Downy mildew of Buddleja

The causal agent of downy mildew in various Buddleja species, including Buddleja davidii and B. globosa, is the oomycete Peronospora hariotii. The pathogen attacks the foliage, causing the appearance of angular chlorotic or yellowish spots on the upper side of the leaf, which eventually turn brown and dry out. Under conditions of high humidity, a downy growth forms on the underside of the leaves, ranging in color from white to grayish-brown.

Severe infection leads to curling and premature abscission of leaves, shoot deformation, and death of terminal buds. In severe cases, systemic infection may occur, causing general growth inhibition of the plant. Data on specific races of the pathogen or resistance genes in Buddleja are currently unavailable, and breeding is conducted mainly by selecting less susceptible forms.