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.

Rudimentary key to major groups

I hope this works for most things you find. I am still developing the key so take a look at the list of things that don't key out at the bottom.

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 Cecidomyiidae
• no
    → other


5. White, purplish grey, or beige down present on leaves? This includes leaf spots with a flat white colour that do not kill leaf tissue.
• yes
    → 6
• no
    → 9


6. Types of down
• Down 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 (note these sometimes need microscopy to distinguish)
   • No structure visible under hand lens, flat white.
       → White Leaf Smuts Entyloma
   • Conidiophores arranged in small, spikey-looking bunches (caespituli) emerging from a stoma.
       → White Moulds Ramularia, Phacellium
   
   
9. Round dead spots on leaf?
• yes
    → Other fungal leaf spots
• no
    → 10


10. Are there orange, beige, or dark pustules or blisters?
• yes
    → 11
• no
    → 13


11. Orange, beige, or dark pustules or blisters
• Dark brown to black, powdery spores emerging from rather large blisters (generally >5mm) in the leaf or stem; spores easily rub off on the fingers.
    → Blister Smuts Urocystis
• Smooth black rounded to linear pustules on the leaves of grasses; no spores visible under hand lens.
    → Phyllachora
• Orange, beige, or dark spores produced in small (<5mm) structures. Individual spores may not be visible under the hand lens.
    → 12


12. Rusts vs False Rusts
• Small, wartlike structures with no spores visible under hand lens; colourless or coloured by pigments inside swollen plant cells. The warts have a smooth shiny surface and do not break the surface of the leaf.
    → Chytrids Synchytrium or False Rusts (Note that the spermogonia of true rusts may also key out here; microscopy may be required)
• Spores visible under hand lens.
    → Rusts Pucciniales


13. White raised 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
    → Gall Wasps or Gall Midges
• no
    → 16


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

Stuff that doesn't key out

• Exobasidiales
• Polythrincium trifolii
• Ergots