Most animals that could be considered pathogens of plants are arthropods, in particular insects and mites. However, in the soil there is a whole world of pathogenic nematodes (microscopic worms), which form galls on roots. Some nematodes even form galls on leaves, perhaps dispersing through water splashing between plants.
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I have left the vast majority of clades out of this tree. Those included are key groups of gall-formers.
Most midge galls consist of rolled-/curled-up leaves that enclose the larvae in a small chamber. Some form fully enclosed galls inside stems (right). The larvae are generally bright to pale orange.
Note I am not including non-galling mites such as Red Spider Mite here. These are also diverse and apparently at least somewhat host-specific.
Gall mites often cause the leaves of a plant to turn in on themselves and grow more hair than normal. On trees there are many species that form more classic ‘gall’-looking structures, also on the leaves (right).
Gall wasps are extremely diverse. Some wasp galls can be host to many different species, including the initial galler, commensals, parasitoids, and hyperparasitoids.
Note I am only including galling species here.
Most plant-parasitic nematodes form galls on the roots of plants, so you are unlikely to encounter them without searching. However, some form galls on leaves, such as Subanguina graminophila (pictured).
Thrips are often host-specific and can cause gall-like symptoms.