Identification of Sources of Resistance and Yield Loss Assessment for Aerial Blight and Anthracnose/Pod Blight Diseases in Soybean

Soybean [Glycine max (L.) Merrill] is a highly valuable legume which ranks first among the oilseeds in the world. The area and production of soybean in India was 10.83 million ha and 10.93 million ton during 2018-19 (SOPA, 2020). In this, Madhya Pradesh is the key state which grows and produces more than 50 per cent of country’s soybean. In India, nearly ninety eight per cent areas under cultivation of soybean is rainfed (Dupare et al., 2014) and therefore, it has to face many biotic and abiotic stresses from germination to harvesting. The yield potential of recent varieties of soybean is higher (about 2.1 t/ha) as compared to average productivity (1.2 t/ha) of the country (Agrawal et al., 2013). Soybean is known to be affected by more than 100 pathogens (Sinclair and Shurlieff, 1975) across the world and currently about eighteen diseases are infecting the soybean crop in India (Anonymous, 2018).  

Among this, aerial or foliar blight caused by Rhizoctonia solani Kuhn, which also infect the soybean crop across the world (Jones and Belmar, 1989; Wan et al., 2005 and Williamson et al., 2006) and causes significant yield loss in top eight soybean producing countries (Wrather et al., 2010), is a prominent soybean disease in India. It is more severe particularly in the state of Uttarakhand, Madhya Pradesh and Chhatisgarh (Wrather et al., 2010 and Mathpal and Singh, 2017) and favors by high humid field conditions (Yang et al., 1990). Another disease, soybean anthracnose caused by Colletotrichum truncatum (Schw.) Andrus and Moore, infects above ground parts and pods (Pod blight) of soybean and favors by warm humid weather (Sharma et al., 2011) is presently the second most wide spread disease in the country (Anonymous, 2012). Anthracnose is also reported from many soybean growing countries (Manandhar and Hartman, 1999) and causes considerable monetary losses (Wrather and Koenning, 2006).

Varietal resistance is always very effective and best way to manage and reduce the yield losses caused by diseases. At present, there are few reports of resistance/tolerance against aerial blight and anthracnose/pod blight diseases of soybean in India (Sharma et al., 2016) and this may be varied under different environmental conditions. In view of this, the present investigation was undertaken to identify the field resistance against both the diseases so that these genotypes may be utilized in contemporary resistance breeding programme. Moreover, the yield loss assessment was also worked out to quantify the losses due to higher level of disease severity in different varieties.
 
 

Experimental details
 
An experiment was carried out to identify source of resistance against aerial blight or Rhizoctonia aerial blight (RAB) and Anthracnose/pod blight diseases of soybean in the field of All India Coordinated Research Project (AICRP) on Soybean at J.N.K.V.V., Jabalpur (Latitude: 23°14 N, Longitude: 79°56 E, Altitude: 411.5m) during Kharif 2017, 2018 and 2019. The AICRP on soybean has identified this centre as hot spot for Aerial blight. Whereas anthracnose and pod blight were also occurred every year with varying severity at this place. In this field trial, a set of one hundred twenty one valuable soybean genotypes (released varieties, exotic lines and advanced lines) including six susceptible checks i.e JS 93-05, JS 335, JS 72-280, Punjab 1, RKS 18 and VLS 58 were evaluated for resistance. Primarily, seeds were treated with thiomethoxam 30% FS @ 10 ml/kg to protect the insect pest i.e. white fly and others at initial stage of crop. Thereafter showing was done in two rows (4 m length and 40 cm spacing) in augmented plot between 28^th to 30^th June each year. All the genotypes were taken care as per the recommended packages and practices except application of fungicides. The crop was regularly observed for incidence and severity of aerial blight and anthracnose and or pod blight.
 
Observations for Aerial blight
 
The severity of aerial blight was recorded from fifteen randomly selected plants at 80-90 days after sowing (before maturity). Selected plants were approximately divided into three positions as bottom, middle and top and from each position two to four leaves were graded. In grading, 0-9 scores were given on the basis of per cent leaf area infected (Mayee and Datar, 1986). The per cent disease index (PDI) was calculated for aerial blight by using the formula given by Wheeler (1969).

PDI ={Sum of individual rating/(Number of examined leaves x Maximum disease rating)}x 100
 
Observations for Anthracnose/pod blight
 
Anthracnose primarily affects stem and petioles and thereafter pods or in some cases only pods. Therefore, both plants (stems, petioles and leaves) and pods were observed critically and 0-9 ranting were given separately for plant and pods on the basis of area affected by anthracnose lesions. Firstly, 15-20 randomly selected plants and thereafter, 45-60 randomly selected pods from these plants were assigned infection rating. Both these infection grades were then utilized in the calculation of per cent disease index (PDI) for anthracnose/pod blight by using the formula given by Wheeler (1969).

Infection rating of anthracnose/pod blight
 


On the basis of per cent disease index (PDI), the genotypes were categorized in different class of resistance and susceptibility as given below
 

 
Estimation of yield losses
For yield losses assessment, ten to fifteen highly infected plants (for each, aerial blight and anthracnose/pod blight) of each susceptible check varieties (six) from each replication (three replications/blocks) were selected and per cent disease index (at 90 days) was worked out for both the diseases separately. Numbers of pods and bulked yield from affected/diseased plants were recorded. Simultaneously, pods and yield were also recorded from healthy plants (free from any disease) from similar plot. Thereafter, numbers of pods and yield per affected/diseased plant (Actual outcome) and per healthy plant (Expected outcome) were worked out. By using this, per cent pods and yield losses [((Expected outcome - Actual outcome)/ Expected outcome) x 100] were calculated. Correlation and regression functions were also derived between per cent disease severity and per cent (pod and yield) losses.

