Interaction between root-knot nematode and fungal pathogens in black pepper, Piper nigrum L.

Abstract

The study entitled " Interaction between root-knot nematode and fungal pathogens in black pepper, Piper nigrum L." was conducted at the Department of Nematology, College of Agriculture, Vellayani, Thiruvananthapuram during 2021- 2023. The objectives were documentation of nematodes and fungal pathogens in the rhizosphere of black pepper, the interaction between root-knot nematode and fungal pathogens, histopathological and biochemical changes due to nematode-fungal disease complex in black pepper. A survey was conducted in the major black pepper growing areas of Kannur (AEU-15), Idukki (AEU-16) and Wayanad (AEU-21) districts to document the nematode fauna and fungal pathogens associated with black pepper. The plant parasitic nematodes identified were Meloidogyne incognita (Kofoid and White) Chitwood, Rotylenchulus reniformis Linford and Oliveira, Radopholus similis (Cobb), Helicotylenchus dihystera (Cobb) Sher and Macroposthonia sp. The highest population of M. incognita and R. reniformis was recorded in Padiyoor and Malapattam areas in Kannur district with prominence value (PV) of 14.99 and 12.29 respectively. In Idukki and Wayanad districts highest population of M. incognita was recorded in Marayoor and Panamaram with PV of 14.76 and 15.48 respectively. Fungal pathogens viz. Fusarium solani, F. oxysporum, Phytophthora sp. Rhizoctonia sp. and Pythium sp. were also isolated from soil and plant samples. Occurrence of F. solani and high population of M. incognita was recorded in Padiyoor and Irikkur areas in Kannur, Marayoor and Mattukatta areas of Idukki and Panamaram and Pulpally regions of Wayanad. Severe galling and yellowing were observed in plants infested with nematode and fungi. A pot culture study was conducted to investigate the interaction between M. incognita and F. solani, since F solani was observed to be the major fungal pathogen associated with nematode infested plants in surveyed locations. The interactive effect of nematode and fungus in plant growth parameters, nematode multiplication, and disease incidence in black pepper were observed. The experiment was laid out in a completely randomized design and the treatments were M. incognita alone, F. solani alone, simultaneous inoculation of M. incognita and F. solani, M. incognita + F. solani two weeks after nematode inoculation, F. solani + M. incognita two weeks after fungus inoculation, and uninoculated control. M. incognita was inoculated @one J2 g soil-1. F. solani was mass multiplied in sand maize medium and mixed with soil (0.5% w/w). After 45 days of inoculation, 55.00 to 70.21 per cent reduction in in plant growth parameters over uninoculated control was observed in plants inoculated with M. incognita fourteen days prior to fungus inoculation. Number of galls, females, egg 119 masses and nematode population were found significantly higher in plants inoculated with nematode alone followed by M. incognita + F. solani fourteen days after nematode inoculation. Reproduction factor of nematode was higher (4.12) in the treatment where, M. incognita was inoculated alone and in all other treatments it ranged from 2.72 to 3.39. Plants inoculated with M. incognita fourteen days prior to F.solani exhibited highest disease incidence (100 per cent) and early symptom development (24 days). The results point out that the nematode infestation can predispose plants to fungal infection and thereby can increase the disease severity in field conditions. After 45 days of inoculation, histopathological and biochemical changes due to M. incognita and F. solani was analysed. Largest (909.2 µm2) and highest number (5) number of giant cells in vascular bundles was observed in plants inoculated with M. incognita alone followed by M. incognita inoculated two weeks prior to fungus inoculation. Fungal mycelium plugging in xylem vessels were observed in all the treatments in which the M. incognita and F. solani were inoculated sequentially and simultaneously. The plugging in xylem vessels were comparatively less in the treatment where F. solani was given alone, showing that presence of nematode increases the activity of fungus. Regarding biochemical changes, the highest amount of phenol was observed in leaves (0.49 mg g tissue -1) and roots (1.75 mg g tissue-1) of plants inoculated with M. incognita + F. solani fourteen days after nematode inoculation. Defence enzymes viz., peroxidase (PO) (8.93 fresh weight), phenyl alanine ammonia lyase (PAL) (16.31 µg of cinnamic acid fresh weight) and polyphenol oxidase (PPO) (6.98 fresh weight) in leaves also increased in plants inoculated with M. incognita + F. solani fourteen days after nematode inoculation. Similar trend was observed in root samples also. Higher activity of phenol and defense related enzymes can be attributed to the defense induced in plants due to the increased infection by nematode and fungus. The protein content was found higher in leaves (3.22 µg of BSA g-1 fresh weight) and roots (3.46 µg of BSA g-1 fresh weight) of the uninoculated plants compared to plants inoculated with nematode and fungus. The decrease in protein content on inoculated plants can be due to the increased use of amino acids by galls or giant cells thereby reducing their availability for protein synthesis. Investigations of the present study highlighted that plant parasitic nematodes viz. M. incognita, R. reniformis, R. similis, H. dihystera and Macroposthonia sp and fungal pathogens viz. F.solani, F. oxysporum Phytophthora sp., Pythium sp. and Rhizoctonia sp. were found associated with black pepper grown in Kannur, Idukki and Wyanad districts of Kerala. Prior inoculation of M. incognita enhanced nematode multiplication and 120 disease incidence as evidenced by increased number of giant cells, distortion of vascular bundles and plugging of fungal mycelia in xylem vessels.