Infection and Pathogenesis

In crucifers, the downy mildew infection may be either general or local. In the former case, all or most of the leaves, and inflorescence, may bear conidiophores. Although some parts (especially the stem) may show no external injury, microscopic examination shows the mycelia are in the tissues. Generalized infection is restricted to young tissues, and seedlings may show completely infected leaves. Localized infection also occurs in young tissues, especially those still in active division. In the hypertrophy caused by Albugo, the cells of the epidermis and cortex are dividing and may readily give entrance to Hyaloperonospora. Young inflorescence may wholly or partly be infected, while normal tissues of older stems and leaves below the initial site of infection may remain free.