Epidemiological Assessment of Charcoal Rot (Macrophomina phaseolina) on Mungbean in Central Punjab, Pakistan ^†

Abstract

Charcoal rot caused by Macrophomina phaseolina limits mungbean yield around Faisalabad. Fields surveyed during Kharif 2024 showed 43–58% disease incidence. At the research farm, disease severity rose from 8.6% (14 days after sowing) to 62.4% (maturity). Plants with 40–55% infection lost 42% of grain yield (1182 to 684 kg/ha). Soil temperature at a 10 cm depth best predicted disease (r = +0.86). Each 1 °C above 27 °C added 8% more severity. Early sowing in April and resistant varieties were recommended for farmers to cut losses.

1. Introduction

Mungbean (Vigna radiata L. Wilczek) is one of the important pulse crops in Pakistan and it grows throughout the summer (Kharif) season. The area of mungbean under national area is 307,000 hectares, which yields 211,000 tons with an average yield of 687 kg/ha. This yield is considerably lower than the genetic potential of 2000–2500 kg/ha with disease-free irrigated conditions [1]. Insect pests, weed and diseases cause major yield gaps whereby the soil fungi are also found to love the hot dry weather of plains in Punjab [2].

Charcoal rot of Macrophomina phaseolina (Tassi) Goid. is one of the diseases that present serious threats to mungbean. Fungus attacks to roots and the lower stem, causing wilting and premature death of plants, was first reported in Pakistan in 1970s. The symptoms are dark brown color change of vascular tissues and the appearance of black microsclerotia in cortex—hence the name “charcoal rot”. A disease will be observed when the temperatures are more than 35 °C and soil moisture is less than 20% [3].

Faisalabad district is also the center of mungbean production in Punjab. Maximum temperatures are 42 °C in summer and the rainfall is low (350 mm) per year, which are ideal conditions that support M. phaseolina. Farmers have stated a 30–60% loss in yield of the affected fields but have no specific data on the disease distribution, course of severity or exact damage in the economy. The earlier surveys were only done Punjab-wide but omitted tehsil-level information around Faisalabad.

There are limited controlled experiments to determine the yield losses by growth stage. Knowledge of the time to maximize the damage caused by a disease would be used to make decisions by farmers on when to irrigate or apply chemicals [4]. The effect of soil temperature on the germination of microsclerotia is shown but the threshold temperatures should be locally tested.

The three aims of current study were (1) to survey the incidence of charcoal rot in five mungbean-producing tehsils around Faisalabad, (2) to determine the severity of the disease progression and yield losses in the changing conditions of the research farm, and (3) to correlate the disease parameters with weather parameters. The project was conducted in the season of Kharif 2024 (May to September) and creates the background information to facilitate the extension program in agriculture in Punjab.

2. Materials and Methods

2.1. Disease Incidence Survey

The Faisalabad survey was completed in five tehsils within an 80 km radius of Faisalabad: Jaranwala, Sammundri, Tandlianwala, Gojra and Jhang. A total of 54 commercial mungbean fields were evaluated at the stage of pod-filling (70–94 days after sowing) when the symptoms were fully expressive. Fields were selected purposively based on farmers’ reported cultivation history [5].

Five points were labeled with a diagonal transect at each field. Sixty plants each (300–720 plants/location) were analyzed consecutively. Infection was characterized by root/stem rot, crown lesions and microsclerotia following longitudinal split of lower stem. Incidence was based on the following standard formula:

$$\mathrm{Disease} \mathrm{Incidence} (\%) = {\displaystyle \frac{number of infected plants}{total plants examined}\times 100}$$

Data was analyzed by analysis of variance. Means were separated by LSD test at the 5% level.

2.2. Experimental Design

The experiment was done at Pulses Section, Ayub Agricultural Research Institute (AARI), Faisalabad (sandy loam soil with a pH of 7.8–8.1 and 0.64% organic matter). The sown NM-2011 was susceptible to the disease and planted on 25 May 2024 in a randomized complete block design with three replications. The plot dimensions were as follows: 5 m × 4 m, interrow distance of 30 cm, and interplant distance of 10 cm. Basal fertilizer was used as follows: 25 kg N + 50 kg P₂O₅/ha. Furrow irrigating based on AARI recommendations was conducted after every 10 days.

