Bean Anthracnose (Colletotrichum lindemuthianum) in Kashmir: Epidemiology and Yield Loss Assessment

Common bean (Phaseolus vulgaris L.) is an annual legume crop of Fabaceae. It has originated in South Mexico and Central America (Vavilov, 1951) and has gained increasing popularity due to its quality proteins and nutritional balance besides certain medicinal properties (Adikshta and Sharma, 2017). This crop has become third most important food legume after soybean and peanut in production and is also the most utilized crop for direct consumption in the world (Broughton et al., 2003). According to Food and Agriculture Organization, global production and productivity of dry bean and green bean in 2019 cropping season was about 31.40 million tonnes and 24.22 million tonnes, respectively. In India dry bean and green bean are cultivated on an area of 15.42 and 0.24 million hectare, respectively. India alone produced 6.39 million tones of dry bean with a productivity of 4.14 quintals ha^-1 (Anonymous, 2019). Jammu and Kashmir is one of the most important pulse growing regions where different pulses are grown over an area of about 26.57 thousand hectares with an annual production of 8.41 thousand tonnes and productivity of 0.317 tonnes ha^-1 (Anonymous, 2015). Bean crop suffers due to a number of fungal and bacterial diseases. Among these bean anthracnose caused by Colletotrichum lindemuthianum is considered an important disease for causing huge yield loss year after year (Junaid et al., 2014). The yield losses due to this disease can reach 100 per cent when badly contaminated seeds are sown and environmental conditions remain favorable for disease development for long periods (Sharma et al., 1994). The mean incidence and intensity of this disease in Kashmir during 2011 cropping season varied from 28.47-40.18 per cent and 10.69-16.42 per cent, respectively (Junaid et al., 2014). However, it may vary from season to season depending upon the weather. Therefore, region specific epidemiology that can provide base line data for deciding any disease suppressive changes in crop geometry and sowing date besides helping in predicting disease in a given season has a scope. Moreover, the extent of yield loss vis-à-vis this disease has to be determined through a systematic approach. In view of importance of this crop in Kashmir, both these aspects of bean anthracnose were studied as follows.

The present investigations on bean anthracnose (Colletotrichum lindemuthianum) in Kashmir vis-a-vis epidemiology and yield loss assessment were conducted in experimental farm of the Division of Plant Pathology, Sher-e-Kashmir University of Agricultural Sciences and Technology, FoA, Wadura Sopore, located at 34^o-20' north latitude and 74^o-24' east longitude at an elevation of 1610 meters above mean sea level.
 
Temporal dynamics of bean anthracnose
 
Bean crop (Cv. Shalimar Rajmash-1) was raised over an area of 100 m2 with 30 cm and 10 cm row to row and plant to plant spacing, respectively. To ensure the presence of inoculums, The boarder rows inoculated with Colletotrichum lindmuthianum spore suspension (10⁷ conidia ml^-1) in the evening hours at 1^st trifoliate stage and the entire crop was then maintained unsprayed under natural epiphytotic conditions. The crop was examined periodically for disease scores until it reached physiological maturity. The observations on disease incidence was worked out as follows:
   
For disease intensity, the sampled units were categorized using 0-9 severity scale of Mayee and Dattar (1986). Where 0 = No symptoms 1= Small, round brown spots covering < 1 per cent area, 3=  spots covering 1-10 per cent area, 5 = spots covering 11-25 per cent area, 7 = spots covering 26-50 per cent area, 9 = spots covering > 50 per cent area. It was then worked out by using the following formula:
 
Where,
n = Number of units in each category, v = Numerical value of category, N = Number of units examined and G = Maximum numerical value i.e. 9.
       
In order to see the role of temperature RH and rainfall in disease development the weather data for the period was obtained from Indian metrological department Srinagar through Division of Agronomy FoA, SKUAST-K.
 
Disease simulation and yield loss assessment
 
The experiment comprised of five treatments with four replications in randomized block design. Variable levels of disease were created by inoculating different unit populations of bean at different phenological stages viz., 1^st trifoliate, 3^rd trifoliate, flowering and pod filling representing five treatments. Until inoculation, the plants were maintained disease free by regular chemical intervention, using combo formation of Carbendazim and Mancozeb @ 0.25%.
       
The observation on terminal disease were recorded on a random sample of 10 plants per replication. Disease incidence and intensity were worked out as given above. However, the seed yield was calculated from net yield of four 3 meter rows in each plot.

First symptoms of bean anthracnose were recorded in the 22^nd standard metrological week (SMW) and it reached maximum of 77.06 per cent incidence and 54.51 per cent intensity towards the end of growing season by 29^th SMW. Weekly average temperature, RH and total precipitation recorded during the corresponding period ranged from 16.6 to 23.4oC, 62.2 to 76.5 per cent and 1.7 to 48.2 mm, respectively. Periodic increase in disease incidence was higher (>10%) during 24^th, 26^th and 29^th SMWs. Average temperature, RH and total precipitation recorded during the corresponding period were ≥ 17.5 ≥ 67.5 ≥ 15.4, respectively. Comparatively, the periodic increase in disease intensity was higher (>10%) during 26^th and 29^th SMWs. The prevailing average temperature, RH and total precipitation recorded during the corresponding period were ≥ 18.9^oC, > 70 per cent ≥ 15.4 mm (Table 1 and Fig 1).
 

