Help! Sudden death strikes annual flowers...!

I have noticed an alarming trend regarding some of my annual that grow multiple stalks, esp. snapdragons and Victoria blue salvia. A single stalk on an otherwise healthy blooming plant suddenly wilts and eventually falls over. The rest of the plant seems OK...but then a few weeks later another stalk goes! I have large numbers of these plants that I put down last fall so I really don't want them all to die by inches! Is this some kind of fungus? There are no spots or any other signs on the leaves.

It sounds like some sort of virus.....since you're in such a warm climate, these plants act almost perennials, right? I'd guess something is in the soil, and the problem is perpetuating itself each season since there's no real soil freezing going on.

You might need to do one of two things: rotate in some different plants, or try solar sterilization of your soil.

Soil sterilization isn't pretty, but it isn't difficult, and will kill the "bad stuff" in your soil. Just remove all the plants and any dead material, deeply moisten the soil - you want it damp down to a foot deep, rake smooth and cover with clear plastic, sealing the edges tight by tamping the soil back around it, or laying boards or bricks all along the edge. Since your days are undoubtedly already quite warm, you can probably get away with 4 weeks of sterilization. When the time is up, remove the plastic, and your soil will be ready to plant. Unfortunately, the timing would mean you'd be ready to plant by mid-May to early June, which is pretty late for you to put in new plants and have them establish a root system without too much stress from the heat.:(

Or, if you don't want to have the bed lay dormant this summer, you can dig out all the soil and replace it with new, which sounds drastic, but I'd do it if the bed was small and/or in a very visible spot (like next to my front door.)

I wonder also if it could be something eating the roots!

Exactly what I thought when I first read the symptoms. If you have moles they will creates air pockets and the plants decline like you described. It's easy to check. Just probe around in the soil next to the plant and see if you can find the tunnels. If that happens to be what it is, let me know how you deal with it as it is an ongoing fight here!!

If it's not moles, could it be some kind of boring insect? I have to remind myself that you're in a much warmer climate than I am, and your growing season is several weeks ahead of mine. You might check one of the stalks that has succumbed, and slit it open lengthwise to check for signs of tunneling.

I'm in Oregon and we have a hard time growing snaps here because of what I've been told is Tomato Spot Wilt Virus (see this link):
http://vegetablemdonline.ppath.cornell.edu/factsheets/Virus_SpottedWilt.htm

I grow cut flowers and last year I had two long rows of Snapdragons coming along just fine and then they got these little black spots on the back of the leaves and then the plant died. I have since found this is very common and it is spread by thrips. You didn't say you have spots, but you might consider this possibility.

This is a great site - thanks! I don't think this is what is affecting my snaps but considering it seems to be widespread in the south I've probably had it as well (or will have soon!) Now at least I'll know what it is if I see it.

We don't have moles here but go-vols your "boring insect idea sounds likely since only one stalk of the plant is affected at at time. Sometimes the rest of the plant goes right on growing just fine, sometimes another stalk will eventually succumb. Assuming its a virus - sterilization is not really an option since I have snaps and salvia in EVERY bed in the garden! Next time I see an affected stalk I'm going to pull it right away and check inside for a bug.

You might pull up the next victim and examine its roots for signs of insect damage there.....let us know what you find, and good luck in your detective work!