Powdery mildew (PM) is a common disease of hemp in greenhouses and high tunnels. A limited number of fungicide products are available for use on hemp in Tennessee. Still, there is no information regarding the efficacy of the products against PM on hemp plants in the greenhouse. In a series of four greenhouse trials conducted between 23 Oct 2020 and 19 Aug 2021, the efficacy of 39 fungicide treatments was evaluated against PM using ‘BaOx2’ (South Central Growers,
Hemp cultivars inoculated with PM conidia are Springfield, TN) or ‘Sweetened’ (PWP Greenhouse, Pall Mall, TN).
Method
Plants were grown in 1-gal pots filled two in. from the top with Lambert Peat Moss potting mix (Quebec, Canada) and watered at least once daily or as needed and fertilized using Miracle-Gro Plant Food (The Scotts Company, Marysville, OH) once per week. Thirteen of the fungicide treatments were commercial products. Luna Experience was not labeled for use on hemp when the studies were conducted, but it was included as a positive control. Twenty-six of the treatments were experimental products from industry partners, and plants sprayed with water served as untreated controls. Experimental units consisted
of one 6 to 7-week-old hemp plant per pot, and pots were arranged in a randomized complete block design with four
replicates.
One day before, or one after, PM conidial suspension (2x105 conidia/ml) inoculation, the first fungicide application was sprayed on the plants. Treatment applications that were made after inoculation were considered curative treatments. During each trial, three fungicide applications were made at 7-day intervals. The fungicide applications and conidial inoculations were made using 1-liter Solo hand-held sprayers (Solo Inc., Newport News, VA). All applications were made to thoroughly cover the plants.
Greenhouse temperatures during trials ranged from 63°F to 78°F for trial 1, 68°F to 81°F for trial 2, 58°F to 87°F for trial 3, and 73°F to 87°F for trial 4. The relative humidity values averaged 51%, 48%, 56%, and 76% during the trials, respectively. The greenhouse photoperiod was 16h light and 6h dark. Data on PM incidence and severity for each plant was collected three times at 7-day intervals for each trial. Powdery mildew incidence was recorded as the percentage of leaves showing PM symptoms, and PM severity was recorded as the average percentage of leaf area expressing symptoms of PM. The data were used to determine the Disease Index (DI) to summarize PM levels for each plant at each data point, and the Area Under the Disease Progress Curve (AUDPC) was calculated for each trial.
Analysis of variance was conducted using mixed models in SAS (SAS Institute, 9.4, Cary NC), with treatment as the fixed effect and replicate as the random effect. Least-squares means were separated using the Tukey test at α = 0.05.
Results
All treatments reduced the PM index and AUDPC in every trial, with the exception of IP-2 (1%), Exile, Luna Experience, Regalia, Sil-matrix, and Stargus in trial 1. All commercial products were not significantly different from each other, with the exception of Defguard, which was significantly less effective than Bonide sulfur, Cinnerate, Exile, Luna Experience, MilStop, and Regalia in Trial 2. The curative treatments were not significantly different from the protectant treatments.
Phytotoxicity was observed in the following treatments: Luna Experience, TPCS, TPNS, TPOS, and Tween 80. Subsequent tests showed phytotoxicity in TPCS, TPNS, TPOS, and Tween 80 treatments that resulted from high concentrations of surfactant.
Akinrinlola, Rufus & Hansen, Zachariah & Gwinn, Kim & Wang, Toni. (2022). Efficacy of fungicide treatments against hemp powdery mildew in the greenhouse in Tennessee, 2021 V049.
To read the complete study, go to www.researchgate.net
Four greenhouse trials