MSc

The study entitled “Role of phytohormones and oxidative stress in Piriformospora indica mediated tolerance to Ralstonia solanacearum inciting bacterial wilt of tomato” was conducted during 2019-2022 at College of Agriculture, Vellayani and Integrated Farming System Research Station (IFSRS), Karamana with an objective to evaluate the beneficial fungal root endophyte, Piriformospora indica for the management of Ralstonia solanacearum inciting bacterial wilt of tomato and elucidating the role of phytohormones and oxidative stress in the tripartite interaction.
Ralstonia solanacearum, the causative agent of bacterial tomato wilt, was isolated from the stem of an infected tomato plant showing typical wilt symptoms collected from a farmer’s field at Nanniyod, Thiruvananthapuram and tested for the presence of ooze for confirmation. The disease symptoms were initial drooping of leaves followed by sudden wilting of whole plants and brown discoloration of vascular region. A white fluidal colony with a red center was obtained on the triphenyl tetrazolium chloride (TZC/TTC) medium and pathogenicity was tested by two methods viz., soil drenching and root dip method to prove Koch's postulates. The symptoms were observed within 5 days after artificial inoculation and complete wilting was observed in 8, 10 days in root dip and soil drenching respectively.
Morphological studies revealed R. solanacearum as gram negative and rod-shaped. By choosing the most virulent colony and diffusing it in distilled sterile water, the bacteria might be preserved for a long time. The maintained culture exhibited virulence up to 5 months. P. indica , the beneficial root endophyte was grown on potato dextrose agar (PDA) medium, On 9 days following inoculation, the fungus had grown completely in a petri-plate (9 cm dia.). The fungus grows into a mat in potato dextrose broth (PDB) after 16 days of inoculation. The fungal mat was transferred to a sterilized potting mixture and under sterile circumstances; the fungus reached full growth in one week.
Tomato seedlings were co-cultivated with P. indica in a pro tray mixture made up of coir pith and dried farm yard manure (FYM) with two per cent gram flour. The roots were examined under a microscope at 10, 15, 20, 25 and 30 days after co-cultivation (DAC) and the presence of fungal spores was seen. The spore size grew over time. P. indica colonization reduced the time for seeds to germinate by two days compared to the control and increased the germination percentage 21 per cent over the non-colonized seeds.
An in vivo study was undertaken at IFSRS, Karamana for the management of bacterial wilt disease in tomato (var. Pusa Ruby) by P. indica-colonised seedlings inoculated with R. solanacearum at 0 days after colonization (DAC), 2 DAC, 5 DAC, 10 DAC and P. indica-colonised seedlings, Seedlings inoculated with R. solanacearum, R. solanacearum inoculated seedlings drenched with copper hydroxide @ 2g L-1, uninoculated non colonized seedlings as checks. Seedlings inoculated with R. solanacearum and P. indica-colonised seedlings inoculated with R. solanacearum at 0 days after colonization revealed 100 per cent disease incidence whereas, P. indica-colonised seedlings inoculated with R. solanacearum at 2 DAC, 5 DAC, 10 DAC showed no disease incidence (0 %). The treatment P. indica-colonised seedlings inoculated with R. solanacearum 10 DAC revealed the minimum number of days taken for flowering (35 DAT), fruit weight (41.80 g), yield per plant (590 g), leaf area (2.0 cm2), number of leaves (26), root length (27 cm) and plant height (25.2 cm) followed P. indica-colonised seedlings inoculated with R. solanacearum 5 DAC with number of days taken for flowering (36 DAT), fruit weight (43.93 g), yield per plant (620 g). The treatments P. indica-colonised seedlings inoculated with R. solanacearum at 2 DAC, 5 DAC and 10 DAC were effective than P. indica non-colonized seedlings. The colonization of P. indica has significant effect on the growth and development of the tomato plant.
Field study was conducted at College of Agriculture, Vellayani and Integrated Farming System Research Station (IFSRS), Karamana. Bacterial wilt incidence in two treatments viz., P. indica-colonized and P. indica non-colonized was studied. Both the treatments were got infected by R. solanacearum under natural condition but P. indica-colonized seedlings could prolong the longevity of tomato seedlings (var. Pusa Ruby) for 5 days compared to P. indica non-colonized seedlings. P. indica colonization has significantly reduced the number of days taken for flowering (20 DAT) and increased the plant growth parameters viz., plant height (31.74 cm), number of leaves (25), root length (26.44 cm), leaf area (4.89 cm2). According to the study, P. indica colonization plays a key role in the growth and development of tomato plants, but its colonization alone is ineffective against bacterial wilt disease of tomato (var. Pusa Ruby) in a sick plot condition.
Biochemical studies on identifying the mechanism of disease tolerance P. indica colonized plants were studied at 18 DAS and 20 DAS. P. indica colonized plants challenged with R. solanacearum showed highest activity of the enzymes viz., PO, PPO, PAL, SOD, catalase, IAA and GA. P. indica colonized plants showed reduction in the ROS production.
Thus, the present study revealed that bacterial wilt of tomato caused by R. solanacearum is an important disease. The colonization of P. indica in tomato (Var. Pusa Ruby) roots significantly improved a number of plant development characteristics, but its colonization alone was unable to control bacterial wilt. Therefore, from the study it is inferred that the management of bacterial wilt disease of a highly susceptible variety of tomato Pusa Ruby in a heavily infested soil is not possible by P. indica colonization.

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