A: Your plants have two issues, and one led to the other.
Your bed has likely stayed far too wet for the tastes of this drought loving plant! Texas sage typically thrive on dry and hot weather, and this year has been wet further into the season than normal. These plants are often difficult to use in the main landscaping around homes, tending to thrive better in berm areas and spots in the landscape that aren’t irrigated at all! The irrigation that has made your begonias look so good this year (rain, gutter run off, and direct watering) is generally more than your Texas sage wants. Adjust your sprinklers in this area to water very infrequently, and do some occasional hose watering to keep your begonias perky.
Because the plants have been unhealthy due to excess soil moisture, they have been infested with either mealybugs or scale. A treatment with Bonide Systemic Insect Drench will work its way throughout the plant and kill these pests, but there are also sprayable solutions as well. There’s more than one right answer.
In the future, try to plant Texas sage in the hottest, driest areas you have, and be stingy with the irrigation. There’s a reason this plant is frequently grown down highway planting areas. At watering time, if you’re in any doubt whether the plant needs water, the answer is, “Not quite yet.”