First Report of Leaf Spot Caused by Phoma costarricensis on Delphinium malabaricum in Western Ghats of India

Abstract

Delphinium L. is a genus of more than 300 species of perennial flowering plants belonging to the family Ranunculaceae and is native throughout the Northern Hemisphere. In India, 24 species are found mainly in the Himalayan regions. Delphinium malabaricum (Huth) Munz is the only species of the genus endemic to northern Western Ghats. Its mediumsized violet-to-metallic blue spurred flowers have considerable ornamental value as a floriculture crop (3). There is no report of serious diseases of this genus in India. Since 2008, a severe foliar disease was observed each year on D. malabaricum cultivated on the experimental plots located at Shivaji University, Kolhapur, India. Symptoms were small, necrotic spots on the abaxial and adaxial sides of infected leaves, with spots enlarging to form round areas that were 6 to 9 mm in diameter and well defined by a dark black margin. Severely infected leaves wilted without abscising. No symptoms were observed on other plant parts. From infected leaves, a fungus was isolated on Czapek Dox agar (CDA) amended with 25 mg/liter of streptomycin sulfate. The fungus was grown on CDA and cultures were maintained at 4°C for further studies. After 6 days, black pycnidia developed, which were variable in size, releasing abundant hyaline, elliptical conidia measuring from 3 to 4 × 1.5 to 2 μm. On the basis of cultural and morphological characteristics, the fungus was identified as Phoma costarricensis (1,2). The identification was verified by sequencing a region of 28S ribosomal RNA with the geneOmbio LSU gene sequencing primers and ABI BigDye Terminator v3.1 Cycle Sequencing Reaction Kit (Applied Biosystems, Carlsbad, CA). The sequence was deposited as Accession No. HE608244 in EMBL-Bank. Blast analysis of the sequence obtained showed a 99% homology with several isolates of P. costarricensis in the GenBank database (Accession No. GU238058.1). Pathogenicity tests were performed by spraying leaves of 10 healthy plants with spores (~10,000 spores or mycelial fragments per ml) on the abaxial and adaxial surface of leaves. Noninoculated plants served as control. Symptoms identical to those on field samples developed on all inoculated plants 1 week after inoculation but controls remained asymptomatic during the observation period. P. costarricensis was reisolated from inoculated symptomatic plants and the identity was confirmed, which completed Koch's postulates. This experiment was repeated three times in a greenhouse, confirming the pathogenicity of P. costarricensis on D. malabaricum. P. ajacis (Thum.) Aa & Boerema, P. delphinii (Rabenh.) Cooke, P. aquilegiicola M. Petrov, and P. xanthina Sacc. are reported to cause leaf spot and stem rot in Delphinium spp. (1). However, to our knowledge, there are no previous reports of leaf spot of D. malabaricum caused by P. costarricensis. Leaf spot severity induced premature defoliation, leading to reduction in flower setting and ultimately the yield. The disease could lead to an economic crisis in the farmer community of western India. The only other known host of P. costarricensis is Coffea arabica (2). References: (1) G. H. Boerema et al. Phoma Identification Manual. CABI Publishing, Wallingford, UK and Cambridge, MA, 2004. (2) E. Echandi. Rev. Biol. Trop. 5:83, 1957. (3) S. R. Pai et al. Cytologia 72:319, 2007.