First Report of Powdery Mildew Caused by Golovinomyces spadiceus on Green and Gold (Chrysogonum virginianum) in the United States

Green-and-gold or Goldenstar (Chrysogonum virginianum L.: Asteraceae: Heliantheae) is a low-growing, shade-tolerant, perennial flowering groundcover that occurs naturally in moist, woodland environments in the eastern United States. Several variants of the plant are available commercially, and are generally considered easy to grow and relatively disease-free, except for some anecdotal reports of “mildew.” The fungus causing powdery mildew on this plant has never been identified and Koch’s postulates have not been completed. Plants growing in ornamental settings from Knoxville, TN, in 2015 to 2017 exhibited signs and symptoms of powdery mildew that included superficial, scanty and patchy, white mycelia, and hyaline conidiophores and conidia on both surfaces of the laminae, which were slightly cupped and upturned from the midvein to the margins compared with uninfected leaves. By June, mycelia were tightly appressed to the leaf surface, gray and crusty, and sporulation was greatly reduced. Infected leaf pieces were mounted in water or stained with Trypan blue. Conidia (n = 30), which lacked fibrosin bodies and were borne in chains, were ellipsoid or doliiform, rarely cylindrical, had a L:W ratio between 1.7 and 2.1, and measured 36.6 µm (32.9 to 42.0 µm) by 18.6 µm (15.6 to 20.5 µm). Conidia germinated subterminally and produced nipple-shaped appressoria. The morphology of the fungus agreed with the description for Golovinomyces spadiceus that occurs on species in the Heliantheae (Braun and Cook 2012). Chasmothecia were not detected during any of the three growing seasons. Total genomic DNA was prepared from mycelium, conidiophores, and conidia from leaves and amplified with ITS1 and ITS4 primers (White et al. 1990). Amplicons were purified before sequencing (Trigiano et al. 2016) and the sequences deposited in GenBank (MF460422). The sequence was 100% identical to G. ambrosiae reported on Helianthus annuus (KM657962) and H. verticillatus (KM657962). The sequence for C. virginianum was also entered in GenBank (MF460421). However, ITS sequencing is not sufficient to delineate G. ambrosiae from G. spadiceus (Takamatsu et al. 2013). Morphologically, G. spadiceus conidia are ≤20 µm wide, whereas G. ambrosiae conidia can be wider (Braun and Cook 2012). Five C. virginianum powdery mildew-free plants were inoculated by rubbing the adaxial surface of leaves with leaves having signs of powdery mildew; two plants were rubbed with healthy leaves as controls. Plants from all treatments were covered with plastic bags for 2 days and then removed. After 10 days, four plants inoculated with powdery mildew leaves from C. virginianum exhibited the same signs and symptoms as described originally from infected C. virginianum plants. All control plants remained disease-free. Our conclusion is that G. spadiceus is the cause of powdery mildew on C. virginianum. The disease on C. virginianum appears to be more cosmetic than detrimental as flowering and growth of the plants are not noticeably affected. This is the first report that provides completed Koch’s postulates for this disease in the United States.