Oomycetes

MICROBIOLOGY

1. TOPIC OOMYCETES Presented By Samadrita Banik ST. GEORGE COLLEGE M.Sc Microbiology 1 st semester

2. The  oomycetes , also known as “water molds”, are a group of several hundred organisms that include some of the most devastating plant pathogens. The diseases they cause include seedling blights, damping-off, root rots, foliar blights and downy mildews. INTRODUCTION

3. Oomycota , phylum of fungus like organisms in the kingdom Chromista . Oomycetes may occur as  saprotrophs  (living on decayed matter) or as parasites living on higher  plants  and can be aquatic, amphibious, or terrestrial. The  species   Phytophthora infestans  famously destroyed  Ireland’s   potato  crop with  late blight  and caused the  Great Famine  of 1845, which resulted in a mass migration of Irish people to the United States.  Oomycetes

4. "Oomycota" means "egg fungi", referring to the large round  oogonia , structures containing the female gametes, that are characteristic of the oomycetes. The name "water mold" refers to their earlier classification as fungi and their preference for conditions of high humidity and running surface water, which is characteristic for the basal taxa of the oomycetes. Etymology of OOMYCETES

5. Class:  Oomycota; Arx, 1967 Scientific name:  Oomycota Phylum:  Heterokontophyta Rank:  Class Higher classification:  Heterokont Domain:  Eukaryota Scientific Classification

6. The oomycetes rarely have septa , and if they do, they are scarce,appearing at the bases of sporangia, and sometimes in older parts of the filaments. Some are unicellular, while others are filamentous and branching. Morphology of OOMYCETES

7. The  Saprolegniales  are the most widespread. Many break down decaying matter; others are parasites. The  Leptomitales  have wall thickenings that give their continuous cell body the appearance of septation. They bear chitin and often reproduce asexually. The  Rhipidiales  use rhizoids to attach their thallus to the bed of stagnant or polluted water bodies. The  Albuginales  are considered by some authors to be a family (Albuginaceae) within the Peronosporales, although it has been shown that they are phylogenetically distinct from this order. The  Peronosporales  too are mainly saprophytic or parasitic on plants, and have an aseptate, branching form. Many of the most damaging agricultural parasites belong to this order. The  Lagenidiales  are the most primitive; some are filamentous, others unicellular; they are generally parasitic. Classification

8. Oomycetes  can  reproduce  asexually, by forming a structure called a sporangium or zoosporangium. Inside these sporangia, zoospores are produced, first the primary zoospore and then the secondary zoospore, which is laterally flagellated. Their flagellum allow the zoospores to move rapidly through water. Reproduction of oomycetes

9. (i) Most of the members are aquatic but some grow on the soil and the others attack the shoots of terrestrial plants. (ii) The primitive forms are unicellular, whereas the advanced forms have a well-developed branched, filamentous, and coenocytic mycelium. (iii) The cell wall is mainly composed of cellulose- β- glucan and chitin is altogether absent. (iv) In primitive aquatic form, zoosporangia are not well differentiated from somatic hyphae, whereas in advanced forms distinct zoosprangia are found. (v) Some forms also produce non-motile asexual spores, known as conidia. (vi) Most of the Oomycetes are eucarpic, but the members of the order Lagenidiales are holocarpic. (vii) The sexual reproduction is of oogamous type. The male and female sex organs are known as antheridia and oogonia, respectively. The sex organs are either terminal or intercalary. Salient features of oomycetes

10. Many terrestrial Oomycetes cause serious plant diseases such as late blight of potato , downy mildew of grape-wine , white rust of crucifers , damping off of seedlings and others.   Late blight of potato and downy mildew of grape-vine are historically very important. The late blight of potato resulted in Irish Famine’ in mid-19th century that caused death of millions of people and Europe. Downy mildew of grape-vine helped Prof. Millardet to discover the first fungicide called ‘Bordeaux mixture’. Many members of this group help understand the evolutionary migration of fungi from aquatic to terrestrial environments. Significance of oomycetes

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