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In this article we will discuss about the life cycle of plasmopara with the help of suitable diagrams.
Mycelium of Plasmopara:
It is freely branched and coenocytic. The hyphae are numerous and vary in size. The fungus is strictly intercellular (Fig. 6.36 H). Some of the hyphae give off lateral outgrowths which penetrate the host cells and swell inside to form vesicular haustoria. The latter secrete enzymes by the help of which they absorb food from the protoplasts of the host cells.
Reproduction in Plasmopara:
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Asexual Reproduction in Plasmopara:
Once established within the host, the mycelium forms pads of hyphae in the substomatal cavities (A). A group of hyphae grows upright from the hyphal pad and emerges through a stoma to function as sporangiophores (A).
Each sporangiophore is a long, monopodially branched structure. The lateral branches vary in number from three to six. Each branch bears branchlets. The branches and the branchlets lie more or less at right angles to the axis bearing them.
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The growth of the sporangiophore is determinate and thus bears sporangia when fully developed. A single sporangium is formed at the tip of an ultimate delicate branchlet called the sterigma (plural sterigmata).
All the sporangia in the crop are of about the same age. They are hyaline and ovate. Each sporangium is attached to its respective branchlet (sterigma) by the small end. At the opposite end, where sporangial wall is the thinnest, is a papilla (B).
The young sporangium is a small ovoid, uninucleate structure. With the increase in size its nucleus undergoes repeated divisions. The multinucleate granular protoplast is then divided into several uninucleate daughter protoplasts (B).
The sporangia are shed at this stage and are dispersed by air. If the detached sporangia which float in the air, happen to land on a suitable, moist substrate (leaf of the host), they germinate. A circular pore appears at the papilla by the dissolution of its thin wall. The uninucleate daughter protoplasts emerge through the opening (C).
After emergence each protoplast furnishes-itself with a pair of flagella (D). The liberated zoospores are plano-convex in form. There is a ridge along the flat side, near which lies the nucleus. The two flagella arise one on either side of the nucleus and are of unequal length. The short flagellum which is of tinsel type is directed forward and the longer one which is of whiplash type trails behind when the zoospore is in motion.
The liberated zoospores swim about in a thin film of water for a while (about half an hour) and then come to rest in the vicinity of stomata. Each quiescent (resting) zoospore rounds up and secretes a thin wall around it (E). Soon the globose cyst produces a germ tube (F). The latter gains entry into the host tissue (G) through a stoma. Rarely the sporangium germinates directly by a germ tube like a conidium.
Sexual Reproduction in Plasmopara:
It is oogamous. Like Albugo the sex organs are developed within the host tissue at the tips of adjacent hyphae.
The oogonium arises as a globular swelling at the tip of a female hypha (B). It is then cut off by a cross wall at its base. The young oogonium is uniformly multinucleate. As it advances towards maturity a central, rounded dense ooplasm with many nuclei is separated from the peripheral, vacuolated, multinucleate periplasm. The mature ooplasm is uninucleate and functions as an oosphere or egg.
The antheridium is developed at the end of male hypha lying near the oogonium (B). The tip of the male hypha enlarges into a clavate (club-shaped) swelling which is later cut off by a cross wall from the supporting hypha. The clavate cell is the antheridium. It contains about a dozen male nuclei and is applied to the side of the oogonium.
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Fertilisation:
At the point of contact with the oogonium the antheridium sends forth a delicate tube known as the fertilisation tube (C). The latter pierces the oogonial wall, passes through the periplasm and enters the oosphere (C). At this stage the tip of the fertilisation tube dissolves. A single male nucleus migrates through it to fuse with the egg nucleus.
The fusion between the two gamete nuclei may be delayed until spring. The egg with the two nuclei is called the zygote. The zygote soon secretes a thick, rough wall to become an oospore (E). The latter enters upon a period of rest. The oospore wall is differentiated into an inner thick smooth endospore surrounded by a thin rough exospore.
Germination of oospore (Fig. 6.37 G):
The following spring when the conditions favourable for growth set in, the oospore germinates. Prior to germination the male and female nuclei fuse. At the time of germination the fused nucleus undergoes repeated division to form 50 or more nuclei.
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Where meiosis takes place is not definitely known. Some hold that first two divisions are meiotic. Bose (1946), however, reported that the reduction of the nucleus to initiate the haplophase takes place in the third or fourth division of the oospore nucleus.
The prevalent view favours gametangial meiosis and diploid life cycle. At this stage the oospore wall bursts and germ tube protrudes through the small crack. The tip of the germ tube swells up to form a terminal ovate sporangium (G). The multinucleate protoplast of the oospore flows into the sporangium.
The latter is then cut off from the germ tube by a septum. Within the sporangium are differentiated 50 or more uninucleate, biflagellate, reniform zoospores. They escape through an apical pore in the sporangial wall. The liberated zoomeiospore germinates in the usual manner on the same or a different host to which it may be carried.
