New Disease Reports
New Disease Reports (2008) 18, 37.
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Peronospora causing downy mildew disease of sweet basil newly reported in Cameroon

H. Voglmayr 1* and M. Piątek 2

*hermann.voglmayr@univie.ac.at

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Accepted: 16 Dec 2008

Sweet basil (Ocimum basilicum) is commonly grown in Central Africa and used as a condiment, in traditional medicine and for essential oil production. In March 2007, numerous plants of sweet basil infected by downy mildew were observed at plantations in Obala town (Centre Province), Cameroon. In initial stages of infection, the pathogen caused chlorotic leaf spots turning brownish with age, and finally the leaves curled (Fig. 1). On the underneath of the leaf spots, a brownish felt developed. Microscopic examinations revealed conidiophores typical of Peronospora; they were 4-6 times pseudodichotomously branched, hyaline, 250-400 µm high, emerging from stomata and connected with an aseptate, endoparasitic, intercellular mycelium bearing irregularly branched haustoria (Fig. 2A, B, G). The strongly sinuous ultimate branchlets of the conidiophores were 6-25 µm long and 2.5-3.5 µm wide at the base (Fig. 2C), at their tips bearing single, dark brown, broadly ellipsoidal to subglobose conidia measuring 28-37 x 25-33 µm, with a l/w ratio of 1.1-1.3 (Fig. 2D-F), surface smooth in LM but verruculose in SEM (Fig. 3). Except for slightly broader conidia, these data agree well with those given for Peronospora sp. of sweet basil in McLeod et al. (2006). Specimens are deposited at HUYI (University of Yaoundé 1, Cameroon), KRAM and WU herbaria.

Recently, the Peronospora from sweet basil was shown to represent an unnamed species distinct from Peronospora lamii (Belbahri et al., 2005). To reveal the identity of the Cameroonian basil pathogen in light of these data, DNA was extracted from infected plant tissue according to Riethmüller et al. (2002). The complete ITS rDNA region was amplified and sequenced with primers ITS5-P2 (Voglmayr & Constantinescu, 2008) and ITS4, and deposited in GenBank (Accession No. EU863410). The sequence differed by only one base substitution in the ITS2 region from sweet basil Peronospora sequences AY831719, AY831720, AY831721 and AY919301 and clustered with other sweet basil Peronospora GenBank sequences in a phylogenetic analysis, confirming their conspecificity (Fig. 4). To our knowledge, this is the first record of Peronospora sp. of sweet basil from Cameroon and the fifth record from Africa, after previous reports from Benin, Uganda, Tanzania and South Africa.

Figure1+
Figure 1: Downy mildew symptoms on Ocimum basilicum infected with Peronospora sp. Arrows denote chlorotic leaf spots.
Figure 1: Downy mildew symptoms on Ocimum basilicum infected with Peronospora sp. Arrows denote chlorotic leaf spots.
Figure2+
Figure 2: Peronospora sp. from Ocimum basilicum as seen by light microscopy. A & B. Conidiophore (Bar = 50 μm); C. Ultimate branchlets (Bar = 20 μm); D-F. Conidia (Bar = 20 µm); G. Aseptate hypha and haustorium stained with Congo red; arrows denote callose sheath of haustorium (Bar = 10 µm).
Figure 2: Peronospora sp. from Ocimum basilicum as seen by light microscopy. A & B. Conidiophore (Bar = 50 μm); C. Ultimate branchlets (Bar = 20 μm); D-F. Conidia (Bar = 20 µm); G. Aseptate hypha and haustorium stained with Congo red; arrows denote callose sheath of haustorium (Bar = 10 µm).
Figure3+
Figure 3: Peronospora sp. from Ocimum basilicum as seen by scanning electron microscopy (SEM). A. Ultimate branchlets of conidiophores and conidia (Bar = 50 µm); B. Ornamentation of conidial wall (Bar = 5 µm).
Figure 3: Peronospora sp. from Ocimum basilicum as seen by scanning electron microscopy (SEM). A. Ultimate branchlets of conidiophores and conidia (Bar = 50 µm); B. Ornamentation of conidial wall (Bar = 5 µm).
Figure4+
Figure 4: Phylogram showing the phylogenetic relationships of Peronospora sp. from sweet basil obtained from a Maximum parsimony (MP) analysis of ITS1-5.8S-ITS2 sequences selected from GenBank, with Peronospora lamii as outgroup. Numbers above/below branches indicate MP bootstrap values from 1000 replicates. Within Peronospora sp., the geographic origins are colour-coded: green labels denote Asian origins, blue labels European origins, and red labels African origins; the Cameroonian accession is in bold type.
Figure 4: Phylogram showing the phylogenetic relationships of Peronospora sp. from sweet basil obtained from a Maximum parsimony (MP) analysis of ITS1-5.8S-ITS2 sequences selected from GenBank, with Peronospora lamii as outgroup. Numbers above/below branches indicate MP bootstrap values from 1000 replicates. Within Peronospora sp., the geographic origins are colour-coded: green labels denote Asian origins, blue labels European origins, and red labels African origins; the Cameroonian accession is in bold type.

Acknowledgements

We thank Prof. D. C. Mossebo for help to obtain the research permit and A. L. Njouonkou for help during the field trip.

References

  1. Belbahri L, Calmin G, Pawlowski J, Lefort F, 2005. Phylogenetic analysis and Real Time PCR detection of a presumbably undescribed Peronospora species on sweet basil and sage. Mycological Research 109, 1276-1287.
  2. McLeod A, Coertze S, Mostert L, 2006. First report of a Peronospora species on sweet basil in South Africa. Plant Disease 90, 1115.
  3. Riethmüller A, Voglmayr H, Göker M, Weiß M, Oberwinkler F, 2002. Phylogenetic relationships of the downy mildews (Peronosporales) and related groups based on nuclear large subunit ribosomal DNA sequences. Mycologia 94, 834-849.
  4. Voglmayr H, Constantinescu O, 2008. Revision and reclassification of three Plasmopara species based on morphological and molecular phylogenetic data. Mycological Research 112, 487-501.
This report was formally published in Plant Pathology

©2008 The Authors