The best way to get rid of spider mites is simply spraying them with a good blast of water. They like it hot and dry. Several repeats and they'll be gone... Insecticides are for insects, and mites are not insects. So by killing off the insects that may help keep the mites in check, you'll get a big bloom of mites - like you experienced.
It's hard to tell what could be wrong with your Allamanda. If it were mine, I would take it out of the pot and look at the roots. Check to see if they are healthy. They should be a light tan and firm. If they are dark brown or black or mushy (try pulling a root through your fingers - if the outside is firmly attached to the core it should be OK, but if the outside comes off as you pull on it, it is rotten), you just may be watering too much. Most plants in pots are killed by too much water, not too little. The soil in the bottom part of the pot - sometimes the whole 1/2 or more of the pot - often stays water logged. Those holes in the bottom of the pot are often not large enough to get adequate drainage. Check to see if the soil is water logged. If there are any healthy roots left, I would cut off the dead (source of fungus) and repot in new soil. After watering it in, don't water again until it starts to wilt. Plants wilt when their roots are rotten, and they wilt when they need water. We want them to wilt from lack of water because that can be fixed.
I really do think this may be your problem, but without checking the roots, we won't know for sure.
What is the main ingredient in your Bayer whatever....? It would be in the lower left of the package.
