Hop Downy Mildew is a fungus specifically related to hops (Humulus Lupulus), its only host. Downy mildew can be diagnosed in many different ways, but at this stage in the season downy mildew appears as dark blackish spots on the underside of leaves near the crown, hopefully not at all though. In the worst cases if downy mildew is not controlled it will rise up through the bines where it will start to infect the cones, if not addressed before bloom.
Downy mildew can attack hops very rapidly when there are conditions of high humidity and stagnant air in the hop yards (see Burn Back and Stripping). Most of the damage inflicted by downy mildew occurs at the base of the hop, wrecking havoc on the crowns themselves promoting root rot and infecting the shoots in early spring when they rise out of the ground. Here on the farm if we see downy mildew in the yards we do our best control it with increased airflow and the removal of infected crowns from the yards.
Luckily I do not have any pictures of downy mildew at the moment (this is a good thing for us), powdery mildew is another story though. Please feel free to look at the following sites for more detailed information on downy mildew:
Oregon State University Plant Disease Control:
http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=599
University of Idaho Department of Plant, Soil, & Entomological Sciences:
http://www.ag.uidaho.edu/PSES/Research/r_ent_hoppest_downymildew.htm
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