Classification of Fungal Plant Pathogens

Introduction

Fungi, which were traditionally classified under the Thallophyta division of the Plantae kingdom, have been subject to evolving classification systems over the years. Initially, four classes were recognized in the traditional system: Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes (also known as fungi imperfecti). Over time, as scientific understanding advanced, particularly with the development of molecular techniques, the classification of fungi became more refined.

Historical Classification Systems

1. Ainsworth’s Classification (1966, 1973):
   • Ainsworth’s system included the kingdom Fungi, subdivided into two divisions: Myxomycota (slime molds) and Eumycota (true fungi).
   • The Eumycota division was further divided into five sub-divisions:
     • Mastigomycotina (Flagellated fungi)
     • Zygomycotina (Zygomycetes)
     • Ascomycotina (Ascomycetes)
     • Basidiomycotina (Basidiomycetes)
     • Deuteromycotina (Fungi imperfecti, including fungi with only asexual reproduction)
2. Hawksworth et al. (1995) & Kirk et al. (2001) Classification:
   • This system, based on phylogenetic relationships derived from 18S ribosomal RNA gene comparisons, is widely accepted today.
   • Fungi are placed into three kingdoms: Fungi, Protozoa, and Straminopila.
   • Taxa (from kingdom to species) are italicized in print and underlined when handwritten.
   • Specific endings for taxonomic ranks are as follows:
     • Phylum: -mycota
     • Class: -mycetes
     • Sub-class: -mycetidae
     • Order: -ales
     • Family: -aceae
     • Genus and species: No fixed endings.
   • Genus is capitalized and species is in lowercase.

Characteristics of Kingdom Fungi

• Somatic Phase: Unicellular or filamentous.
• Cell Wall: Made of chitin and glucans.
• Mode of Nutrition: Absorptive (heterotrophic).
• Flagella: Whiplash type (in some groups).
• Mitochondria: Flattened cristae.
• Peroxisomes and Golgi Bodies: Present.

There are four major phyla in the kingdom Fungi:

1. Chytridiomycota
2. Zygomycota
3. Ascomycota
4. Basidiomycota

Deuteromycota (Imperfect Fungi)

• Fungi that only reproduce asexually were once grouped in Deuteromycotina (Ainsworth, 1973).
• However, they are no longer considered a distinct taxonomic category, as they are anamorphs (asexual stages) of Ascomycota and Basidiomycota.
• Fungi not shifted to Ascomycota or Basidiomycota are classified under mitosporic or anamorphic fungi.

Kingdom Straminopila

Fungi in this kingdom exhibit characteristics that place them closer to brown algae and diatoms based on 18S rRNA phylogenetic studies. They include:

• Somatic Phase: Unicellular or filamentous.
• Cell Wall: Made of cellulose.
• Mode of Nutrition: Absorptive.
• Flagella: Tinsel type (whip-like structures with lateral hairs).
• Mitochondria: Tubular cristae.
• Peroxisomes and Golgi Bodies: Present.

Three major phyla are placed under Straminopila:

1. Hyphochytridiomycota
2. Labrynthulomycota
3. Oomycota (including plant pathogens like Phytophthora and Pythium)

Kingdom Protozoa

Organisms in this kingdom are characterized by:

• Somatic Phase: Plasmodial or amoeboid.
• No Cell Wall in the somatic phase.
• Nutrition: Ingestive, except for certain plant parasites like Plasmodiophoromycota, which obtain nutrition through absorption.
• Mitochondria: Tubular cristae.

There are four major phyla under Protozoa:

1. Acrasiomycota (e.g., cellular slime molds)
2. Dictyosteliomycota (e.g., social slime molds)
3. Myxomycota (e.g., true slime molds)
4. Plasmodiophoromycota (plant pathogens like Plasmodiophora brassicae, causing clubroot disease in cruciferous plants)