Natural infection occurred due to the uniform disease history of the experimental site. Each plot was rated on fifty plants at six developmental stages: 14 (seedling), 28 (vegetative), 42 (pre-flowering), 56 (flowering), 70 (pod-filling) and 84 days after sowing (maturity). A standard 0–5 scale was used which is given below.

Rating	Description
0	No symptoms
1	1–10% root/stem rot
2	11–25% rot + slight wilting
3	26–50% rot + moderate wilting
4	51–75% rot + severe wilting
5	>75% rot + plant collapse

DSI (%) = [Σ(s × n_s)/(S × N)] × 100

Percentage data were arcsine-transformed before ANOVA. Means were separated by LSD (p ≤ 0.05).

2.3. Yield Component and Loss Assessment

At maturity (90 DAS), the yield characteristics were measured on 15 randomly tagged plants per replication (45 plants per treatment). Parameters involved pods/plant, seeds/pod, the weight of 100 seeds in an oven (dried to 12% moisture), biological yield/plant and grain yield (kg/ha).

In the case of yield loss relationships, there will be six microplots (2 × 2 m) set up per disease rating category. The final DSI was affirmed at DAS 70. Yield loss is calculated as follows:

$$\mathrm{Yield} \mathrm{Loss}\% = {\displaystyle \frac{healthy yield- disease Yield}{diseased yield}}\times 100$$

Linear regression was modeled on the relationship between disease rating (X) and grain yield (Y) through Y = a + bX. Goodness of fit was measured by the coefficient of determination (R2).

2.4. Weather Monitoring and Statistical Analysis

Temperature of soil (AMS) during 10 cm depth with soil thermometer (three soil temperature measurements/plot on one rating date). Air temperature (maximum and minimum), relative humidity, rainfall, and wind speed were sent to the AARI meteorological observatory. Pearson correlation coefficient (r) between biweekly averages and data on the disease was calculated:

$$\mathrm{R} = \Sigma [({\mathrm{X}}_{\mathrm{i}} - \overline{X})({\mathrm{Y}}_{\mathrm{i}}-\overline{\mathrm{Y}})]/\surd \{\Sigma ({\mathrm{X}}_{\mathrm{i}} - \overline{X}{)}^2 \times \Sigma ({\mathrm{Y}}_{\mathrm{i}}-\overline{\mathrm{Y}}{)}^2\}$$

Linear regression was used to model the soil temperature versus DSI. All were analyzed with Statistix 8.1 software. Significance level were set at p ≤ 0.05 and p ≤ 0.01.

3. Results

3.1. Weather Conditions During Experiment

Typical hot dry weather was experienced in Kharif 2024 and this was conducive to charcoal rot (Table 1). The highest air temperatures (42.8 °C, June) and soil temperature at 10 cm depth (37.6 °C, July) were recorded. Relative humidity remained low (38–62%) except in the case of the August monsoon (62.4%). Mean soil temperature (34.64 °C) was higher than the pathogen minimum infection temperature of 27 °C during crop growing.

Table 1. Charcoal rot incidence in surveyed mungbean fields (n = 60 plants/field).

Month	Max Air Temp (°C)	Min Air Temp (°C)	Soil Temp 10 cm (°C)	RH (%)	Rainfall (mm)	Wind Speed (km/h)
May	40.2	24.6	31.4	42.8	18.6	12.4
June	42.8	26.4	34.8	38.4	12.2	14.6
July	41.6	28.2	37.6	56.2	44.8	10.8
August	40.4	27.6	36.8	62.4	38.4	9.6
September	37.2	24.8	32.6	54.8	22.4	11.2
Mean	40.4	26.3	34.6	50.9	27.3	11.7

Means followed by same column not significantly different (LSD, p ≤ 0.05).

3.2. Disease Incidence Across Tehsils

Charcoal rot was present in all fields surveyed that showed an endemic condition (Table 2). There were great differences in incidence among locations (p ≤ 0.05). Tandlianwala showed the highest (58.33 ± 2.46) and Jaranwala was the second best in (52.22 ± 2.14) percentage. Jhang had the lowest (43.33 ± 1.87%) but economically significant incidence. The grand mean of 49.64444 percent shows moderate–high levels of disease pressure in Faisalabad district.

Table 2. Monthly agro-meteorological conditions during Kharif mungbean season.

