Thanks for contacting Ask an Expert. Based on your description and the picture you have included I would lean towards some form of wilt, either Verticillium or Fusarium. Both are caused by a fungus but fungicides are not recommended. The very best way to determine what is happening is to get a sample over to the diagnostic lab so the precise issue can be determined. Please go to the Extension website: http://extension.udel.edu/lawngarden/
and look for "Our Services" - select "Plant Diagnostic Clinic" - this will give you instructions for submitting a sample.
Please let us know if you require additional information or if you have further questions.
You will receive a brief survey to let us know how we are doing and what we can do to improve our service. Please take a moment to fill it out!!
UD NCC Master Gardener