Influence of weather factors on the development of leaf spot of betel vine caused by colletotrichum capsici

Betel vine (Piper betle L.) is an important cash crop in India, Bangladesh, Malaysia, Sri Lanka, Pakistan, Mauritius and Myanmar. Plants of betel vine are cultivated in conservatories under shady and humid condition that favours the development of many diseases (Chattopadhayay and Maiti, 1990). Leaf spot caused by Colletotrichum capsici is one of the major disease of the crop (Bhale et al. 1987) that in severe condition lead to death of the plants producing up to 25-90 percent losses of the crop in different parts of India (Dastur, 1935; Chattopadhayay and Maiti, 1990). This disease of betel vine was first identified by (Roy, 1948) and it is also known as anthracnose of ‘Pan’ (vernacularly called as ‘Jhalma’). The epidemic form of the disease in the country has also been reported by (Basak et al. 1992) and (Hossain et al. 1986), causing 10-60 percent yield loss (Singh and Joshi 1971, Maiti and Sen 1982). It reduces market value of the crop (Maiti and Sen, 1971). Environmental factors play an important role on the development of disease as they help the pathogen for growth, dissemination and infection as well as influences on expression of susceptibility/ resistance of the host plant after infection (Walker, 1965). Among the environmental factors, temperature, humidity and rainfall are the most crucial for the development of leaf spot of betel vine (Maiti and Sen 1982). So, the present study was undertaken to find the effect of weather factors on the development of the disease severity and to formulate suitable prediction equation.

The field experiment on betel vine was conducted in the betel vine garden during the year 2009, 2010 and 2011 at Baliapal block of Balasore district in coastal belt of Odisha. Five betel vine cultivars viz. Bangla, Chandrakona, Balipan, Aima Bangla and Sanchi were planted in the garden. Young cuttings of vines of each cultivar were used as planting materials. The cuttings were planted in 3.0 x 3.0 m plots maintaining 45 cm (R×R), 15 cm (P×P) and 1.0 m plot to plot distance. After establishment of the cutting the field was fertilized with only mustard oil cake and neem cake @ 16q ha -1 per year.
The oil cake was applied in equal splits during the months of May and September. All the agronomic and cultural practices such as weeding, watering, earthing up, pruning, tieing, vine lowering and other operations were done as and when necessary to maintain congenial environment for proper growth and development of the plants. Shed was provided with a thin roof made of straw and bamboo sticks. Normally betel vine growers of Balasore district raise their betel vine garden exclusion of chemical fertilizers. So, no chemical fertilizers were used in experimental field. The experiment was laid out in a 4x3 factorial in randomized complete block design with three replications. To study the development of leaf spot observations were taken in betel vine garden during three consecutive years viz. 2009, 2010 and 2011. Every year plants of all five cultivars were checked regularly with 10 days interval during January to December to record severity of the disease. Severity was indexed on a subjective scale based on leaf area diseased. The severity of the disease was computed following a modified 0-5 rating scale (Basak et al. 1992), where 0= healthy, 1= up to 5%, 2= 6-15%, 3= 16-30%, 4= 31-50% and 5=>50% diseased leaf area. It was expressed as Percent Disease Index (PDI), which was computed using a standard formula (Basak et al. 1992) as shown below: Sum of all numerical ratings PDI = ------------------X 100 Total numbers of leaves counted X Maximum rating For studying the multiple effects on the dependent variables the multiple regression analysis was done. The monthly average temperature (X 1 ), average relative humidity (X 2 ) and total rainfall (X 3 ) were recorded every month during the study period. The meteorological data was collected from 'Meteorological office, Chandanmahal, Sunhat, Baleswar'. Before analysis the data, square root transformation of PDI value was calculated. To predict the disease development, multiple regression equations were computed for each cultivar by using SPSS computer

