Downy mildew diseases cause considerable losses to crops throughout the world (Clark & Spencer-Phillips, 2000; Dang & Panwar, 2004). The organisms that cause these diseases belong to the oomycetes, a group of fungal-like microbes that belong to the kingdom Straminipila (Dick, 2002), and are genetically distinct from the true fungi which are from the kingdom Mycota. This difference probably accounts for the incomplete effectiveness of control of downy mildews by chemicals specifically designed to target fungi. Despite this, approximately 17% of global fungicide sales are directed at the downy mildew diseases (Gisi, 2002), with control in the UK aimed at crops such as brassica, leek, lettuce, onion and pea. The downy mildews in all of these crops, except for lettuce, are caused by Peronospora spp. (and the closely related Hyaloperonospora in brassicas), so the Downy Mildew Research Group at UWE is focusing on this genus, and in particular on Peronospora viciae which infects pea (Pisum sativum) as a model system. Typical symptoms of infection of pea by P. viciae are a downy mass of asexual spores on the underside of leaves and stipules, and on tendrils and stems.