Peronosporaceae

“Downy mildews”

The dowwny mildews differ from the Albuginaceae by the fact that the “conidia” are not formed below the epidermis of the host plant, but externally, borne on a conidiopore: a vertically erect, thickened hypha, up to almost a milimeter in height, that apically is repeatedly branching, each terminal branch bearing a ± ovoid conidium. In this family too the terms conidium/conidiophore and sporangium/sporangiophore are used exchangably in the literature. The conidiophores generally are formed at the underside of the leaves; in some specialised species they occur on the corolla.

Within the tissues of the host plant oospores are formed: thick-walled, globular resting spores, sometimes with a characteristic surface structure. They are long-lived, surviving the death of the host plant. They may even be formed in fruits and seeds, thereby effectively contributing to the dispersal of the fungus.

The damage to the hostplant is quite variable. Something all that can be seen is a slight discolouration, perhaps limited by the venation, of the upper side of the leaves. At the other extreme strong growth disturbances and malformations may occur, especially in those parts of the plant where oospores are being formed.

Powdery and downy mildews actually have nothing more in common than a superficial similarity. Powdery mildews (Erysiphaceae) have no conidiophores but globular fruiting bodies (often missing); the mycelium mostly is at the upper side of the leaves.

references

Alexopoulos, Mims & Blackwell (1996a), Gäumann (1964a), Webster (1980a).

04/10/2015