Hi Jackie, I'll try to give you some info based on general interpretations, be aware that my quesses may be off the mark. I'll attach a PDF on Sodium Chlorate, not sure why they chose to use it, there are less potentially damaging products, note esp the info on page 3; persistence in soil and ground water.
SodiumChlorate.pdf
The symptoms you describe relate to a breakdown in the connection between roots and leaves, this could be caused by physical damage/problems in the roots, stems or leaves themselves. The physical problems could be caused by biological agents ie fungi, insects etc... or abiotic factors ie; damage to roots before/during planting, damage to stems etc.
Now since the symptoms appear to be affecting a wide range of plants, different species and different locations in the bed, it is unlikely (but not impossible) that it relates to poor planting technique. It is also unlikely that is being caused by damage to stems you should be able to see that yourself.
My quess is that the Sodium Chlorate was still active in areas of the bed when the plants were put in, and this has resulted in the damage. If this is the cause the rainfall you have experience should have leached the remainder from the soil, additional watering may help to speed this up, but it may not save your plants it all depends on how much of the root system has been dessicated by the salt. As I say this is my best quess based on your descriptions..additional watering to leach the remainder from the soil profile is all you can do, beyond pulling the plants and inspecting the root ball to seee if you can identify "burnt" areas. I could be wrong and it could be another cause (fungi, insect, physcial damage) but they should be more obvious. Try watering in a very dilute solution of fish emulsion and seaweed extract every day for a week, a watering can for each affected plant, if things are still getting worse pull one of the affected plants up and carefully inspect the roots. (Obviously before that carefully inspect the stems and foliage).