The white powder is powdery mildew which is the result of humid/moist conditions and is not the cause of your plant's dry back. My guess, without more cultural info, is the soil is moist/wet and the plants in the picture are showing sign of this condition (leaf tip dry back).

It looks to me like your leaves are sunburned. How are you watering them? Where are you in California (might want to be more specific, some parts are cool in the summer, some parts unbelievable hot and dry.)

melissa 1, the shrub, Viburnum?, looks like it is suffering from a watering and/or disease problem. The tipping can be due to too much or too little water. You know how much and how often it has been watered, therefore you can adjust the program. If the condition occured suddenly and evenly, you might have a disease, especially if it has not progressed much further. Cercospora is a possibility although the pattern is very strange. Moreover, the youngest leaves seem to be unaffected so I believe the plant may have been subjected to a brief exposure to an adverse condition. Sunburn is a possibility especialy if some shade was removed eg. the pruning or removal of a shade tree or removal of a fence. Perhaps you can check with the previous owner but I do not think that you will lose the shrub(s).

The purple tips strongly suggest a nutrient deficiency. Start with a good, reliable soil test of the area these are planted in.