Romneya coulteri, known as matilija poppy or coulter's matilija poppy, is a perennial woody shrub (family Papaveraceae) that is native to southern California and northern Mexico. The plant is used in landscape settings and has the largest flowers, measuring up to 16 cm across, of any plant native to California. In August 2006, in Monterey County, CA, landscape plants of R. coulteri were found affected with a powdery mildew disease. Initial symptoms consisted of chlorotic, irregularly shaped, vein-delimited lesions. As disease progressed, grayish white sporulation was visible on lesions on adaxial and abaxial sides of affected leaves. In advanced stages of the disease, lesions expanded up to 2 cm long and the center tissue of the lesions turned necrotic. Symptoms and signs were observed on both mature and younger foliage. The small matilija poppy planting consisted of six or seven plants and all plants were affected by the disease. Epidermal strips from both sides of leaves and leaf cross sections were mounted in drops of lactophenol and aniline blue and examined with a light microscope. These preparations showed that conidiophores developed from endophytic mycelium and emerged through stomates. Conidiophores were sometimes branched and carried one or two conidia. Hyaline, single-celled conidia were dimorphic. Primary (terminal) conidia were lanceolate with distinct apical points and measured 53 to 61 × 17 to 25 μm. Secondary conidia were ellipsoid-cylindric and measured 53 to 67 × 16 to 22 μm. On the basis of these characters, the pathogen was identified as Leveillula taurica (1). Only the anamorph Oidiopsis taurica stage was observed on plants. To prove pathogenicity, diseased leaves were collected from the landscape plantings. One diseased leaf was gently rubbed against one leaf of a potted, healthy matilija poppy. Twelve leaves were inoculated in this way, and plants were kept in a humidity chamber for 48 h and then maintained in a greenhouse (24 to 26°C). After 14 days, chlorotic lesions appeared on inoculated matilija poppy leaves and sporulation of L. taurica was observed several days later. Untreated control plants did not develop powdery mildew. The experiment was repeated and the results were the same. To my knowledge, this is the first report of powdery mildew caused by L. taurica on matilija poppy. There was no indication that the disease affected the growth of the host; however, powdery mildew reduced the quality of the appearance of this ornamental plant.
Reference: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980.
