Incidence of charcoal rot across the zones
The charcoal rot was found in all the major soybean growing areas of Madhya Pradesh (Table 1). In Kymore Plateau and Satpura hills agroclimatic zones, the incidence of charcoal rot ranged between 1.75-44.50 (in 2018) and 10.75-34.25% (in 2019), with the maximum average of 39.38% in JS 95-60 at Seoni block, Seoni. In Satpura plateau agroclimatic zone, it was distributed from 1.25-42.50 and 0.50-32.00% during 2018 and 2019, respectively. Variety JS 20-29 (34.50%) was highly affected at Mohkhed, Chhindwara. In Central Narmada Valley, it ranged between 0.75-42.0 and 0.50-32.25% during 2018 and 2019, respectively, with the highest average (37.13%) in JS 95-60 at Hoshangabad. In the Vindhyan plateau, charcoal rot incidence varied from 2.25-35.0 and 0.75-31.0% during 2018 and 2019, respectively, with the maximum average (30.75%) in JS 95-60 at Vidisha. In the case of the Malwa plateau, charcoal rot was very low and ranged from 4.5-10.5 and 1.5-16.25% during 2018 and 2019, respectively. The JS 20-98 fields were free from charcoal rot infection. In the Northern hill region, it was also comparatively low and ranged between 13.0-24.5 and 1.75-19.25% in 2018 and 2019, respectively.
Zone-wise average incidence was higher in Satpura Plateau (19.07%), followed by Kymore Plateau and Satpura Hills (18.81%) and central Narmada Valley (15.60%). At the same time, it was lowest in the Malwa Plateau (5.94%),
i.e. Rajgarh and Ujjain district. Incidence was higher in 2018 (16.48 %) than in 2019 (12.26%). Across the zones, JS 95-60 was found to be the most affected (27.76 %), followed JS 93-05 (25.38%) and JS 20-29 (23.96%). At the same time, JS 20-34 (0.77%), JS 20-98 (0.83%) and JS 20-69 (1.70%) were least affected by charcoal rot. Our results have been corroborated with the findings of some recent investigations.
Mishra and Dantre (2017) indicated that charcoal rot (30-60%) have also been infecting soybean varieties to varying extent in different districts of Chhattisgarh. High severity of charcoal rot (up to 100 %) in most presently growing varieties and germplasm at the experimental area in Jabalpur has also indicated its prevalence in the state
(Amrate et al., 2018; Amrate et al., 2019). Charcoal rot is severe under high temperatures and erratic rainfall during reproductive stages in Indian conditions
(Wrather et al., 2010; Gupta et al., 2012). The high incidence of charcoal in 2018 might be attributed to low rainfall and dry spells in most of the region of Madhya Pradesh, whereas in 2019, the conditions were not favourable.
Influence of cropping pattern and soil type
Through cropping pattern revealed that the incidence of charcoal rot was highest in the fields that had maize-chickpea (26.32%) cropping pattern in the previous year, followed by Maize-Wheat (23.75 %), Soybean - Pea (23.63%) and Soybean - Chickpea (18.87%) in the same land (Fig 4). The lowest average incidence of charcoal rot was recorded in the cropping pattern of rice-chickpea (12.00%) and Rice - Wheat (12.45%). The charcoal rot incidence was comparatively higher in the field with single/both crops (
kharif and
rabi) as hosts of charcoal rot pathogen. Maize and chickpea are the prominent host of
M. phaseolina, which causes considerable losses in Indian conditions
(Kaur et al., 2022; Bankoliya et al., 2022). Hence, it might be possible that these cropping patterns enhanced the pathogen load in soil, resulting in high disease. In contrast, a low incidence of charcoal rot in the Rice - Wheat system might reduce the microsclerotial of
M. phaseolina, which could result in low infection in other hosts. Previous reports also indicate that crop rotation with non-host crops or less susceptible varieties have been influential in reducing inoculums of
M. phaseolina (Gupta et al., 2012). Besides this, charcoal rot was partially higher in the field with light soil (6.5 - 21.3%) than in heavy soil (6.6 - 19.0%) across the agroclimatic zones and years (Fig 5). However, some fields with heavy soil also had a higher incidence than light soil in the same zone. Therefore, it could not be concluded that soil types influenced the incidence of charcoal rot disease in soybean.
Cultural and morphological characteristics of isolates
Mycelial growth among all isolates (16) was significantly varied from 76.33 (MP-13(SHD)) to 90.00 (MP-9(SGR)) mm (Table 2). Seven isolates
i.e. MP-1(JBP), MP-2(NAR), MP-4(CWA), MP-6(HBD), MP-3(SEO), MP-9(SGR) and MP-14(SHR) were designated as Fast-Growing Isolates (FGI). Colony elevation patterns of most of the isolates were partially fluffy to fluffy (Fig 6). The colony colour of the isolates varied from dark black or black [MP-1(JBP), MP-3(SEO), MP-4(CWA), Mp-11(VDS) and Mp-13(SHD)] to greyish [Mp-5(BET) and Mp-10(RSN)] (Fig 6). Most of the isolates had oblong shape microsclerotia, except some had round. The number of sclerotia /microscopic fields (10x) varied significantly in most of the isolates and ranged from 41.07 [Mp-10(RSN)] to 77.20 [Mp-4(CWA)]. Four isolates,
i.e. Mp-4(CWA), Mp-1(JBP), Mp-2(NAR) and Mp-14(SHR), had more than 70.0 sclerotia per microscopic field. Our findings revealed that isolates were variable concerning growth pattern, colony appearance and morphological traits, which are in agreement with the previous finding that also depicted variabilities in
M. phaseolina isolated from different areas
(Aghakhani and Dubey, 2009; Iqbal and Mukhtar, 2014; Gade et al., 2018).
Pathogenic variability
Progression of downward necrotic lesion through cut stem inoculation of all isolates differed at 3, 6, 9 and 12 days after inoculation in the variety JS 20-29. The Jabalpur isolates Mp-1(JBP) was found to be highly aggressive as recorded high length necrotic lesion of 15.30 cm, followed by Hoshangabad Mp-6(HBD) and Sagar Mp-9(SGR) isolates (12.80 cm) at 12 days after inoculation (Table 2). The isolates from Damoh Mp-8(DMH), Raisen Mp-10(RSN) and Umaria Mp-12(UMR) were the least aggressive, with the most downward, about 3.00 mm lesion. Similar trends were also depicted in the area under the lesion progression curve (AULPC). Through this, it was concluded that isolates from Jabalpur and Hoshangabad were the most aggressive among all sixteen. Previous to this, significant difference in necrotic lesion development and relative area under the disease progress curve by isolates of
M. phaseolina from soybean was well described by
Twizeyimana et al. (2012). Varieties also differed in necrotic lesion development after cut stem inoculation by
M. phaseolina (Amrate et al., 2019).