Hi, 

We can't be entirely sure the cause of the die back from your photos as leaf discoloring and dieback can be a few different things. 

Since you had the asters do similar things last year, if you wanted to you can get the soil tested from a laboratory. It won't test from specific pathogens but can tell you if it is deficient of any nutrients (sometimes that is what purple edges show). They typically cost around $20-$30. There are a few different labs listed on our page, you can choose one and follow the directions on their webpage. Sometimes purple edges on leaves is a sign of a Phosphorus deficiency. (Garden Fertilizer Basics Webpage)

It could be a root rot or crown rot issue. Is the soil consistently wet? Dianthus and asters tend to not like excessively wet soil. Does it get full sun? The Echinacea ( white variety in the background) and marigolds will like the full sun and dryer soil as well. The Caladium that is shown on the ends is more of a part shade plant so if its full sun, they may tend to get a bit crispy in the summer heat, needing more water, where the other plants wouldn't like that much moisture. 

You can try to remove any of the discolored or dying leaves and anything that is brown now and see if that helps. Make sure to feel the soil before watering. It shouldn't still be wet. You can stick your finger down to about your knuckle - 4 inches or so - to see if the bed needs additional water before adding it. 

To help retain soil moisture between watering, you could add some pine fines, or hardwood mulch. This will also add organic material to the soil as it breaks down and decomposes. 

There could be a fungal or bacterial issue coming into play, but those tend to be opportunistic problems. If the soil and air circulation isn't ideal for the plants, then the plants can be susceptible to different issues, either on the leaves or in the soil itself. 

Let us know if you have further questions or notice other signs and symptoms. If you choose to get the soil test and would like help interpreting the report, feel free to reach back out to this thread and include the results. 

Emily