Phytophthora ramorum
Department of Crop Sciences
University of Illinois at Urbana-Champaign
Many species of Phytophthora are considered common or established in Illinois, but P. ramorum is not one of them. For example:
P. sojae is a common pathogen of soybean.
P. medicaginis is a common pathogen of alfalfa.
P. cactorum, P. citricola, P. heveae, P. nicotianae var. parasitica and P. syringae attack leaves and twigs of rhododendron and azalea. In addition, several of these species plus P. cinnamomi may also attack azalea, fir, pine and rhododendron root tissue, causing root and crown rot. These are significant pathogens in nurseries.
Although you may correctly conclude that your rhododendron shows "Phytophthora blight" symptoms, the only way to determine which species is causing the disease is to submit a plant sample for culture or PCR testing.