Peronospora viciae (downy mildew: legumes)

The mould occurring on infected leaves, consisting of conidiophores emerging from the stomata, is the most characteristic symptom and makes it easy to recognize the presence of a downy mildew pathogen such as P. viciae. The host plant species is further important to determine the species and/or forma specialis. Differences in symptoms also occur in different host plants. For example, the characteristic stunting of oospore-infected seedlings usually occurs in pea but may be less common in faba bean. Van der Gaag and Frinking (1997a) did not observe stunting of oospore-infected faba bean seedlings but Jellis et al. (1998) indicated that soilborne infection results in stunted and distorted seedlings. Stunted growth of faba bean may also occur after conidial infection above soil. Leaf lesions on faba bean become brown and necrotic and shot holes may be formed which do not occur in pea. Mould appears on infected pea stems at high humidity but not on faba bean stems.

Removal of crop debris containing oospores to avoid a build up of soilborne inoculum is important (Reiling, 1984; Stegmark, 1992). Seed treatment with, for example, metalaxyl reduces the number of primary infections and can even reduce the number of secondary infections on young plants (Rossignol, 1988; Harvey et al., 1992). The best control is probably obtained by avoiding growth of the host plant in fields heavily infested with oospores and by using (partially) resistant cultivars (see also Stegmark, 1992).

In general, P. viciae does not cause high crop losses (Pegg and Mence, 1972; von Heydendorff, 1977; Bugiani and Giovoni, 1996). High crop losses can be obtained with soils heavily infested with oospores. Crop losses of 30 and 45% in Sweden and in the UK, respectively, have been reported (Oloffson, 1966; Biddle et al., 1988). Ryan (1971) reported that whole pea fields had to be ploughed in due to high levels of primary infection. Local infection of leaves does not usually lead to yield losses (Pegg and Mence, 1972). Infection of the pods may lead to considerable yield losses. Hagedorn (1974) reported a severe downy mildew epidemic in Wisconsin, USA, where fields with 62-85% diseased pods occurred. Diseased pods produced few peas and were of a poor quality.