The below mentioned article provides a note on peronospora which causes diseases like downy mildews. This will also help you to draw the structure and diagram of peronospora.
This is the most advanced family of order Peronosporales. It comprises 6 genera with numerous species. All of them are important plant pathogens which are obligate parasites of flowering plants such as onion, grape, bajra, jowar, maize, pea, turnip, beet, tobacco and many crucifers.
The diseases caused by the Peronosporaceae are called downy mildews. The mycelium is branched, tubular and coenocytic. The hyphae are strictly intercellular and occur in the stem but more commonly in the leaf.
They absorb nutrition by sending vesicular (Fig. 6.36 A) or branched finger-like haustoria (Fig. 6.40 A) into the host cells. The houstoria do not perforate the plasma membrane of the host cell but simply push it in and invaginate. The sporangiophores (or conidiophores) are distinct and of determinate growth.
They arise from an endophytic mycelium, emerge through the stomata to come into the air when humidity is sufficiently high and branch characteristically. The individual genera of the family in fact distinguished on the basis of the form of the sporangiophore. Each branch bears a few to many short, delicate branchlets. A single ovoid or pyriform sporangium is formed at the tip of an ultimate branchlet called a sterigma.
The sporangia are deciduous and thus get detached when mature. The detached sporangia are disseminated by wind. On landing on a suitable host the sporangium in the genera except Peronospora behaves in two different ways.
It germinates either discharging a number of small biflagellate, reniform zoospores or directly forming a short germ tube under warmer or dry conditions thus assuming the role of a conidium. The germ tube formed in whatever manner enters the host through a stoma and forma the mycelium.
The sex organs are formed within the tissues of the host on the same or adjacent hyphae. The antheridia are paragynous. The protoplast of the oogonium is differentiated ooplasm. The oospore has a thick wall. The outer layer or epispore is sculputured.
The oospore, as a rule germinates by germ tubes. The only exceptions are Peronospora tabacina and Plasmopara in which the oospore liberates zoospores in a sporangial vesicle.
The genera placed in the family are known by the common name Downy mildews hairy fungi, because the sporangiophores of the members look like hairs emerging from the stomata. Since the oospores and sex organs are not much different in various genera asexual reproductive structures—the sporangiophores have been used as the criterion of delimitation, of genera.
Important genera placed in the family include Plasmopara, Peronospora, Basidiospora, Bremia and Sclerospora whose characteristic sporangiophores are shown in fig. 6.35. Of these Plasmopara, Peronospora and Sclerospora are discussed in detail.
