There is a single class, Oomycetes, which is further divided into 7 orders. We shall study some genera belonging to the orders Saprolegniales, Pythiales and Peronosporales.
Order Saprolegniales (Water Molds):
The term ‘water molds’ is generally used for genera belonging to this order. The zoospores are formed in zoosporangia. A peculiar feature of this order is the production of two types of zoospores, the primary and secondary zoospores, in succession. This is called dimorphism, or less accurately, diplanetism. Sometimes, one form of zoospore is represented by a cyst. Another important feature is the absence of periplasm in the oogonium. There are 2 families- Saprolegniaceae and Leptolegniellaceae.
Family Saprolegniaceae:
The oospores have two layers, the outer ooplast and the inner endospore. The zoosporangia are filamentous or cylindrical. We shall study 2 genera, Saprolegnia and Achlya. In Saprolegnia, both the zoospores are vigorously motile, while in Achlya, the primary zoospores encyst at the mouth of the zoosporangia.
Genus Saprolegnia:
Habitat:
It is an aquatic submerged fungus found in well aerated fresh water ponds. There are 13 species which occur frequently on submerged plant and animal debris, especially dead fish. S. parasitica is pathogenic on fish and other aquatic vertebrates. The species are primary sugar fungi and they fail to decompose the complex polysaccharides like cellulose.
Thallus:
It consists of branched, coenocytic hyphae.
Isolation and Culture:
House flies, ants’ eggs or Brassica seeds are used as baits to isolate the fungus from water. Seeds should be boiled for 2-3 minutes until the coat splits. The baits should be put in large Petri dishes containing pond water. Within 2-3 days characteristically coarse, septate hyphae appear which can be taken in a Potato Dextrose Agar (PDA) Petri plate.
Genus Achlya:
Habitat:
Achlya, with 50 species, is the largest genus of the family. It is found in ponds and moist soils. It can be isolated in the laboratory like Saprolegnia, using hemp seeds.
Thallus:
Hyphae are coenocytic, highly branched and have cellulose in the cell wall. Some of the hyphae penetrate the substratum and form rhizoids, while the rest remain above and form the hyphal system.
Order Pythiales:
Genus Pythium:
This is a large genus having 127 species which cause ‘root rot’ and ‘damping off of seedlings.
Some important species of Pythium and the diseases they cause are the following:
1. P. debaryanum- damping off of tobacco and chillies.
2. P. aphanidermatum- soft rot of papaya and damping off of potato.
3. P. graminicolum- rhizome rot and root rot of turmeric.
4. P. myriotylum- foot-rot of ginger
Habitat:
Although some species are aquatic, parasitising algae, majority of Pythium species are soil inhabitants causing damping off disease of seedlings of several economically important crops. In damping off diseases, the seedlings topple down due to rotting of tissues of basal portion of stems or roots. The fungus can grow saprobically in soil, as well as parasitically in plants.
Isolation and Culture:
Pythium can be obtained by germinating mustard seeds in soil contained in Petri dishes. Some seedlings topple down. The fungus can be isolated from such seedlings by aseptically placing small rotted basal tissues on PDA (Potato Dextrose Agar) plates or tubes. Pythium grows very fast. Some species form sporangia only on natural substrates like sterilized grass, oat meal or when supplied with certain sterols in the medium.
Thallus:
The mycelium consists of slender coenocytic and branched hyphae which contain cellulose in their cell walls. Septa are formed only to delimit reproductive organs or to cut off older hyphae. In host tissues the hyphae are both intercellular and intracellular. Haustoria are not produced.
Genus Phytophthora:
Phytophthora, which shows much resemblance to Pythium, however, differs in several ways.
The important differences lie in the following characters:
1. The method of sporangial germination (no vesicle is formed and the zoospores are fashioned in the sporangium itself).
2. The production of distinct sporangiophores, which are easily distinguishable from the somatic hypha.
3. Sporangia are lemon-shaped with a distinct papilla.
4. The resumed growth of the sporangiophore tip pushes the terminal sporangium to lateral position. Repeated occurrence of this results in a sympodial zig-zag growth of the sporangiophore, bearing swelling marks of detached sporangia.
5. Phytophthora completes its life cycle only within its host tissues. It has no saprobic existence in soil. It lives as dormant mycelium in the dead host-remains lying in the soil.
6. Shows requirement for thiamine.
Habitat:
Phytophthora survives in the dead host tissues, left in the soil. P. infestans, as a rule, survives as dormant mycelium in infected potato tubers. However, it can be cultured in laboratory.
Thallus:
The hyphae are coenocytic and highly branched, which grow mainly intercellularly in the host tissues. The fungus derives nourishment from the host cells through specialised food-absorbing organs, called haustoria (singular haustorium). These are peg-like projections of the hyphae into the host cells. The optimum temperature for fungal growth is around 20°C; above 26°C the hyphae die.
Isolation and Culture of Phytophthora:
Phytophthora can be isolated from the soil with the help of an apple, used as bait. Take an unripe apple. Stab one side of it with a needle and press it into the soil. After a few days, take it out. A brown rot appears on the fruit. Wash the fruit thoroughly and place a portion of the rotted tissue in a dish containing tap water. A fluffy mycelial growth appears in 24-28 hrs. Some hyphae are lifted from the periphery with a needle and transferred to a PDA tube. A pure culture of Phytophthora is obtained.
Order Peronosporales:
Genus Albugo:
The genus has 44 species, all obligate parasites of higher plants, causing white blisters (areas of raised host epidermis) on the foliage. The name ‘white rust’ is a misnomer, because the name ‘rust’ is used only for the fungi belonging to order Uredinales of class Urediniomycetes (Basidiomycota) and the diseases they cause. In addition to the blisters, the fungus causes hypertrophy of different floral parts, and forms tumors of varying sizes on all parts – stems, leaves, veins, petioles, etc.
The important species are the following:
1. A. Candida – causes white blisters on members of Cruciferae like Brassica, cabbage, Raphanus, ‘Horse radish’, Capsella, etc. Sometimes these diseases cause considerable damage.
2. A. Albugo ipomae-panduranae – on Convolvulaceae.
3. A. bliti – common on Amaranthaceae.
4. A. tragapogonis – on members of Compositae.
5. A. occidentalis – infects spinach.
6. A. portulaceae – on Portulaca.
Habitat:
The fungus is an obligate parasite and grows in the intercellular spaces of host tissues. In the absence of the host, the fungus survives on host debris as oospores – or dormant mycelium. It has no saprobic growth.
Thallus:
Coenocytic tubular hyphae lie in the intercellular spaces of the host and derive nutrition from the host cells through knob-like haustoria.
Physiological or Biological Specialization in Albugo:
Any species of Albugo, e.g., A. Candida, which is identified by certain morphological criteria, like size of sporangia and the markings on the oospore wall, attacks only members of the family Cruciferae, and not others. But, A. Candida that infects one host, e.g., Brassica, will not infect radish.
This means that A. Candida obtained from different hosts are not the same and, therefore, have to be named differently. Thus, the species are divided into groups, below species level. These are called biological forms and the phenomenon biological specialization.