Hi folks! I haven’t found a previous discussion on this topic, but please share a link if you know of one.
I’ve been spending my time sorting through the Unknown pile in my area and have come across a few observations where I am not even sure where to start with a coarse ID. These mostly have to do with observations of damaged plants, like this one: https://www.inaturalist.org/observations/126551972
I know that many insects cause various kinds of plant damage, as do fungi, and I’m sure other things as well.
As a non-expert in this specific area, I’m wondering if there are any helpful guidelines or tips for things to look for that can be used to get a broad classification for what is causing the damage? Usually if a plant part seems to have been eaten away, I assume insect damage. But in the case of spots or other markings, I’m not sure where to start.
If you know any experts in a group you think it can be it may help to tag them, but if you just don’t know, it’s better to skip it or id as life, it can be fungal, viral, bactrial infection.
You might find Charley Eisman’s Tracks and Signs of Insects and Other Invertebrates useful for feeding damage. He also posts other information on his blog. (Information for both here.)
It also looks like there are some (mostly geographic-specific) plant pathogen projects. If one applies to your area, you might be able to add Life-level observations and see if the people involved in the project can at least get further on the ID
Here is a very good site for various plant damages. And many things are well illustrated. You can browse by host species: https://bladmineerders.nl/scientific-plant-names-genera/ Though mainly it is dedicated to mines and galls, there are some illustrations of fungi damage - for these you will need to go down to the page end of a key for mines for “all parasites per species”.
I’m a retired university plant pathologist with a specialty in tree diseases and have ID’d several fungal “plant damage” posts on iNat. One of the questions that I almost always have to ask the person who made the post is “do you know the host plant?” If the plant can be ID’d then the list of possibilities is narrowed considerably.
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:
Immersed small dots/pimples of the sexual ‘perithecia’ or the similar looking tiny ‘flasks’ of asexual ‘coelomycetes’ on the discoloured patch.
A minute forest of asexual spore-bearing ‘conidiophores’ sticking up from the affected leaf tissue (google these technical terms)
The ‘sori’ of rusts causing scabs on the surface and filled with yellow/orange/brown spores.
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.