Fundamentals of Plant Pathology

INTRODUCTION

• Traditionally, fungi were classified under Thallophyta division of Plantae Kingdom under 4 classes, viz., phycomycetes (also called lower fungi including Oomycetes, Chytridiomycetes and Zygomycetes), ascomycetes, basidiomycetes and deuteromycetes (including fungi imperfecti).
• Ainsworth’s classification (1966, 1973) included kingdom Fungi with Myxomycota and Eumycota divisions. The Eumycota division was divided into 5 sub-divisions, viz., mastigomycotina, zygomycotina, ascomycotina, basidiomycotina and deuteromycotina which were further sub-divided into (classes, orders and so on).
• Classification system based on information on phylogenetic relationships of fungi obtained by small submit (18 S) ribosomal RNA gene comparison are now used. The classification given by Hawksworth et al. (1995) and partially modified by Kirk et al. (2001) is now universally accepted. According to it, the fungi are placed in three kingdoms: Fungi, Protozoa and Straminopila
• All the taxa (kingdom to species) are now italicized in print and underlined when hand written.

There are standard endings for the various taxa. Phylum ends in -mycota, class in –mycetes, sub-class in -mycetidae, order in –ales, family in –aceae, and genus and species have no fixed endings.

• Genus is always written with the first letter in capital (proper noun).
• Species in small letters as adjective qualifying the noun.

Kingdom Fungi
They are characterized by:

• Unicellular or filamentous somatic phase
• Cell wall made of chitin and glucans
• Absorptive mode of nutrition
• Presence of only whiplash type of flagella
• Mitochondria with flattened cristae, and
• Presence of peroxisomes and golgi bodies

There are four Phyla in Kingdom Fungi: Chytridiomycota, Zygomycota, Ascomycota and Basidiomycota

• The imperfect fungi that reproduce only asexually and were earlier placed in Deuteromycotina (Ainsworth, 1973) are no more accepted as a distinct taxonomic category. They are not monophyletic unit, but are fungi which are anamorphs (asexual stages) of Ascomycota and Basidiomycota.
• Those which have not been shifted to Asco- or Basidiomycota are placed under mitosporic or anamorphic fungi.

Kingdom Straminopila
These are characterized by:

• Unicellular or filamentous somatic phase
• Cell wall made of cellulose
• Absorptive mode of nutrition
• Presence of tinsel type of flagella
• Tubular mitochondrial christae
• Presence of peroxisomes and golgi bodies

They showed closer genetic relationship to brown algae and diatoms rather than the Fungi in 18 S r RNA phylogenetic studies.
There are 3 phyla: Hyphochytridiomycota, Labrynthulomycota, Oomycota.

Kingdom Protozoa
These are characterized by:

• The somatic phase is plasmodial or amoeboid.
• There is no cell wall in the somatic phase.
• Nutrition is by ingestion except in plant parasites (e.g., Plasmodiophoromycota), where the plasmodium obtains nutrition by absorption.
• Mitochondrial cristae are tubular.

There are 4 phyla: Acrasiomycota, Dictyosteliomycota, Myxomycota, Plasmodiophoromycota.