Answer: Salvias can be subject to some diseases and infections, but most commonly they are healthy plants if grown under the right conditions. They need full sun, good air circulation and soil with excellent drainage. They are drought tolerant and will develop root problems in overly wet soil. In an unamended heavy clay soil or if planted too deeply or with excess mulch applied, or if subjected to overhead watering on a constant basis, they may develop problems.
In my experience too these plants do better if they are divided about every second year in order to keep the plants growing vigorously. If your plants were a large size and have been in the ground for some time they may have needed that treatment last spring. It is a bit late in the season (nearly October) for dividing plants. It would be better to do it very early next spring.
At this point you might want to consult with your county extension to see if they can determine the exact cause for the deterioration. In the meantime, remove any damaged foliage/stems to avoid leaving a possible source of reinfection in the garden. Avoid overhead watering, especially in the evening, and make sure any mulch is pulled away from the crowns.
Bougainvillea and mandevillas both require full sun to grow and bloom their best. They also bloom on new growth of the season, and need to build their strength prior to beginning to bloom. Mandevilla will bloom over a very long period, but bougainvillea is a cyclical bloomer. The most common reason for lack of bloom, especially if they have luxurious foliage, is usually lack of light.
I hope this helps you trouble shoot.
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