Downy mildew — bulb onion

The causal agent of the disease is the oomycete Peronospora destructor (syn. Peronospora schleideniana). The pathogen is highly specialized to the genus Allium and affects the leaves, scapes, and seeds of the bulb onion. The infection manifests as a systemic lesion where the mycelium penetrates the growing point, causing curling and yellowing of the leaves, or as local chlorotic spots that become covered with downy sporulation under high humidity conditions.

Completely immune onion varieties do not exist; however, breeding efforts are focused on utilizing the resistance gene Pd1, derived from the wild species Allium roylei. To identify this gene in the breeding process, molecular markers such as DMR1 are employed, and in professional documentation, the level of resistance to the pathogen is designated by the code Pd.