In addition, as you all know, WNC is very humid and sometimes wet! As a result, we struggle with the control of plant diseases. Hops are no exception. Upon my last visit to the hops yard, I found some leaves exhibiting symptoms that had me concerned. I had discovered what I initially thought was downy mildew, caused by the fungal-like organism, Pseudoperonospora humuli, on some lower hops leaves.
Note: the large circular lesion is not caused by downy mildew
Upon inspection using my hand lens, I was able to confirm that there was downy mildew sporangia emerging on the underside of the leaf surface. Because I love learning about plant diseases, I had to take a closer look using a dissecting microscope (about 20-45x magnification).
Sporangia (spores) and sporangiophores of P. humuli on underside of hops leaf. You can really see the football/lemon shaped spores at this magnification.
Strategies for the Management of Hops Downy Mildew
To manage downy mildew of hops it will take an integrative approach using some or all of the following strategies.
- Host Resistance. One of the most effective management strategies for hops downy mildew is to plant resistant varieties. There is no variety that is immune to downy mildew, but 'Fuggle', 'Cascade', 'Newport' and 'US Tettnang' are resistant. 'Centennial', 'Willamette', 'Chinook', 'Liberty', 'Cascade', 'Bullion' and 'Brewer's Gold' are tolerant. 'Late Cluster', 'Galena', 'Horizon' and 'Nugget' are susceptible. Interestingly, many of the varieties that are resistant to downy mildew are susceptible to powdery mildew and vice versa. For a complete listing of varieties inlcuding disease susceptibility, pedigree and chemical characteristic, see the Compendium of Hop Diseases and Pests edited by Walter Mahaffee, Sarah Pethybridge and David Gent. This book is a must have for a hops producer!
- Disease Free Stock. It is always important to purchase disease free plant stock! Ask your supplier.
- Pruning. P. humuli overwinters in dormant crowns and buds of hops. During the winter and spring, P. humuli may spread into developing buds and cause the new shoots to be systemically infected. Pruning helps to reduce downy mildew levels in already infected plantings. Pruning hops yards as late as possible in the spring can help to reduce the amount of downy mildew in the yard, however if you do it too late you may reduce yield by dealying training.
- Manage Moisture. Moisture on the leaves favors disease development, so using practices that reduce leaf moisture will also help to manage the disease. Avoid overhead irrigation.
- Reduce Inoculum. Stripping diseased leaves after training may help reduce disease in the upper plant canopy.
- Early Harvest. Early harvest may be beneficial in minimizing cone infection. Downy mildew favors the cool, wet weather that occurs late season in WNC.
- Timely Fungicide Application. Many hops growers rely on fungicide use for downy mildew control. Because P. humuli produces so many spores (progeny), it is impoprtant to employ tactics to prevent fungicide resistance. To read more about fungicides for the management of downy mildew of hops visit Oregon State Extension's Factsheet. Some organic fungicides include copper products and Sonata (Bacillus pumulis strain QST 2808).
I would like to thank Dr. Kelly Ivors and Landis and Dreama for letting me use their awesome microscopes and camera to take these incredible pictures!!