It is not possible to determine whether or not Phytophthora has killed this plant simply by looking at a photo.
To help you to more accurately determine the problem, I include a link to "Diagnosis and Control of Phytophthora Diseases" by J.W. Pscheidt, Extension Plant Pathology Specialist, OSU. http://pnwhandbooks.org/plantdisease/pathogen-articles/pathogens-common-many-plants/oomycetes/diagno...
Yes, Phytophthora can kill one plant among several. There are also many other causes of plant death so a more complete history of this plant and the area in which it is planted is needed. Is this area well drained? Are there root weevils present? Could it have sustained some sort of mechanical injury or winter injury? Is a sprinkler broken thus causing the plant to not receive water?
This plant should be removed. Once any irrigation and drainage problems are solved, then replant with a root rot resistant plant.
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