I can't tell you what's infecting the arabidopsis, but I've had recurring
problems with a bacterial disease in my tobacco plants and here's what I
do. I autoclave my soil, and wash/soak the pots in 1 to 5% bleach. I also
bag infected plants and I do not compost them or the soil they're in. If I
am diligent, I keep the infection down to a very manageable level. If I
get lax, it comes back with a vengence. Those steps might be worth a try.