First report of root rot and damping off disease in soybean (Glycine max) caused by Pythium deliense in India.

Abstract

Seedling rot symptoms were observed at Research Farm of ICAR-Indian Institute of Soybean Research, Indore, India. The infected seedlings had water-soaked lesions on the cotyledons and hypocotyls that gradually developed into brown lesions and further progressed to soft rot. These seedlings could be easily pulled-off from the soil. The diseased seedling samples were rinsed thoroughly in flowing tap water and eventually in double-distilled water and were subjected to surface sterilization with NaOCl(1%). The samples were further washed thrice with sterilized double distilled water. The root fragments were properly sterilized and placed on V8 juice agar as well as potato dextrose agar (PDA) media plates. These plates were incubated at 27± 2°C for 48 hours. After incubation, white fluffy mycelial growth was observed on both the media. The fungus was observed to produce brown round vesicles with mycelial attachment when observed under a compound microscope magnification of 20X. Subcultures of these fungal isolates were placed on PDA media and incubated for 7 days at (27±2°C). The pure fungal culture along with PDA media was cut into small pieces and mixed with a sterilized soil mix (70% soil and 20% sand and 10 % vermicompost) at the rate of one petri dish per pot (plastic pots of 10 cm depth) and covered properly with tin foil. These pots were subjected to substrate colonization for 10 days at room temperature and the substrates were shaken occasionally to improve infection efficiency of pathogen by enhacing inocula production. Seeds of soybean variety, Gaurav were sown in three replicates, each with 10 seeds in the inoculated pots. The control was established by sowing seeds in the soil mix, amended previously with plain PDA. The pots were maintained at 25 to 30 ºC with 45 to 50 % of soil moisture content under glasshouse conditions. In the inoculated pots, the fungus killed soybean seeds before and after germination. Some of the plants that emerged developed lesions were initially yellow and gradually turned to necrotic later. These lesions were found on the roots of the plant and at the base of the hypocotyl region. The soybean seeds planted in un-inoculated soil emerged but did not develop any necrotic lesions. When the causal organism was re-isolated from the diseased plant part it was found to be morphologically and culturally similar to theoriginal culture. The isolated pathogen was thus classified as Pythium deliense based on morphological and cultural characters as well as the pathogenicity test. (Plaats-Niterink 1981). For further confirmation of pathogen's identity, complete genomic DNA of the fungus was extracted using the HiPurA Fungal DNA Purification Kit (HiMedia, India). The nuclear rDNA region of the internal transcribed spacer and 5.8S rDNA was amplified by universal primers ITS 1 (5' TCCGTAGGTGAACCTGCGG 3') and ITS 4 (5' TCCTCCGCTTATTGATATGC 3') as mentioned by White et al. (1990). Amplification was performed in a 12.5 μL reaction volume containing 1.5 μL of 10X PCR buffer, 3 μL of 25 mM MgCl2, 1.2 μL of 2.5 mM deoxyribonucleotide triphosphates (dNTPs), 0.7 μL of 10 pM each primer (ITS 1 and ITS 4), and 1 μL of DNA template, 0.3 μL of 1 units of Taq DNA polymerase. The thermal cycle consisted of 4-minute initial denaturation at 94°C, followed by 35 cycles of 1-minute denaturation at 95°C, 30-second primer annealing at 57 The PCR products were sequenced and submitted to NCBI (GenBank Acc. MT2665888). The BLAST study of the fungal isolate showed 100% similarity with reference sequences of Pythium deliense (MT126658.1) in the GenBank. The isolate was identified as Pythium deliense on the basis of molecular analysis, corroborating the above morphological identification. Further, the beta-tubulin gene (Bt) was amplified with primers BtF (5'GCTGGCCTTGATGTTGTTCG3') and BtR (5'CGTGA AGAGTACCCAGAC CG3'). Similarly, the cytochrome oxidase gene was amplified with primers COXF (5'GGTGCTTTTTCAGGTGTAGTTGG3') and COXR (5'GCTCCTGCTAATACTGGTAATG T3'). The PCR products were sequenced and submitted to GenBank with accession numbers MW196444 and MW196445 respectively. In BLAST analysis, the beta-tubulin gene exhibited 100 percent sequence homology with Pythium deliense (MK752986.1) and cytochrome oxidase gene also showed 100 % sequence homology with Pythium deliense (HQ708566.1). Pythium deliense has been recorded worldwide causing disease in many agricultural crops including soybean but to our knowledge, this is the first study in India of the genus Pythium and Pythium deliense causing root rot and damping off of soybean.