Location	Fields Surveyed	Plants Examined	Infected Plants	Incidence (%)	SE (±)
Jaranwala	12	720	376	52.22 a	2.14
Sammundri	10	600	274	45.67 c	1.98
Tandlianwala	12	720	420	58.33 a	2.46
Gojra	10	600	292	48.67 b	2.08
Jhang	10	600	260	43.33 c	1.87
Total	54	3240	1622	49.64444	2.11

Means followed by same superscript letter within column not significantly different (LSD, p ≤ 0.05).

3.3. Disease Severity

The DSI grew progressively between the seedling and maturity stage (Table 3). Initial infection (8.6% at 14 DAS) was low but increased rapidly post-flowering. The sharpest increase (+22.9) was 28–56 DAS when the soil temperatures were the highest (34.8–37.6 °C). At 84 DAS of high level of severity, the final DSI was 62.4%.

Table 3. Disease Severity Index (DSI) of charcoal rot at successive growth stages (n = 50 plants/plot).

Growth Stage	Days After Sowing	DSI (%)	SE (±)
Seedling	14	8.6 e	0.82
Vegetative	28	18.4 d	1.14
Pre-flowering	42	28.7 c	1.83
Flowering	56	41.3 b	2.24
Pod filling	70	53.8 a	2.61
Maturity	84	62.4 a	2.89

Means followed by same superscript letter within column not significantly different (LSD, p ≤ 0.05).

3.4. Impact on Yield Components

Diseased plants had all yield parameters that were significantly lower (Table 4). Grain yield was reduced by 42.13 percent between 1182 kg/ha (healthy) and 684 kg/ha at 40–55 DSI. The number of pods per plant decreased by 38.64, which indicated the establishment of fewer reproductive sinks during infection stress.

Table 4. Yield attributes of healthy vs. charcoal rot-infected mungbean plants (40–55% DSI).

Parameter	Healthy (Mean ± SE)	Diseased (Mean ± SE)	% Reduction
Pods/plant	26.4 ± 1.42	16.2 ± 1.18	38.64
Seeds/pod	9.8 ± 0.64	7.1 ± 0.52	27.55
100-seed wt (g)	4.62 ± 0.18	3.48 ± 0.15	24.68
Bio. yield (g/plant)	38.6 ± 2.16	24.3 ± 1.87	37.05
Grain yield (kg/ha)	1182 ± 38.4	684 ± 27.6	42.13
Harvest index (%)	42.8 ± 1.62	34.2 ± 1.34	20.09

Differences between means were assessed using an independent t-test at p ≤ 0.05.

3.5. Yield Loss vs. Disease Rating

The decrease in yields was directly associated with disease rating (Table 5). A rating of 3 (26–50% tissue affected) resulted in a 40.27% loss and a rating of 4 resulted in a 58.04% loss. The regression model is as follows: grain yield = 1188.46 −183.2 × rating (R² = 0.987, p = 0.000). Each rating unit use 183 kg/ha.

Table 5. Grain yield loss associated with charcoal rot disease rating classes (n ≥ 6 microplots/class).

Rating	Description	DSI Range (%)	Yield (kg/ha)	Loss (%)
0	Healthy	0	1182	0
1	Trace	1–10	1048	11.34
2	Mild	11–25	882	25.38 *
3	Moderate	26–50	706	40.27 **
4	Severe	51–75	496	58.04 **
5	Collapse	>75	258	78.17 **

Means differ significantly (independent t-test): * p ≤ 0.05, ** p ≤ 0.01.

3.6. Weather Correlation

There was the highest correlation between soil temperature and all disease parameters (Table 6). Correlation coefficients are as follows: incidence (r = +0.89), DSI (r = +0.86), yield loss (r = +0.82), with all correlations significant at p < 0.001. A ninety-seven point six percent variation (R² = 0.976) was explained using the linear model DSI = −220.4 + 8.24 × soil temperature. The threshold of disease was soil temperature of about 27 °C.

Table 6. Pearson correlation coefficients between weather variables and disease parameters.

Variable	Incidence (r)	DSI (r)	Yield Loss (r)
Soil temp 10 cm (°C)	+0.89 **	+0.86 **	+0.82 **
Max air temp (°C)	+0.83 **	+0.79 **	+0.76 **
RH (%)	−0.71 **	−0.68 **	−0.64 **

** Correlation significant at p ≤ 0.01.

4. Discussion

The survey findings affirm that charcoal rot is an established disease in Faisalabad mungbean tehsils, with an average incidence of 49.64444 that is within the range of 42–59%, found by Pandey between 2016 and 2018 [4]. The tehsil-level analysis shows a much greater incidence of 58.33 in Tandlianwala than in Jhang of 43.33, which can be attributed to the difference in soil texture. M. phaseolina microsclerotia were shown to survive 4–5 years in sandy soils with less than 0.5% organic matter and not to survive 2 years in clay loams [6]. The light-textured profile of Tandlianwala is suitable to the environment that supports the existence of pathogens over time, light soils are suitable in Jhang, and deposited silt in the Chenab river supports the enhancement of moisture retention [7,8].

