ADVERTISEMENTS:
In this article we will discuss about the characteristics and classification of peronosporales.
General Characteristics of Peronosporales:
The order is represented by 12 genera and about 325 species. In habit they range from aquatic to the terrestrial ones through the species with amphibious way of life.
Many of the terrestrial species are highly specialized obligate plant parasites which cause serious diseases to important economic plants. A few, however, occur as parasites of thallohytes. The rest are saprophytic. Some parasitic species can live as saprophytes after the death of the host (pythium).
ADVERTISEMENTS:
The mycelium is well developed, branched and coenocytic. The septa normally remain suppressed in the vigorously growing vegetative hyphae but appear to wall off injured part of separate reproductive cells. The mycelium shows no distinction into rhizoidal and aerial hyphae.
The somatic hyphae in the downy mildews which are parasitic inhabit the intercellular spaces of the host tissue with only the haustoria entering the host cells. In most of the Pythiaceae, however the hyphae are intracellular and the haustoria, with the exception of a few advanced species of Phytophthora, are rarely found.
Asexual reproduction takes place typically by the formation of motile asexual spores called the zoospores within sporangia. The sporangia are usually globose or oval in form sometimes shaped but not elongated. They are developed hyphae but in more advanced complex forms the sporangia are borne at the tips reproductive hyphae known as the sporangiophores.
The zoospores are typically kidney-shaped, biflagallate and monoplanetic. The two flagella are inserted laterally on the concave side; one is of tinsel (pantonematic) type and the other whiplash (acronematic) type.
ADVERTISEMENTS:
In some species the sporangium may behave either as a zoosporangium and produces zoospores or directly germinates by putting out a germ tube and thus functions as a conidium. The sporangia which behave as conidia are sometimes called as conidiosporangia.
Sexual reproduction is oogamous and takes place by gametangial contact. Both kinds of sex organs (antharidia and oogonia) may be borne on the same or different hyphae. The globose oogonium contain a single oosphere usually surrounded by periplasm. The antheridium is usually short and club-shaped. It is delimited by a basal septum and usually arises from below the oogonium. The antheridium is of limited growth.
Fertilisation is usually effected by means of a fine penetration tube called the fertilisation tube. This kind of fertilisation is described as paragynous. In Phytophthora, however, fertilisation is of amphigynous type the oospore germinates either directly by a germ tube (Peronospora) or produces zoospores (Albugo).
The Peronosporales show an advance over the Saprolegniales in the following respects:
ADVERTISEMENTS:
1. The Peronosporales show a wide range of habit from aquatic, through amphibious way of life to the land habit whereas the Saprolegniales are mostly aquatic. Some occur in damp places.
2. Many peronosporales are parasites and some are saprophytes. A few parasitic species can live as saprobes after the death of the host. The Saprolegniales are mostly saprophytes rarely parasites.
3. The sporangia in the Peronosporales usually get detached and generally function as conidia. They are globose or oval in form whereas in the Saprolegniales they are tubular or club-shaped in form and do not get detached and never function as conidia.
4. The sporangia in many species are borne on special reproductive hyphae called the sporangiophores whereas in the Saprolegniales they are always borne on hyphae usually similar to the somatic hypae which are hardly distinguishable from the sporangia.
ADVERTISEMENTS:
5. The zoospores in the Peronosporales are typically reniform in shape and biflagellate with the flagella arising from the concave side. They are thus monoplanetic whereas in the Saprolegniales the zoospores are generally diplanetic to polyplanetic. The primary zoospores are pyriform with the two flagella inserted apically and the secondary ones are reniform with the two flagella arising from the concave side.
6. The oogonium in the Peronosporales contains a single egg which is surrounded by periplasm whereas in the Saprolegniales the oogonium lacks periplasm and contains more than one egg.
Classification of Peronosporales:
The order is generally divided into three families on the basis of charactarastics of sporangiophore and sporangia. A summary of the salient features and key to these families viz.
Pythiaceae, Peronosporaceae and Albuginaceae are as follows:
ADVERTISEMENTS:
Pythiaceae:
Sporangiophores undifferentiated from the mycelium, indeterminate in growth, branched, haustoria absent or branched, facultative parasites or saprophytes.
Peronosporaceae:
Sporangiophores easily distinguishable from the mycelium, branched determinate in growth, sporangia singly or in clusters, borne at the characteristically branched sporangiophores; haustoria varied or branched; obligate parasites.
ADVERTISEMENTS:
Albuginaceae:
Sporangiophores unbranched clavate, each producing a chain of conidia, in basipetal succession; haustoria knob shaped; obligate parasites.
The representatives of these families are discussed in detail.
