Death touch against plants... wow, I don't think I've ever actually battled a plant creature in D&D, ever. My experience aside, I think plant death touch is... mostly useless. =D
My favorite idea so far is the always on +1 CL to Desert domain spells with the blight effect for flavor.
Here's my official proposal, then.
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Desert Domain
Granted Power
You cast spells on the Desert domain spell list at +1 caster level.
Also, when casting a spell from the Desert domain spell list while in an area with plant life, you may choose to blight the life around you, creating dead spot like a limited cancer. The boundary of the dead spot is a circle centered on you, with a radius of 1 foot per level of the spell being cast.
Within the boundary of the dead spot, all non-sentient plants instantly die and shrivel and all water evaporates, leaving behind an area of dry, dead husks and dust. Living creatures (and sentient plants) within the boundary of the dead spot are not directly affected, but plants can find no sustenance in the dust of the dead spot, water-breathing creatures die quickly when water turns to dust, and mobile animals know enough to leave the area alone. After 30 days, the effect ends, and the dead spot may be colonized naturally over the course of several years by bacteria, plants, and animals.
1st Endure Elements
2nd Soften Earth and Stone
3rd Diminish Plants
4th Hallucinatory Terrain
5th Blight
6th Move Earth
7th Control Weather
8th Horrid Wilting
9th Elemental Swarm (any, except Water)