|This is the second plant that I have bought and almost immediately a single leaf will begin to turn|
|Dieffenbachia can be a bit temperamental and does best in conditions that might be described as "jungle-like.". Keep in mind that it does best in indirect light, high humidity (but with good air circulation) and warmer temperatures. The soil should be evenly moist, never saturated/sopping wet and never dried out. You also need to fertilize carefully to avoid over or under-supplying nitrogen. Also, if you routinely repot newly purchased plants, make sure you are using a soil mix that is a good match with the soil it came in and that you are moving it up only slightly in pot size. Based on your description I would tend to think watering is possibly not as consistent as it should be, or there might be a problem with spider mites. Maintaining humidity will help slow spider mites, as will rinsing the plant twice a week in tepid water. You can also use insecticidal soap to treat spider mites, be sure to read and carefully follow all of the label instructions. In the meantime, here is detailed information about foliage symptoms you may find interesting. You may need to cut and paste the complete url into your browser to make it work correctly.
If none of these scenarios seem plausible, you might want to consult with your local county extension and see if they can help with a diagnosis, and then knowing that specifically, determine how to proceed. I'm sorry you are having trouble with your plants.