Answer: Unfortunately, based on your description I can't tell what the exact problem is. It might simply be drought stress, but it could also be a fungus exacerbated by overhead watering and/or poor air circulation. You might try thinning out some of the stems to increase the air flow and certainly remove any affected foliage and stems as they occur (and all stems at the end of the season) to try to reduce reinfection. You might also take a very close look for evidence of insects, being sure to look at the undersides of the leaves and along the stems.
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