Appearance and distribution of diseases
 
In three years, it had been noticed that symptoms of aerial blight (Fig 1) and anthracnose/pod blight (Fig 2) usually appeared between first to second week of August {bloom initiation (R1) to full bloom (R2) stages} and last week of July to first week of September {vegetative third node (V3) to full pod (R4) stages}, respectively. The severity of aerial blight (0.0-21.8, 0.0-42.3 and 0.0-46.8 per cent) and anthracnose/pod blight (0.0-24.6, 0.0-38.7 and 0.0-56.2 per cent) varied in different genotypes during 2017, 2018 and 2019, respectively (Fig 3 and 4). In 2019, the disease severity was higher and most of the genotypes were found to be affected by aerial blight and anthracnose/pod blight with mean severity of 14.08 and 15.11 per cent, respectively.









Resistance to aerial blight
 
Out of one hundred twenty one, only six  namely JS 20-30, JS 20-57, JSM 222, MACS 1407, PS 1611and Cat 2126 B were found to be highly resistant against aerial blight disease of soybean (Table 1).Whereas fifty genotypes reacted as moderately resistant and rest were found to be moderately susceptible (54) to susceptible (11). The check varieties VLS 58 and RKS 18 were reacted susceptible with more than 25 per cent severity. Before this, Patel (2011) and Amrate et al., (2018) were also evaluated several genotypes and found highly resistant to highly susceptible reaction against aerial blight in central Indian conditions. Amrate et al., (2020) also screened sixty important soybean genotypes for major diseases and found that aerial blight was severe and most of them reacted as moderately susceptible.


 
Resistance to anthracnose and pod blight
 
For anthracnose/pod blight, only fifteen, therein eight i.e. JS 20-69, JS 20-57, JSM 222, AMS 2014-1, MACS 1407, PS 1225, PS 1611, Cat 330 and seven i.e. JS 20-96, JSM 126, DCB 137, SL 900, JS 20-98, JS 20-116, Cat 2126 B reacted as absolute and highly resistant, respectively (Table 2). The other genotypes were found to be moderately resistant (58), moderately susceptible (38) and susceptible (10). In checks, JS 93-05 was most affected with 53.5 per cent severity. Previously, Gawade et al., (2009) evaluated 184 genotypes of soybean and none was found to be highly resistant against Anthracnose. Shrirao et al., (2009) found absolute and highly resistant genotypes and Sajeesh et al., (2014) reported moderate resistant in most of the genotypes (64 per cent) against anthracnose and pod blight.


 
Resistance to both diseases
 
In one hundred twenty one, only five genotypes namely JS 20-57, JSM 222, MACS 1407, PS 1611, Cat 2126 B were found to be highly resistant against both diseases aerial blight plus anthracnose/pod blight (Table 3). Whereas only one genotypes (JS 20-30) was found to be highly resistant (aerial blight) + moderately resistance (anthracnose/pod blight) and nine (JS 20-69, AMS 2014-1, PS 1225, JS 20-96, JSM 126, DCB 137, JS 20-98, JS 20-116, Cat 330) were also reacted as highly resistant (anthracnose/pod blight) + moderately resistance (aerial blight), respectively.


 
Yield loss due to aerial blight and anthracnose/pod blight
 
The results revealed that each variety were responded differently in term of per cent pod and yield losses at different level of severity of aerial blight and anthracnose/pod blight in soybean during 2018 and 2019 (Fig 5 and 6). Both the disease reduced pod and yield significantly in all the varieties (Table 4). In case of aerial blight, per cent pod and yield losses were ranged from 6.7 to 30.7 and 14.1 to 41.0, respectively as against 28.9 (in JS 93-05) to 55.6 (in RKS 18) per cent severity at 90 days old crop. Whereas, in case of anthracnose/pod blight per cent pod losses was found to be little as comparison to aerial blight in most varieties. The highest pod (39.4 per cent) and yield (64.8 per cent) losses were recorded in JS 93-05 on 75.2 per cent severity of anthracnose/pod blight at 90 days old crop. Severity of RAB and Anthracnose was highly significantly correlated with per cent pod (0.966** and 0.957**) and yield (0.995** and 0.995**) losses, respectively (Table 5). Linear regression equations (Fig 7 and 8) for yield reduction from aerial blight (y = 1.021x-16.00, R² = 0.989) and anthracnose/pod blight (y=1.018x-12.44, R² = 0.989) revealed that both the disease can cause huge yield losses at high level of severity and one unit increase in the severity may cause almost one unit yield loss in soybean. In previous findings, Fenille et al., (2002) also reported 31–60% yield loss in North and Northeast Brazil due to RAB and Stetina et al., (2006) also reported pod loss in aerial blight infection and found significant positive correlation between foliar blight and pod losses. Backman et al., (1982) reported 16 to 26% seed yield loss due to anthracnose in the southern area of the United States. Sharma et al., (2011) found no or few seed formation in soybean in case of early stage infection of anthracnose.

The study concluded that most of the evaluated soybean genotypes were moderately resistant to moderately susceptible against aerial blight and anthracnose/pod blight. Only five genotypes namely JS 20-57, JSM 222, MACS 1407, PS 1611, Cat 2126 B were found to be highly resistant against both the diseases. These identified genotypes may further be utilized in resistance breeding programme and for confirmation of resistance at molecular level. It has also revealed that pod and yield losses increased with severity of aerial blight and anthracnose/pod blight. Early infection coupled with high severity of any, aerial blight or anthracnose/pod blight, disease could result in huge yield reduction in soybean.

We are highly thankful to ICAR-Indian Institute of Soybean Research, Indore for providing some of valuable genotypes for field screening and also expressing our gratitude to Head, Department of Plant Breeding and Genetics at university for their guidance and support.