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Savanna S. Shelnutt, Ali R. Cala, George M. Stack, Chris D. Smart, and Larry B. Smart
What is hemp powdery mildew? (Golovinomyces ambrosiae)
Hemp powdery mildew (PM) is a fungal pathogen belonging to the order ascomycetes. Infections are characterized by the appearance of white spore patches on the adaxial (upper) surface of leaves (Fig. 1), though infections can also manifest on stems. G. ambrosiae is biotrophic, meaning that it only grows on living plant tissue. PM thrives in warm, dry climates with high humidity making high tunnels an ideal environment.
Rationale and Goals
In 2021 we observed incidence of powdery mildew in our high tunnel trials. For hemp production in tunnels to be viable, growers must have a simple, effective means of controlling infection. Studies in pumpkin, wheat (Fig. 2), and cucurbits have shown that the application of powdered silicon (Si) rock dust (Wollastonite) to the soil pre-planting is an effective means of controlling PM throughout the season. We are testing soil (Vansil CS1) and foliar (Sil-MATRIX) applications of silicon, a known fungicide control (Trigo), a biological product (Milstop + CEASE) labelled for PM management in hemp all compared to a no treatment control to determine which are effective in controlling PM in tunnels (Table 1).
Methods
The cultivar ‘White CBG’ was selected for this trial as it is highly susceptible to PM. Plants were started from seed in the greenhouse on 5/11/2022 and transplanted into tunnels the week of 6/12/2022. Landscape fabric and drip tape was installed pre-planting and granular 19-5-19 was broadcast and incorporated into the soil at a rate of 100 lb N/acre. At the same time, Wollastonite was incorporated into the top six inches of soil in the specified plots (Fig. 3). Post-transplant, plants were allowed to grow vegetatively for five weeks prior to the first foliar fungicide applications the week of 7/18/2022. Each location was inoculated with PM 24 hours after the first spray treatments. Ratings for PM incidence are taken weekly and recorded as the percentage of total adaxial leaf area covered in visible spores. Ratings will continue for five to six weeks after the first inoculation.
| Product | Active Ingredient(s) | Rate |
| Milstop SP + CEASE | Potassium bicarbonate + QST 713 strain of Bacillus subtilis | 14 g/gal + 1.6 oz/gal weekly |
| Sil-MATRIX | Potassium silicate | 1% v/v weekly |
| Wollastonite Vansil CS-1 | Calcium silicate mineral (calcium metasilicate) | 4 tons/acre (0.18 lb/ft2) incorporated prior to planting |
| Trigo | Trifloxystrobin and Triadimefon | 0.0425 oz/gal biweekly |
| Control | Water | n/a |
Table 1. Fungicide treatments and rates
Acknowledgements: We would like to thank Oregon CBD for contributing seed and Vanderbilt Minerals for donating Vansil CS-1, as well as the lab members of Larry Smart and Chris Smart for their assistance. This work was funded by the New York Farm Viability Institute and New York State Ag and Markets through a grant from Empire State Development Corporation.