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software. Coefficient of determination (R 2 ) was also calculated and tested for significance at 1% level of probability.
The pooled data obtained from betel vine on leaf spot disease severity for the three consecutive years showed that the mean PDI values of leaf spot on five betel vine cultivars namely Bangla, Chandrakona, Balipan, Aima Bangla and Sanchi were 3.3,5.4, 4.4,3.2 and 1.9, respectively. Each year the highest incidence of the disease was recorded on Chandrakona followed by Balipan, Bangla, Aima Bangla and Sanchi (Table 1). Similar type of findings was recorded by Goswami et al. (2002).
To develop the prediction equation of leaf spot of betel vine multiple regression equation was computed for taking into account the disease severity of each cultivar and the three weather parameters (Table 2). In these equations predicted severity (dependent variable) was calculated by using weather parameters i.e. average monthly temperature, average RH and total rainfall as independent variables. The goodness of fit of the equation was evaluated by coefficient of determination (R 2 ), adjusted coefficient of determination (R 2 adj.) and error mean square.
The predicted disease severity depicted for the five cultivars showed different prediction equations (Table 2). In case of cultivars Bangla and Sanchi it was observed that among the three weather parameters only average temperature was positively and significantly affected the disease severity. Coefficient of determination (R 2 ) value indicated that the predicted value of PDI can be explained 55.3 percent and 55.5 percent of the total variation in the PDI in case of Bangla and Sanchi respectively.
On the other hand, it was observed that the disease severity of the cultivars Chandrakona and Aima Bangla depended upon both average temperature and RH positively and significantly. The coefficient of determination (R 2 ) and adjusted coefficient of determination showed that among the two equations Chandrakona were good fit and it was confirmed high coefficient of determination (R 2 =0.738) and low adjusted coefficient of determination and standard error estimate.

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Average temperature and total rainfall were significantly and positively correlated with the disease severity for the cultivar Balipan and here coefficient of determination (R 2 ) value showed highest variation in disease severity that can be explained upto 75 percent with combined effect of weather variable (Table 2). Similar type of experiment on stem rot of betel vine under close conservatory condition was carried out by Datta et al.(2009) and on Phytophthora wilt of betelvine by Singh (1996).
PDI of leaf spot recorded on five betel vine cultivars through out the year (January to December) revealed considerable prevalence of the disease in the month of May which increased gradually up to August (Fig 1). The highest peak of disease severity obtained during August, when all the three weather factors (especially, temperature and rainfall) were higher as compared to other months of the year. In all the three experimental years, maximum rainfall obtained in the month of July -August without much variation in the average temperature so the disease severity found to be at par in these two months then declined up to the month of November. This finding was supported by Das Gupta and Sen (1985). They found that 92 percent RH was the critical moisture level for severe leaf spotting and heavy losses in betel vine. Disease pressure was not appreciable in the month of November, December and January. Basak et al.(1992) reported under dry weather the progress in disease severity was slow whereas during moist weather leaf spot enlarges rapidly causing rottening of whole leaf. Observing the disease situation along with the weather prevalence -fact revealed that the monthly average temperature and RH remained low in November to January and both the weather factors increase gradually from February up to August but maximum rainfall obtained in the month of June to August. These finding presented (Fig 1) clearly showed that the severity of leaf spot of betel vine was directly influenced by the three weather factors. Similar results were reported by Goswami et al. (2002).
Research findings of the present investigation indicated that among the five predominant cultivars of betel vine in coastal belt of North Odisha Chandrakona showed maximal and Sanchi showed minimal leaf spot disease severity caused by Colletotrichum capsici and the degree of which was greatly influenced by the three weather factors i.e. temperature, relative humidity and rainfall. Maximum disease severity was observed during July-August, when all the three weather factors were higher in comparison to other months of the year. Roy (1948) also recorded severe leaf spotting of betel vine due to anthracnose when rainfall was high. Multiple regression equation computed revealed positive correlation with the weather factors. However, the co-efficient of determination was highly significant in case of cultivar Chandrakona, Balipan and Aima Bangla (73.8 percent, 75.2 percent and 72.2 percent respectively).