The development of disease severity of 8.6 percent DSI at 14 days after sowing to 62.4 percent at maturity follows the classical temperature effect pattern reported by Gupta et al. in soybean, with 68 percent of the overall seasonal DSI change in the 4075 days after sowing in the soil temperature of 3538 °C [9]. The present experiment comes up with the same critical period of 42–70 days of sowing which coincides precisely with the July soil temperature peak of 37.6 °C, which matches the thermal optimum range of 30–37 °C in germination and root infection of microsclerotia as set by Abdel-Monaim. The gradual early succession in the seedling phase is an indication of partial vascular tolerance associated with lignification, which is broken down at the flowering stage in bean pathogen systems.

The relationships between yield loss serve as quantitative validation of published multi-location data between legumes. The 42.13 percent decrease in grain yield at 40–55 percent DSI is equivalent to the 38–51 percent range compiled by Wyllie on other hosts. Maximum loss was recorded in pods with a 38.64% loss per plant due to the M. phaseolina mycelial blockage of xylem vessels inhibiting floral initiation and aborting developing flowers to create a classical sink limitation when plants are under root infection stress [10]. The 27.55 percent reduction in the number of seeds per pod indicates the impaired photosynthate delivery to the growing sinks, whereas the 24.68 percent decrease in weight of 100 seeds confirms that there is chronic interference of source–sink associations. Calculation of economic impact is as follows: The 498 kg/ha loss in production has a value of PKR 89,640/ha at a current market price of PKR 180/kg, which is above the recorded production cost of PKR 68–75,000/ha and eradicates the profit margin of Punjab smallholder farmers working on 2–5 hectare plots.

The maximum air temperature correlation of r = +0.79 had been exceeded by the soil temperature at a 10 cm depth, with the correlation coefficient r = +0.86 found to be the most dominant epidemiological driver. This observation confirms observations by Islam who set a 26–28 °C soil temperature as the limit of infection of the Pakistani M. phaseolina isolate in various crops [11]. The existing regression model accounts for 97.6% of the variation in DSI using equation DSI = −220.4 + 8.24 × Tsoil, where a one-unit increase in rating causes a 8.24 percentage point increase in the DSI, which is equivalent to a 41 kg/ha yield loss in rating effect. The predisposing mechanism of drought stress at the pre-infection period is supported by a negative correlation with relative humidity (r = −0.68) [12].

The management implications are directly related to Punjab farming realities wherein the sowing date is adjusted to mid-April because of the 2.5 °C July soil temperature upper limit and 35–42 DSI. Furrow irrigation practices at 42 and 56 days after planting which keep soil moisture above 20% reduce disease propagation by 32 percent [13]. AARI has an average DSI that lowers susceptibility by 28% across NM-2011, NM-2020 and AARI-2016. Chemical management using carboxin + thiram seed treatment on 2 g/kg seed mixed with a single soil drench during flowering will offer 65–72% disease control [14]. Regression analysis resulted in an economic threshold of rating 1.4, which represents 14% DSI, at which the prevented loss value is PKR 4500/ha application cost. This was surpassed by all 54 surveyed fields in the course of Kharif 2024.

Records of Faisalabad climate show 0.8 °C per decade of warming since 1980. The predicted increase of 2 °C by 2050 increases the epidemic-conducive weeks by three, and this may increase the losses in the economy by two folds, unless extension services put the recommended adaptation measures in place.

5. Conclusions

Charcoal rot appears in half of Faisalabad mungbean fields, causing 42% grain yield loss when disease reaches moderate levels. Soil temperatures above 27 °C during flowering drives rapid disease spread. Farmers lose PKR 90,000 per hectare at current prices. Sow seeds in mid-April to avoid the July heat peak. Use resistant varieties from AARI like NM-2020. Apply one fungicide during flowering; spray when 10–15% plants show symptoms. A soil thermometer costing PKR 500 guides the exact timing required. These steps cut losses by half based on farm trial results. Extension workers can use yield loss tables for farmer advice.