First confirmed report of downy mildew caused by Hyaloperonospora parasitica on broccoli, cauliflower and Romanesco-type cauliflower heads in France

In autumn 1997 cauliflower, Romanesco-type cauliflower (Brassica oleracea var. botrytis), referred to as ‘Romanesco’, and broccoli (Brassica oleracea var. italica) were brought to our laboratory for disease diagnosis. The heads of all three species showed brownish to dark discolorations when cut longitudinally (Fig. 1). Broccoli heads also showed numerous black necrotic flecks (Fig. 2) on the base of floret branches. The upper surfaces of cauliflower and broccoli heads sometimes also develop pale brown or greyish discoloration (Fig. 3). There were no visible symptoms present on the surface of Romanesco heads, only dark brownish necrotic flecks seen on the cut floret branches. When samples from all three crops were incubated for 24 h in a humid chamber, typical downy mildew sporulation was seen (Fig. 4) and the pathogen Hyaloperonospora parasitica Constant.(Pers. ex Fr.), syn. Peronospora parasitica was easily isolated. Artificial inoculation of healthy floret branches with the isolated pathogen gave similar symptoms. Characterization of the pathogen on seedlings of a differential set of Brassica hosts along with other H. parasitica isolates confirmed that the pathogen was H. parasitica (Constantinescu & Fatehi, 2002). Infection of broccoli and cauliflower heads, but not Romanesco, by H. parasitica has been documented (Hong et al., 2008). Fourteen derived single spore isolates are now maintained in our laboratory (Godard et al., 1999; Monot & Silué, 2009) and molecular markers for accurate diagnosis of the pathogen have been developed.

Downy mildew on inflorescences was not known in Brittany till 1997 and is now considered as one of the most devastating brassica diseases. Losses due to the disease are most important on Romanesco and may be total. Romanesco was introduced in Brittany in 1994/95 as an alternative to cauliflower as it is more marketable. However, because the production period (September to December) in the west of France is humid with mild temperatures favouring epidemics, farmers nearly abandoned its production in 1998 due to Romanesco’s susceptibility to the disease. Nonetheless, recent efforts in plant breeding and other IPM methods rescued the production of all three crops.