Powdery Mildew of Rudbeckia hirta var. pulcherrima Caused by Podosphaera fusca

Abstract

Rudbeckia hirta var. pulcherrima (syn. R. bicolor, R. serotina),commonly known as “black-eyed Susan”, is a flowering plantbelonging to the Asteraceae. The plant is native to North Americaand was introduced to Korea for ornamental purposesin t he 1950s; itis now naturalized and can be found growing wild here. InSeptember 2009, occurrence of a previously unknown powderymildew was observed on the plants in a public park and also in aprivate garden in Ganghwa, Korea. The leaf surfaces of affectedplants were covered with white mycelia and conidia, occasionallyhaving a necrotic center. Leaf distortions, which often result fromthese infections, reduce the ornamental value of this plant.Symptoms were also observed on stems and flower petals (Fig. 1A& B). Around 30% of the plants surveyed were affected by thedisease. In November 2009, plants with symptoms were also foundin Namyangju and Jeju, Korea. Representative samples (KUS-F24604, F24863, F24869) were deposited in Korea University. Detailed microscopic examination of a representative sample(KUS-F24869) was made to identify the pathogen. Conidiophoreswere unbranched, cylindrical, 80−180×9−11(−13) µm, composedof a basal cell with a slightly constricted base and three to eightbarrel-shaped conidia formed in chains (Fig. 1C). Appressoria werepoorly developed, indistinct and nipple-shaped. Conidia were ellipsoidor doliiform, 24−36×15−20 µm, containing distinct fibrosin bodies(Fig. 1D). No chasmothecia were found but the above characteri-stics are still consistent with Podosphaera fusca (Fr.) U. Braun &Shishkoff [syn. Sphaerotheca fusca (Fr.) S. Blumer] (Braun, 1987).To confirm the identity of the causal fungus, the complete ITSregion of rDNA from KUS-F24869 was amplified with primersITS5 and P3, as described by Takamatsu et al. (2009), and directlysequenced. The resulting sequence, of 477 bp, was deposited inGenBank, with the accession number of HM004615. MEGA4 withneighbor-joining method (using Tajima-Nei model) was used toperform phylogenetic analysis. In the phylogenetic tree (Fig. 2), theITS sequence of the fungus infecting R. hirta var. pulcherrima wasgrouped with those of three isolates of P. fusca retrieved fromGenBank. The present isolate shared 100% sequence similarity withthose of P. fusca from Lactuca indica and Tussilago farfara, and99% (1 bp substitution and 2 bp insertion) with a Japanese isolatefrom R. hirta var. pulcherrima. Powdery mildew infections with Golovinomyces cichoracearum(syn. Erysiphe cichoracearum) have been recorded on severalspecies of Rudbeckia from many countries (Farr and Rossman,2010). However, powdery mildew infections with P. fusca (syn. P.fuliginea) were recorded on R. laciniata var. hortensis from Japanand on R. amplexicaulis from Romania (Farr and Rossman, 2010).Hirata et al. (2000) provided the first evidence that P. fusca isassociated with R. hirta var. pulcherrima, by analyzing the ITSsequence of a Japanese isolate. The present work confirms this host-parasite combination, by providing consistent morphological andmolecular data.