Root Rot:
If a plant is rotting at the roots, - dying from the base up, it's fairly safe to say it's a goner. The best course of action is to take as many cutting as you can. Cuttings from the top of the plant stand the best chance of being rot free. Rooting the cuttings in separate containers will isolate good from bad and improve over all chances of success, but you should do this anyway (see advice on cuttings).

 

Branches Dying: If a plant appears to be dying from the tips of a branch, shedding leaves, with the branch turning brown and eventually dying off, this is not necessarily as serious as it first may seem. It could do this at the summer's end, or just anytime. A good example would be where the growth of leaf on other branches nearer the light source had the effect of blocking out light to lower / further back branches and leaves. In a case such as this, the plant may decide that the game isn't worth the candle and effectively close down operations and redirect it's resource to producing shoots and leaves elsewhere.

Having said that, if this doesn't ring true in your case, if more branches keep dying and your plant does not appear to be making any progress (i.e. when you would expect it to be growing), then it could be that the plant is otherwise unhappy. It may need re-potting. It may be too dry. Check the rest of the plant care advice to see if its over all circumstances could be improved.

 

Leaf Condition: A little leaf yellowing, or browning and mottling, or drying at the edges, again may not be too much to worry about. Rather like the advice for branches dying, this can be simply the plant adapting to its situation. For example yellowing leaf not getting enough sun, being crowded out by growth elsewhere.

The intuitive conclusion for leaf drying and browning is that the humidity is too low. However, before you take the decision that you simply must have a humidity tent, consider other factors too. If the plant has plenty of good leaf on it, and just a few browning in one particular area, then, as with the advice above, this could just be the result of these leaves being shaded out, - this would be particularly true for mottling leaves.

Conversely, truer for leaf drying at the edges, it could be a result of the leaves being too much at the forefront in direct sunlight and suffering while they provide shade for the others. Check the rest of the plant care advice and see if other factors could be improved. If the weather has been unusually dry, i.e. it's not your normal conditions, then you could attend to them with a spray misting gun and also water them a bit more.

A final and very important point to emphasise here is that the leaves do not last forever.  It doesn't matter how ideal your conditions are, the leaf has a life-span.  Older leaf will become mottled and dry and lose it's colour before eventually dropping from the plant.  This is not something to worry about when you have new growth happening elsewhere.  I think the older leaf is better for smoking by the way.  It doesn't seem to lose its potency and seems to burn sweeter and smoother than young leaf.

 

Pests: Salvia divinorum is supposedly a bit vulnerable to pests such as whitefly. This can be more of a problem with having a humidity tent, which can turn out to be a breeding ground for little bugs. As said in the watering section, but worth mentioning again, another thing that I do is I tend to water the plants from the bottom, that is, by adding water to the saucer rather than pouring over the top of the soil. This gives the topsoil a chance to remain relatively dry as opposed to constantly soaked. The topsoil can be wetted occasionally, but if it doesn’t also get a chance to dry out you’ll more likely suffer from bugs, not only that but mould / fungi growing on the soil.

If you do get an infestation of something like whitefly the best first option is to try to pick them all off.  Obviously you need to be very careful about using something like a insecticide.  However, it's not completely out of the question.  Some concoctions will state that they are safe for the treatment of edible plants, given that you do not consume them within a certain period of time.  If you feel you must spray your plants with an insecticide I would err on the side of caution with regard to harvesting any leaves.  Remember that the manufacturer's instructions may be based on the expectation that the plants live outside, exposed to the elements, so add at least a couple of weeks onto the manufacturer's recommended leave period and make sure you wash the leaf thoroughly after harvesting before consumption.

 

A battle with bugs