicide treatments significantly reduced the area under the disease progress curve values compared with the untreated control. Adding a nonionic surfactant did not improve the efficacy of any of the chemical treatments evaluated for reducing downy mildew. Organic basil growers need novel, effective products to minimize damage from basil downy mildew. To aid organic basil growers, two novel, effective biocontrol agents were evaluated, Bacillus amyloliquefaciens AS 43.3 and Papiliotrema flavescens OH 182.9 3C (formerly Cryptococcus flavescens). Greenhouse experiments were conducted with the fungicides quinoxyfen and azox-ystrobin serving as negative and positive fungicide treatment controls, respectively. Azoxystrobin reduced downy mildew according to the greenhouse tests, but neither quinoxyfen nor the biocontrol agents reduced downy mildew severity compared with the untreated control. This study identified 13 fungicide regimens that resulted in less than 10% basil downy mildew disease severity. More studies are needed to identify effective control products for basil downy mildew on organic basil.