In Version Ars_Elemental 1.20.1-0.6.7.8 for 1.20.1, (Lesser) Earth Focus does not work correctly when casting Poison Spore directly on Undead.
In the wiki it says it's meant to apply Hunger before the damage to make sure it can also proc, but it does not.
The only time I have successfully gotten it to affect Undead is when I use it on non-undead enemies and the AoE damage hits undead. This limits the usefulness of poison spores since the damage is already low and the AoE is not as reliable as cold snap and flare.