All pesticides should be used immediately after mixing. Not only do most of them begin to lose their efficacy right away, but when stored for long periods of time, these highly hazardous chemicals can evolve into something quite dangerous.

Only time will tell if you've killed the plant or just fried the foliage. I'd be very patient and play the waiting game. I'm not even sure that I would fertilize at the present time!

Bobber,

A properly diluted batch of Malathion mixed last season probably would not induce the reaction you have experienced. And I doubt it could "evolve" in the traditional sense of that word into anything more dangerous to the plant (or, thankfully, to you). It actually should break down in water. I am not sure of that, but even some rather heavily anti-Malathion web-sites that I have looked at concede that water, soil-born bacteria, and sunlight all break it down fairly rapidly into things that are less toxic to humans and I gather most insects (but not to amphibians, alas).

I hate to ask, but, since you yourself raised the spectre by mentioning it, is it remotely possible that you did in fact spray the plant with a batch of premixed Roundup? Roundup is mixed with water in the bottle, and therefore I suspect it might still be very potent even after a season or two. The main active ingredient in Roundup is Glyphosate. Cornell University has a report (http://pmep.cce.cornell.edu/profiles/extoxnet/
dienochlor-glyphosate/glyphosate-ext.html) that says "In water, glyphosate is strongly adsorbed to suspended organic and mineral matter and is broken down primarily by microorganisms also. Its half-life in pond water ranges from 12 days to 10 weeks."

It seems to me that if half of it can last up to ten weeks in pond water, which is filled with microorganisms, then if you mixed a batch in fairly clean tap water it might remain highly active for many months.

Suggestion: If you think you may have sprayed Roundup on your citrus, cut ALL the effected woody material away ASAP. If the plant is dying from the effects of a sprayed on systemic herbicide, your best hope is to cut away the affected wood. If you are lucky, the plant will resprout. I have a big orange that died to literally the ground due to a lack of water, and it resprouted, so even if you have to cut the plant back to the ground you should not give up all hope immediately. But every moment you wait, if you have indeed sprayed the plant with Roundup, is not helping the odds.

Remember, in any case, that leaving dead wood on the tree does not help the part of the tree that may be alive: it actually is going to further stress the still viable part of the plant, since water will be drawn into the dying cells.

Hope this helps.

Don