Variability in cultural and morphological characteristics of Macrophomina phaseolina isolates associated with dry root rot of mungbean

Introduction

Pulses are recognized as nutritionally valuable food legumes and form an integral part of diets in many developing countries, particularly in India. Their consumption alongside cereal-based foods provides a balanced source of dietary protein and contributes to improved nutritional quality (Ali and Sen Gupta, 2012; Calles et al., 2019; Ramya et al., 2026) [1,3,7] . Beyond their role in human nutrition, pulse crops have considerable agronomic significance owing to their ability to fix atmospheric nitrogen through symbiotic associations, enhance soil fertility and perform relatively well under low-input production conditions. Their importance in sustainable food production and nutritional security was globally acknowledged through the declaration of 2016 as the "International Year of Pulses" by the United Nations General Assembly (Khan, 2016) [6] . Among the food legumes cultivated in Asia, mungbean [Vigna radiata (L.) Wilczek] occupies an important position because of its short growth cycle, nutritional value and relatively low requirement for external inputs. The crop, commonly referred to as green gram, is widely cultivated throughout India and several countries of Southeast and East Asia (Lambrides and Godwin, 2017) [10] . Mungbean belongs to the family Leguminosae and is considered to have originated in the Indian subcontinent, where its domestication dates back several centuries (Vavilov, 1926) [20] . It is predominantly suited to tropical and subtropical climates and can be grown across diverse geographical regions where temperatures remain above approximately 15 °C (Lawn, 1983; Imrie et al., 1981) [11,5] . Its early maturity and flexibility in cropping sequences further enhance its importance in intensive agricultural systems. However, the productivity potential of mungbean is frequently affected by a range of environmental and biological stresses. Among these, diseases represent a major limitation to crop establishment, growth and yield. Several fungal, bacterial and viral diseases have been documented on mungbean, including yellow mosaic disease, Cercospora leaf spot, powdery [8,9,14,18] . The occurrence and intensity of these diseases are influenced by the interaction among host genotype, pathogen population and prevailing environmental conditions, which can result in substantial variation in disease expression across locations and growing seasons. Dry root rot, caused by the soil-and seed-borne fungus Macrophomina phaseolina (Tassi) Goid, has emerged as an important disease of mungbean, particularly in regions characterized by high temperature and inadequate soil moisture. The pathogen possesses a broad host range and has been reported from several important mungbeanproducing areas, where infection may result in considerable yield losses (Allen et al., 2017; Pandey et al., 2020) [2,13] . A major factor contributing to the persistence of M. phaseolina is its ability to produce microsclerotia, which remain viable in soil and infected crop residues for prolonged periods and serve as important sources of primary inoculum (Pandey et al., 2021) [15] . Infected plants commonly exhibit deterioration of the root system, followed by darkening and rotting of roots, yellowing, wilting and eventual death under severe disease pressure (Khan et al., 2017) [7] . Disease development is closely associated with environmental conditions, particularly elevated temperature and reduced soil moisture, which favor infection and colonization by the pathogen (Saleh et al., 2010) [19] . In addition, the movement of infested soil, infected residues and contaminated seed can facilitate the dissemination and establishment of the pathogen within and between production areas. The prolonged survival of microsclerotia in soil further complicates disease management and contributes to the recurring nature of the disease. An important characteristic of M. phaseolina is its capacity to exhibit considerable variation in biological and phenotypic traits among isolates. Such variability may be reflected in colony characteristics, growth rate, pigmentation, microsclerotial production and other morphological attributes. Understanding this variability is particularly important because differences among pathogen populations may influence disease development, hostpathogen interactions and the effectiveness of disease management practices. Characterization of the pathogen population therefore provides useful baseline information for subsequent studies on pathogenicity, epidemiology and management. Management of mungbean dry root rot remains difficult because of the soil-borne nature of the pathogen, long-term survival of microsclerotia and favorable environmental conditions that promote disease development. Various approaches, including chemical fungicides, biological control agents and other cultural or integrated practices, have been explored to reduce disease incidence. Nevertheless, sustainable disease suppression requires a comprehensive understanding of the pathogen and its biological variability before appropriate management strategies can be developed and optimized. Therefore, the present investigation was undertaken to characterize M. phaseolina isolates associated with dry root rot of mungbean, with particular emphasis on their cultural and morphological variability and pathogenic potential. The study is intended to provide a better understanding of the variability present within the pathogen population and generate information useful for the development of effective and sustainable management strategies for mungbean dry root rot.

Materials and Methods

Cultural Studies

The present study aimed to evaluate the cultural variability of thirty isolates of M. phaseolina (Table 1). The isolates of M. phaseolina were purified through the hyphal tip technique on PDA slants and stored in a refrigerator (4 °C). It was then grown separately on PDA in Petri plates for seven days to provide inoculum for the study. Cultural studies for the isolate were conducted on PDA in 90 mm Petri plates. The Petri plates were inoculated centrally with 5 mm culture discs taken from the periphery of a seven-dayold culture and incubated at 28 ± 2 °C. Observations on cultural characters, viz., colony colour, type, growth, size and shape, were recorded one week after inoculation. Morphological characters of the pathogen were studied by examining the culture under a compound microscope.

Morphology of M. phaseolina

Slides of the fungus were prepared by growing hyphae or the respective samples. The fungal material was mounted on a clean glass slide with a drop of lactophenol cotton blue and covered with a coverslip. The morphological characters of the fungus were studied with the help of a compound microscope (40X). After calibrating the microscope, measurements of hyphae, sclerotia, were taken using ocular and stage micrometers.

Overall variability among isolates

Overall, the thirty M. phaseolina isolates exhibited considerable phenotypic variability in both cultural and morphological characteristics. Variations were observed in radial growth, colony texture, colour and margin, as well as in the production, size and shape of sclerotia. Such variability among isolates collected from different locations of Junagadh district indicates the occurrence of considerable intraspecific variation in M. phaseolina and provides a useful basis for further studies on pathogenic variability and disease management.

Conclusion

The present investigation demonstrated considerable cultural and morphological variability among the thirty isolates of Macrophomina phaseolina associated with dry root rot of mungbean under in vitro conditions. The isolates differed considerably in colony growth, texture, colour, margin characteristics, sclerotial production, size and shape, indicating substantial phenotypic diversity within the pathogen population. Colony diameter after 10 days of incubation ranged from 85.55 to 90.00 mm, with Mp 23 (Manpur) and Mp 9 (Khadpipli) recording the maximum growth, whereas Mp 8 (Mahobatpur) exhibited the minimum growth. The isolates showed diverse colony textures, including fluffy, woolly, cottony, velvety and compact types, along with variation in colony colour and margin characteristics. Sclerotial production varied from poor to excellent among the isolates, while sclerotial size and shape also exhibited marked differences. The largest sclerotia were recorded in Mp 23 (Manpur), whereas the smallest were observed in Mp 5 (Bantva).

Overall, the study confirmed pronounced cultural and morphological variability among M. phaseolina isolates from mungbean. Such variability may contribute to differences in the survival, adaptation and disease-causing ability of the pathogen. The characterization of these isolates provides valuable baseline information for further studies on pathogenic variability, genetic diversity, hostpathogen interactions and the development of effective management strategies for dry root rot of mungbean.

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