First Confirmed Report of Powdery Mildew Caused by Erysiphe aquilegiae on Casuarina cunninghamiana in Argentina

Abstract

Casuarina cunninghamiana Miq. (Fam. Casuarinaceae) is native toAustralia and was introduced to Argentina and used as a windbreak,shade tree and for ornamental purposes. In autumn 2009, the pottedseedlings in the nursery of La Plata University were found to beinfected with a powdery mildew. Symptoms first appeared as thinwhite colonies, which subsequently developed into an abundantgrowth on the leavesand y oung stems (Fig. 1A)The . damage causedby the powdery mildew infection has been observed every year afterthis initial infection and was again confirmed in April, 2011. Therehave been a few reports on the presence of powdery mildew onCasuarina spp. (Mantz et al., 2008; Farr and Rossman, 2011). In allthe reports, the pathogen was cited to be Oidium sp. Thus, the aim ofthis work was to identify the causal agent of the powdery mildewobserved on casuarina in Argentina. A detailed microscopic examination of a representative sample(KUS-F24744) deposited in Korea University was made to identifythe pathogen. Conidiophores were cylindrical, 70–145 × 6–8.5 µm,moderately flexuous to straight in foot-cells and produced conidiasingly, followed by 2–3 cells (Fig. 1B–D). Appressoria weremultilobed or moderately lobed (Fig. 1J). Conidia were oblong-elliptical, 28–54 × 11–18 µm, without conspicuous fibrosin bodiesand produced germ tubes in the perihilar position (Fig. 1F–I). First-formed conidia (Fig. 1E) were apically conical, basally subtruncate,and generally smaller than the secondary conidia. No chasmotheciawere found but the above characteristics were consistent with theconidial state of Erysiphe aquilegiae DC. (Braun, 1987; Liberatoand Cunnington, 2006). To confirm the identity of the causal fungus, the complete internaltranscribed spacer (ITS) region of rDNA from KUS-F24744 wasamplified using primers ITS5 and P3, and then directly sequenced.The resulting sequence of 553 bp was deposited in GenBank(Accession No. JN003594). Phylogenetic analysis was performedusing MEGA4 with the neighbor-joining method and the Tajima-Nei distance calculation. A comparison with the sequencesavailable in the GenBank database revealed that the ITS sequenceshared over 99% (552/553 bp) similarity with those of E. aquilegiaeon several host genera of the Ranunculaceae (Fig. 2). Therefore, thesequence analysis verified that the pathogen was E. aquilegiae.Powdery mildew caused by E. aquilegiae has been shown tooccur on plants belonging to the Ranunculaceae with one exception(Farr and Rossman, 2011). Catharanthus roseus (L.) G. Don, whichbelongs to the Apocynaceae, was found to be naturally infected byE aquil. egiae as demonstrated by morphological and moleculardata (Liberato and Cunnington, 2006). To the best of our knowledge, thisis the second report where E. aquilegiae was found on a host outsideof the Ranunculaceae. An occurrence of powdery mildew diseaseon this tree was previously recorded in Argentina; however, in thatstudy, the causal fungus was not identified (Mantz et al., 2008).Their description of the causal fungus was actually identical to thecharacteristics reported here. Therefore, we believe that their funguswas also E. aquilegiae .