Downy Mildew in Lilac
Abstract: Syringa spp. ‘Old Glory’ is a disease-resistant selection of lilac that was developed from a controlled hybridization between Syringa ‘Sweet Charity’ and Syringa × hyacinthflora ‘Pocahontas’. ‘Old Glory’ is credited for high-level disease resistance to powdery mildew, bacterial blight, and other foliage diseases common in the southern region of the United States, but in 2005 and 2006,‘Old Glory’ plants developed symptoms of downy mildew in McMinnville, Tennessee, U.S. Symptoms started in late April as chlorotic lesions and later became necrotic. Upper leaf surface symptoms appeared similar to common leaf spots with necrotic lesions, but underside leaf lesions were covered with masses of sporangiophores and sporangia. Morphologic features of the sporangiophores and sporangia matched that of Plasmopara spp. The leaf lesions were circular or irregular and developed between veins. Coalesced lesions formed large necrotic patches; severely infected leaves defoliated prematurely. Surface-sterilized healthy leaves were spray-inoculated with sporangiospores and placed in Petri dishes over triple-layered wet paper towels at 23°C to 25°C (73.4°F to 77°F). Disease symptoms were reproduced in approximately 12 days. Noninoculated control leaves did not develop disease symptoms. Oospores were not observed. Downy mildew has not previously been reported in lilac and this is the first report of the disease in Tennessee.
Keywords: Plant health care; Plasmopara spp. leaf spot diseases; Syringa species