IDing plant damage - Insects vs. Fungus vs.?

Nature Talk
6

Host identification is critical to naming most leaf-spots. In addition, plant damage might also be due to mites, viruses, phytoplasmas etc.

For fungal damage (i.e., pathogens infecting the plant tissue and not just sitting on the leaf and getting nutrients from elsewhere) then you are looking for evidence that fungal mycelium is growing into the tissue where it usually causes senescence and change in color of the surrounding leaf surface. However, damage by insects can also cause tissue senescence, so in addition you are looking for the spore-bearing structures of the fungus. These will generally fall into the categories of:

  1. Immersed small dots/pimples of the sexual ‘perithecia’ or the similar looking tiny ‘flasks’ of asexual ‘coelomycetes’ on the discoloured patch.
  2. A minute forest of asexual spore-bearing ‘conidiophores’ sticking up from the affected leaf tissue (google these technical terms)
  3. The ‘sori’ of rusts causing scabs on the surface and filled with yellow/orange/brown spores.
  4. The white surface mycelium of powdery and downy mildews.

Often a fungus isn’t at the stage where any of these structures are present, and all you see is tissue damage that cannot be attributed reliably. Even if spore-bearing structures are present you ideally need a microscope to confirm what you are seeing. Species-level identification usually requires that level of examination. Identification by the macroscopic appearance of general symptoms (rather than the causal agent confirmed by 1-4) is usually guess-work and too prone to error for many (not all) spots/hosts.

If the darkened areas of leaf tissue form angular rather than amorphous spots then it indicates a possible plant pathogenic bacteria and not a fungus or an insect/mite. Plant bacteria spread in leaf tissue but initially stop when they reach leaf-veins, so they look angular.

For insect damage then you are looking for obvious signs of browsing without accompanying extensive discoloration due to mycelium growth (like the insect excavated pits in your link, showing no other leaf tisue damage), or you are looking for pin-pricks in the leaf surface where an insect has poked its stylus through the leaf surface to suck sap. These are often also associated with leaf secondary leaf senescence.

Galling of the plant host tissue can also be caused by various agents, and gall symptoms often a reliable guide to identity.

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