Major groups of Plant Pathogens

The first step to identifying a pathogen is to put it into one of a few major groups. The majority of plant pathogens you will encounter will belong to one of these. They are identifiable without microscopy and after a while you will be able to recognise them without thinking. The groups are:

• Fungi
• Rusts
• Smuts
• Powdery mildews
• Leaf spots
• Other fungi
• Oömycetes
• Downy mildews
• White rusts
• Other oömycetes
• Animals
• Midges
• Mites
• Wasps
• Aphids
• Other animals
• Bacteria, Viruses, and Phytoplasmas

Rudimentary key to major groups

Currently missing:

• Exobasidium

I hope this works for most things you find, but there is no guarantee it will.

1. Flowers or leaves?
• flowers
    → 2
• leaves
    → 5


2. Down of small conidiophores visible under hand lens?
• yes
    → Floricolous Downy Mildews Peronospora
• no
    → 3


3. Spores on anthers?
• yes
    → Flower Smuts Microbotryum, Antherospora, Ustilago, Anthracoidea...
• no
    → 4


4. Small orange or yellow larvae inside?
• yes
    → Gall Midges Cecidomyidae
• no
    → other


5. White, purplish grey, or beige down present on leaves?
• yes
    → 6
• no
    → 9


6. Types of down
• Down formed of very small conidiophores on leaf spots
    → 8
• Down not restricted to neat leaf spots (sometimes a difficult distinction)
    → 7


7. Structure of down
• individual conidiophores growing up from leaf surface; white, purplish grey, or beige
    → Downy Mildews Peronosporaceae
• a web of hyphae growing outwards; generally white or greyish
    → Powdery Mildews Erysiphaceae


8. Structure of conidiophores
   • no structure visible under hand lens, flat white
       → Leaf Smuts Entyloma
   • conidiophores arranged in small, spikey-looking bunches (caespituli) emerging from a stoma
       → White Moulds Ramularia
   
   
9. Round dead spots on leaf?
• yes
    → Other fungal leaf spots
• no
    → 10


10. Orange, beige, or dark spores emerging from pustules or blisters?
• yes
    → 11
• no
    → 13


11. Rusts vs Blister Smuts
• Dark, powdery spores emerging from rather large blisters (generally >5mm) in the leaf or stem; spores easily rub off on the fingers
    → Blister Smuts Urocystis
• Orange, beige, or dark spores produced in small (<5mm) structures
    → 12


12. (Rusts vs False Rusts)
• small, wartlike structures with no spores visible under hand lens; colour is given by plant pigments
    → False Rusts Synchytrium
• Spores visible under hand lens
    → Rusts Pucciniales


13. White pustules on leaves?
• yes
    → 14
• no
    → 15


14. White rusts vs true rusts
• small (<2mm) white, powdery pustules
    → Rusts Pucciniales
• large white mass; not powdery
    → White Rusts Albuginales


15. Larvae inside
• yes
    → Insects
• no
    → 16


16. Extra white hair produced on leaves, leaves generally distorted but no obvious larvae present
• yes
    → Gall Mites Eriophyoidea
• no
    → other