Phylum: Chytridiomycota The phylum Chytridiomycota contains the single class Chytridiomycetes. General characteristics of Chytridiomycota: 1. They produce motile cells at some stage in their life cycle. The motile cells of these organisms each possess a single, posteriorly directed whiplash flagellum, exception: few species have poly flagellate cells. 2. Thallus is coenocytic, whether a globose or a well developed structure, an elongated single hypha or a well developed mycelium. 3. The conversion of the zygote into a resting spore or resting sporangium. 4. Cell wall of these fungi is known to contain chitin and glucan. Example: Synchytrium endobioticum causes the disease known as black wart of potato. 1
Phylum: Oomycota General characteristics: 1. The division Oomycota, unlike the Chytridiomycota, is a group of aquatic fungi that have cell walls composed of cellulose and a diploid dominant lifecycle (=gametic lifecycle). 2. The mycelium is coenocytic and produces septa only to separate the reproductive structures from the assimilative portion of the thallus. 3. Asexual reproduction is by zoospores that are produced in zoosporangia. The zoospores produced are biflagellated with a longer tinsel flagellum directed forward and a shorter whiplash flagellum directed backward. 4. Sexual reproduction is heterogamous and occurs by direct injection of the male nuclei (=sperms) from the antheridium into the eggs contained in the oogonium. A swimming sperm is absent in the Oomycota. This type of sexual reproduction is referred to as gametangial copulation that results in the production of a thick walled sexual spore termed an oospore. 2
5. Their Cell walls composed of primarily of ß- glucans but also containing the amino acid hydroxyl prolline as well as small amount of cellulose. 6. The eggs and sperms are products of meiosis and the only parts of the life cycle that are haploid. Only one class, the Oomycetes, is recognized in this division. The combination of cellulose cell wall, biflatellated zoospores, one flagellum of the tinsel type and the other of the whiplash type, and gametangial copulation are characteristics that are shared with some members of the algal divisions Phaeophyta and Chrysophyta. This has recently led to recognition of yet another kingdom, Stramenopila, which includes the divisions Chrysophyta, Phaeophyta, Hyphochytridiomycota and Oomycota. These divisions are now thought to be derived from a common ancestor Class: Oomycetes The characteristics of the class Oomycetes is the same as those of the division. We will examine two representatives in the Oomycota: Pythium spp. and Phytophthora spp. (Peronosporales). 3
Phylum Oomycota has one class Oomycetes. Oomycetes class is grouped into five orders: 1. Leptomitales 2. Saprolegniales 3. Rhipidiales 4. Lagenidiales and 5. Peronosporales Order peronosporales has three well defined families namely, i) Pythiaceae ii) Peronosporaceae and iii) Albuginaceae Family: Pythiaceae The most important genera of this family are Phytophthora and Pythium. Both are plant pathogenic causing many severe diseases of plants. e.g. Phytophthora infestans (Fig. 7) Pythium debaryanum (Fig. 8) Fig. 7. Sporangia of Pythium Fig. 8. Sporangium of Phytophthora 4
Economic importance of Pythium spp. Members of the genus Pythium are soil inhabiting organisms that occur all over the world. They are however, most commonly encountered in cultivated soils where they pose serious problems for various agronomicaly important crops. Pythium species are the most important causes of pre and post emergence damping off diseases of seedlings. The term post emergence damping off refers to a condition in which young seedlings are attacked and rotted near the soil line, causing them to fall over. This condition is particularly troublesome in green house, where large numbers of seedlings are grown in close proximity to one another. Pythium species also attack and rot seeds in the field and may destroy seedlings before they emerge from the soil, this condition referred to as pre emergence damping off. Soft fleshy plants and plant parts such as squash, cabbage, beans and potatoes also may be rotted by certain species of Pythium. Economic importance of Phytophthora spp. Phytophthora species cause a wide variety of diseases on tremendous number of host including both herbaceous and woody 5
plants. Most species cause rots, damping off of seedling and rots of lower stems, tubers and corms. Others cause blight of the foliage, young twigs and fruits. Some plant disease caused by Pythium and Phytophthora species. Diseases caused by Pythium spp. Diseases caused by Phytophthora sp. Disease with Caysal fungus Disease with host Caysal host fungus Damping off of debaryanum Late blight of infestans tobacco, Chilli potato and tomato Rhizome rot of ginger Damping off of potato Soft rot of papaya Rhizome and root rot of turmeric Damping off of vegetables seedlings myriotylum aphanidermatum aphanidermatum graminium ultimum Leaf blight of Colocasia Damping off, root rot fruit rot of many plants Black shank of tobacco colocasiae cactorum nicotianae Bud rot of coconut palmivora Leaf blight of parasitca sesame var.sesami 6
Family: Peronosporaceae: This is the highly specialized family in Peromosporales. All species are in nature biotrophic parasites of higher plants, causing diseases known collectively as downy mildews. The important diseases caused by Peronosporaceae are: Dowry mildew Plasmopora viticola Dowry Peronospora of grape vine mildew of destructor onion Dowry mildew Bremia lactucae Dowry Pseudoprenospora of lettuce mildew of cubensis cucurbits Family Albuginace: Obligate parasites of higher plants, the diseases caused by the fungus known as white blister or white rast. There are about 30 species under the genus. Only one genus Albugo. Some examples of some plant pathogenic species are shown below: Albugo candida attacks crucifers A. occidentalis on spinach A. bliti on various members of Amaranthaceae 7