Powdery Mildew on Hops

Hop powdery mildew, which is caused by the fungus Podosphaera macularis, has become a big concern over the past few decades for growers in the Pacific Northwest. Sulfur has become a popular fungicide to control this pathogen, due to it low costs and relative effectiveness, especially as a preventative fungicide. However, a study from USDA and Oregon State researchers, published in 2009 suggests that there may be some negative aspects of using sulfur on hops, which may not be apparent at the time of spray application.

Sulfur Fungicides Affect Two-Spotted Spider Mite Populations

Based on trials conducted in Oregon and Washington in 2005 and 2006, researchers from the USDA and Oregon State found that Powdery Mildew control practices, in some cases, can significantly affect populations of two-spotted spider mites in hopyards in the Pacific Northwest.( Journal of Economic Entomology(2014) 102 (1): 274-286). They measured their affects on populations of arthropod pests and natural predators, following repeated sulfur sprays early in the season for powdery mildew control.

Adult two spotted spider mites. Photo by J. Holopainen.
Adult two spotted spider mites. Photo by J. Holopainen.

Sulfur is commonly used to control Powdery Mildew infections on hops, but is also used as a broad insecticide. While it does reduce spider mite populations immediately following application, it can actually have negative results over the course of the season. In this study, multiple applications of sulfur for Powdery Mildew control resulted in 1.4-3.3x greater population of spider mites in summer compared to water sprays. The populations that were sprayed with sulfur also grew back up faster after 20-30 day lag time, compared to water spray controls. Plots sprayed with sulfur also had 10x decrease in phytoseiid mites, a natural predator of two spotted spider mites. While they are likely not the only factor, this suggests that phytoseiid predatory mites play a large role in preventing spider mite outbreaks. Similar results were seen with paraffin oils, while synthetic fungicides had less conclusive results.

Integrated Pest Management in a Complex Bio-system

This study highlights the need to consider the sometimes complex bio-systems present in the hopyard when making pest management decisions. Natural predators play a huge role in any integrated pest management, so special care must be given to maintain these beneficial populations and it’s important to consider how management decisions aimed to control one pest may affect another down the road. Frequent sampling for target pests as well as beneficial insects, before and after making spray applications of any kind can help a grower get a better idea of the full picture of his crop, and will encourage better pest management decisions. In this case, growers of hops in the Pacific Northwest may choose alternate fungicides to sulfur, or adjust to lower rates in order to preserve beneficial insect populations and help control two-spotted spider mites later in the season.


Effects of powdery mildew fungicide programs on twospotted spider mite (Acari: Tetranychidae), hop aphid (Hemiptera: Aphididae), and their natural enemies in hop yards

Plant Management Network, 2008