• It is a zoosporic fungi
  • Sub division of divison eumycota
  • Plasmodium or pseudo plasmodium absent.
  • Assimilative phase typically filamentous.


  • True fungi.
  • Approx.75000 known species.
  • Thalli do not posses plasmodi or pseudoplasmodia.
  • Unicellular or filamentous.
  • Definite cell wall
  • Growth of hyphae is apical.


  • Commonly known as “zoosporic fungi”
  • Adapted mostly in aquatic habitat.
  • Most of them are filamentous & have coenocytic mycelium. However unicellular form are present, and some genera show the pseudosepta (falsecrosswall)formation.
  • Centric nuclear division. their centrioles remains functional during nuclear division.
  • Live either saprophytes or parasites.
  • Sexual reproduction by oospore formation. zoospore is flagellated fungi reproduce asexually by means of flagellated spore produced in zoosporangium.

Three types of zoospore

  • Laterally biflagellate
  • Posteriorly uniflagellate
  • Anteriorly uniflagellate .


By ainsworth, 1973  3 classes  On the basis of flagellation of  zoospore

  1. Chitridiomycetes
  2. Hypochitridiomycetes
  3. Oomycetes


  • The vegetative body is unicellular or chain of cells attached with substratum by rhizoids.
  • Cell wall is made up of chitin & glucans.
  • Plant body is normally haploid. except allowmycetes
  • Produces posteriorly uniflagellate zoospore.



  • By zoospore produced in zoosporangium.
  • Zoospore are uniflagellate, flagellum whip lash  type & posteriorly placed


  • By pianogametes developed in gametangia fused to form zygote.
  • Meisios forms new haploid thallus.


  • Halocarpic-primitive forms may consist of a unicellular thallus without specialized vegetative parts which converted to reproductive organ. Eg:  olpidium
  • Eucarpic-reproductive organs arise from only a position of the thallus.
  • Eucarpic monocentric– rhizoidal system bears only a single reproductive structure.
  • Polycentric-rhizoidal system bears more than one reproductive structure.


  1. Chitridiales  -True mycelium lacking rhizoids or rhizomycelium present in some species.
  2. Blastocladiales- sexual reproduction present zoospores with  nuclear cap; thick walled ,resistant sporangia formed.
  3. Monoblepharidales-Sexual reproduction, no resistant sporangia formed.
  4. Spizellomycetales-mainly in soil.
  5. Neocalimasticales- occur in rumen.
examples of  chitridiomycetes :-
Synchytriun endobioticum: cause wart disease of potato.
Olpidium  brassicae: cause  wart diseases in roots  of crucifers.
Urophlyctis alfalfae: causes crown wart of alfalfa (medicago)
Physoderma  maydis: causes brown spot of maize.


  • Aquatic fresh water or marine fungi.
  • Zoospore are anteriorly uniflagellate .
  • Thallus is halocarpic or eucarpic,&monocentric or polycentric.
  • Vegetative system is rhizoidal or hypha like with intercalary swellings.
  • Cellwall contains chitin or chitin with cellulose.
  • The sporangia are inoperculate .
  • The zoospore are either completely differentiated in the sporangium. & zoospore differentiates outside the sporangium.
  • Similar to morphological structure of chytridiomycetes but differ from them in the structure and anterior flagellation of the zoospore.


  1. Hyphochytriales  or  anisochytridiales
  • Based on the organization, morphology & development of the thallus 3 families:
  1. Anisolpidiaceae
  2. Rhizidiomycetaceae
  3. Hyphochytriaceae


  • On the basis of differences in molecular weights of rhibosamal RNA lovett & haselby(1971)found  that  anisochytridse could not be placed in any other major class of fungi.
  • Bartnicky Garcia (1970) also placed the anisochytrids outside the main phylogenitic line of fungi.But nearer the oomycetes.
  • Le John (1972) on the basis of regulation of glutamic dehydrogenase activity proposed that anisochitrids are predessors of both the chytridiomycetes & oomycetes.
  • The hyphochitridiomycetes differ from the  Chytridiomycetes on the basis of the pathways of  lysine synthesis, the nature of the flagella as well as the cellwall  composition (cellulose chitin).
  • In hyphochytridiomycetes diaminopimelic pathway of lysine synthesis occurs, where as in the chytridiomycetes it is aminoadipic.
  • Also difference in the position of the flagellum on the planospores.
  • This justify place in a separate class hyphochytriomycetes.


  • Vegetative body is filamentous and coenocytic except the unicellular lagenidiaies.
  • Halocarpic or eucarpic.
  • Cellwall contains cellulose and glucans &chitin is absent.


  • Asexual reproduction :

  • By biflagellate heterokont & anisokont zoospore that are produced in zoosporangia.
  • Zoosporangia- modified hyphae that are usually terminal delimited by a septum.
  • Zoospores are diploid formed by mitosis.
  • Anteriorly directed flagellum is tinsel type & posteriorly directed is whiplash type.
  • Depending on genera two types of zoospores are formed.
  • Two  types of zoospores are there formed in the  lifecycle:
  • Primary zoospore-First formed pip shaped&the flagella are located anteriorly, which is released from the zoosporangium, encyst & germinates to form the secondary zoospore.
  • Secondary zoospore-The secondary zoospore which is reniform or bean shaped and laterally flagellated.
  • Zoosporangium and zoospores are the major dispersal agents for most species.

Sexual reproduction:

  • Sexual reproduction is heterogamous (oogamous) by oogonia (female) and antheridia (male)
  • Female gamete (oosphere) produced by oogonium.
  • Depending on taxa there may be one too many oospheres per oogonium. male gamate is produced by antheridium and transferred to the oogonium by gametangial contact and migration of male nuclei into oogonia and fertilize oosphers Homothalic- self fertile or heterothalic-opposite mating type required for sexual reproduction.
  • A swimming sperm is absent in the oomycetes.
  • This type of sexual reproduction is referred to as gametangial copulation.
  • In antheridia & oogonia meiosis take place.
  • The eggs and sperms are products of meiosis and the only parts of the lifecycle that are haploid. diploid zygote develops into thick walled resistant oospore that germinates and gives rise vegetaive diploid hyphae that reproduce asexually by production of zoospores.


  • Four  orders

  1. Lagenidiales (salilagenidials)
  2. Leptomitales
  3. Saproleginales
  4. Peronosporales- This order has some of the most well known pathogens cause diseases to many crop plants. It divides into 3 families;
1. Pythiaceae- pithium, phytophthora
2. Peronosporaceae- plasmopara
3. Albuginaceae- albugo
       The most economocally important group of oomycetes is the peronosporales.
       It contain the late blight of potato fungus phytophthora infestans  and relatives such as peronospora, bremia, plasmopara and others that cause ‘downy  mildews’ ,the ‘damping off’ fungi,pythium spp. And the white rust fungi, albugo spp.


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