New Disease Reports (2012) 25, 17. [http://dx.doi.org/10.5197/j.2044-0588.2012.025.017]
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First report of Dactuliophora species causing leaf spot of cowpea in India

S. Mahadevakumar and G.R. Janardhana*

*grjbelur@gmail.com

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Received: 19 Dec 2011; Published: 05 Apr 2012

Keywords: Vigna unguiculata, mycelia sterilia, fungal diseases, sclerotia

Cowpea (Vigna unguiculata) is an important legume crop in developing countries and is cultivated throughout the arid and semiarid tropics (Zohri et al., 1992; Singh et al., 1997). India is a leading cowpea producer (Asiwe et al., 2009). Over the period 2010 to 2011, cowpea plants were found infected by a new leaf spot-causing pathogen in a survey conducted in the Mysore district of Karnataka State. It was observed that leaf spot symptoms increased after rains leading to the death of infected plants. Initially leaf spots appeared as small whitish lesions with concentric rings on upper surface and were pinkish grey on the abaxial surface (Fig. 1). As the disease advanced, the spots coalesced to form larger necrotic areas (Fig. 2). These symptoms were persistent throughout the crop season with sclerotia forming on the lower surface. On the lower leaf surface, sclerotiophores bearing sclerotia were produced from the immersed mycelium (Fig. 3). Sclerotia were generally globose to irregular in shape and dark to grey brown in colour measuring 139.5 x 89 µm. Upon germination, germ tubes were produced over the entire surface of the sclerotium measuring 69.9 x 6 µm (Fig. 4). The symptoms persisted throughout the crop season and affected the overall yield of cowpea.

Fungal colonies were isolated from surface sterilised (2% NaOCl) infected leaf samples on potato dextrose agar (PDA). Aerial mycelium without sclerotia developed on PDA. The colonies were white to brown in colour with mycelia devoid of spores. The fungus was identified as Dactuliophora sp. based on cultural and morphological characteristics (Barnett & Hunter, 1972). On the host, white mycelium was appressed to the cuticle on the lower leaf surface. A septum near each hyphal branch and a slight constriction at the branch were also observed. The isolated fungus was tested for pathogenicity on healthy plants under glasshouse conditions. A twelve-day-old mycelial culture of Dactuliophora sp. was mixed with sterile distilled water and the suspension sprayed on cowpea plants. The plants were kept under high humidity (80%) for five days and at ambient temperature conditions. Leaf spot symptoms appeared on leaves after twenty-five days. The lesions coalesced to form larger concentric rings with sclerotia that developed from the sclerotiophores. The pathogen was re-isolated from infected plants. No such symptoms were observed on control cowpea plants treated with sterile distilled water. Cowpea is susceptible to a wide range of pests and pathogens, which can cause severe loss in all stages of growth and development (Summerfield & Roberts, 1985). The most significant and widespread diseases are caused by rusts (Uromyces phaseoli var. vignae), wilt (Fusarium oxysporum f.sp. tracheiphilum) and powdery mildew (Erysiphe polygoni) respectively. The incidence of Dactuliophora tarrii on cowpea was recorded earlier as a minor leaf spot pathogen in tropical Africa (Leakey, 1964). However, Dactuliophora as a major pathogen of cowpea has not previously been reported elsewhere. This is the first report of leaf spot disease of cowpea caused by Dactuliophora sp. from India.

Figure1+
Figure 1: Dactuliophora leaf spot symptoms on upper and lower leaf surfaces of cowpea.
Figure 1: Dactuliophora leaf spot symptoms on upper and lower leaf surfaces of cowpea.
Figure2+
Figure 2: Advanced stage of Dactuliophora leaf spot on upper leaf surface of cowpea.
Figure 2: Advanced stage of Dactuliophora leaf spot on upper leaf surface of cowpea.
Figure3+
Figure 3: Sclerotia of Dactuliophora sp. produced on host tissue observed under stereo binocular microscope.
Figure 3: Sclerotia of Dactuliophora sp. produced on host tissue observed under stereo binocular microscope.
Figure4+
Figure 4: Germinated sclerotia of Dactuliophora sp. with germ tubes under compound microscope (Scale 20 µm).
Figure 4: Germinated sclerotia of Dactuliophora sp. with germ tubes under compound microscope (Scale 20 µm).

Acknowledgements

The author (S. Mahadevakumar) would like to thank DST, New Delhi, for providing INSPIRE Fellowship.


References

  1. Asiwe JAN, 2009. Insect mediated outcrossing and geneflow in Vigna unguiculata (L.) Walp: implication for seed production and provision of containment structures for GT cowpea. African Journal of Biotechnology 8, 226-230.
  2. Barnet HL, Hunter BB, 1972. Illustrated genera of Imperfect Fungi. Minnesota, USA: Burgess Publishing Company.
  3. Leakey CLA, 1964. Dactuliophora, a new genus of mycelia sterilia from Tropical Africa. Transactions of the British Mycological Society 47, 341-350. [http://dx.doi.org/10.1016/S0007-1536(64)80006-1]
  4. Singh BB, Mohan Raj DRM, Dashiel KE, Jackie LEN, eds, 1997. Advances in cowpea research. Ibadan, Nigeria: Co-publication of International Institute of Tropical Agriculture and Japan International Research Centre for Agricultural Sciences, IITA, Ibadan, Nigeria.
  5. Summerfield RJ, Roberts EH, eds, 1985. Grain legume crops. London: Collins.
  6. Zohri AA, Gawad KAE, 1992. Studies on Mycoflora and Mycotoxins of cowpea cultivars. Korean Mycology 20, 252-258.

To cite this report: Mahadevakumar S, Janardhana GR, 2012. First report of Dactuliophora species causing leaf spot of cowpea in India. New Disease Reports 25, 17. [http://dx.doi.org/10.5197/j.2044-0588.2012.025.017]

©2012 The Authors