 

       
These observations confirmed prior findings of Rathava (2017) who in Gujrat recorded first symptoms of bean anthracnose after 26 days of sowing. Further, he found the disease was continuously increasing upto 16^th October reaching 35.20 per cent from 6.80 per cent recorded on July 24^th and reported quick jump of the anthracnose intensity during 30^th to 34^th standard weeks which he considered a window period for the anthracnose. Our findings are further supported by Patial (2004) who reported (79.48%) terminal disease intensity in the highly susceptible bean cultivar, Jawala.
       
The present findings indicated significant and positive correlation of temperature with disease intensity. However, the correlation of RH with disease incidence and intensity was non-significant but positive. Moreover, rain showed significant and positive correlation with disease incidence although its impact on disease intensity was found non-significant (Table 2). Significant and positive correlation of RH with bean anthracnose was also reported by Rathava (2017). However, he found rainfall and minimum temperature to be more important in the disease development as compared to other weather parameters. Earlier, Hall (1994) reported that bean anthracnose is favoured by 13-26^oC temperature. During the course of present investigation there was not much variation in average temperature due to which its impact on overall disease development was not worked out. The effect of rain being significant in the present investigation is in corroboration of the findings of Tu (1981), who have also reported that periods of heavy rainfall are responsible for the spread of C. lindemuthianum in the field. The underlying reason could be the appearance of conidia in a gelatinous mass which can be more efficiently displaced by rain splashes.


       
Regression analysis was also worked out to know the extent of correlation which is presented in (Table 3). It indicated that for every unit increase in RH and precipitation there was an increase of 0.071 and 0.073 units in disease incidence, respectively. Likewise for every unit increase in temperature and RH, there was an increase of 0.920 and 0.447 units in disease intensity, respectively.
 

 
Disease simulation and yield loss assessment
 
The data generated on disease simulation and consequent yield loss (Table 4 and Fig 2) revealed that the terminal disease incidence and intensity were significantly higher when disease initiated at first trifoliate stage and thus caused maximum yield loss. The yield potential as affected by bean anthracnose was 64.38, 49.04, 20.54 and 10.95 per cent when the disease started at first trifoliate, third trifoliate, flowering and pod development stages, respectively. Yield loss was proportionately least (10.95%) when disease started late in the season at pod filling stage. Bean anthracnose is a polycyclic disease and can also start as early in the season as on seedling stage. Moreover, there were well distributed rains during 2019 growing season which helped disease development to this extent. The proportionate yield losses as revealed by correlation studies are due to the fact that bean anthracnose caused reduction in photosynthetic area of affected plant populations. Earlier, Patial (2004) also reported highest yield reduction of 44.59 per cent in cotyledonary leaf stage infection, which is the most susceptible stage of plant with respect to anthracnose infection followed by 35.52 per cent reduction in pod development stage infection and least reduction of 9.61 per cent in second trifoliate leaf stage infection. The findings of Sharma and Sharma (1994) are in close proximity with our finding who found reduction in number of grains per pod, grain weight per pod and reduction in plant height of anthracnose affected bean plants when compared with the healthy plants. The present finding is further in relevance with that of Guzman et al., (1979), who reported yield losses of 95 per cent in the highlands of Colombia when a susceptible bean cultivar was inoculated one week after plant emergence and 38 per cent when inoculated six weeks after emergence with (107 conidia ml^-1). Similarly, Mukunya and Keya (1979) has also recorded losses up to the tune of 100 per cent in kidney bean due to seed borne infection of bean anthracnose in Kenya.
 

 

       
The analysis matrix indicated significant and positive correlation of disease incidence and disease intensity with yield loss (Table 5). The findings are supported by Amin and Ullase (1981), who reported positive and significant correlation between disease indices and infected fruits and total yield in bean crop. Highest correlations were found between yield loss and disease indices during flowering to fruit formation. Similarly, Ali et al., (1987) reported highly significant positive correlation between loss in grain yield and disease severity in sorghum crop.
 

Although the prevailing temperature during Kharif 2019 was less variable its effect on disease intensity was found significantly positive. The correlation of RH with disease incidence and the disease intensity was also positive but non-significant. Further, rain showed significant and positive correlation with disease incidence. The study thus revealed that a weather set comprising of high relative humidity (>75%), moderate rainfall and moderate temperature 16-24oC with an optimum of 17oC leads to rapid development of anthracnose disease in bean. The yield potential of bean was significantly affected by bean anthracnose causing >10 per cent yield loss even after appearing at pod development stage. Given these facts the bean anthracnose be considered among the major diseases of bean in Kashmir. It can start any time in the vegetative and reproductive stages of the crop and is more importantly favored by frequent rain, thus highlights the significance of wider spacing in minimizing the yield losses in bean.