Archivio Journal of Plant Pathology

Abstract:

Rheum (Rheum australe, family Polygonaceae), a rhizomatous perennial herb with potential medicinal value, is a rare and endangered species distributed in the Himalayas at an elevation of 2000-4500 m above sea level (asl) (Chauhan, 1999). In November 2011, a disease observed during domestication at Palampur (ca. 1300 asl) was characterized by pinkish spots on the leaves that gradually coalesced into large lesions affecting the entire blade, which turned yellowish-brown and withered. A fungus, consistently isolated from diseased leaves on potato dextrose agar (PDA) had a purple-coloured aerial mycelium growing rapidly at 28±2°C. Conidiophores were unbranched with branched monophialides, microconidia were abundant, generally single-celled, oval- to kidney-shaped, produced in false heads. Macroconidia were slightly sickle-shaped, thin-walled and delicate, while chlamydospores were 1-2 celled. These morphological traits identified the fungus as Fusarium oxysporum (Nelson, 1983). This was confimed by PCR amplification and sequencing of the internal transcribed spacer (ITS) region (GenBank accession No. KF986647) that showed 99% similarity with a F. oxysporum isolate from ginger (KJ938022). Pathogenicity tests were done twice in a screenhouse by spraying a conidial suspension (1×106 conidia ml−1 in sterile tap water) from a 7-day-old culture onto the leaves of 10-month-old plants. Inoculated plants and water-sprayed controls were kept in a growth chamber at 20°C for 48-72 h. Symptoms appeared 8-10 days post inoculation on the leaves, from which pathogen was reisolated thus fulfilling Koch’s postulates. Control plants remained symptomless. The culture has been deposited in the Microbial Type Culture Collection (MTCC 11222) of the Institute of Microbial Technology, Chandigarh, India. To the best of our knowledge, this is the first record of F. oxysporum on R. australe.