Effect of sowing dates, varieties and weather factors on the occurrence and severity of Alternaria leaf blight and yield of Indian mustard

Field experiments were undertaken to determine the effect of sowing dates and weather parameters on the disease progression or severity of Alternaria blight of rapeseed mustard and their impact on yield for the two consecutive years 2008-2009 and 2009-2010. Disease severity gradually increased with delay in sowing irrespective of three different varieties (Binoy, Seeta and Bhagirathi). The crop sowing from 20 th October to 5 th November recorded significantly less disease severity with the highest seed yield irrespective of varieties and enhanced in subsequent sowing dates. Bhagirathi showed significantly less disease severity with maximum yield followed by Binoy and Seeta on leaf and siliqua infection on all sowing dates. 90 days old plants showed highest disease severity. From 20 th October to 5 th November, sowing only minimum temperature (9.69 to 12.04°C) had significant negative correlation, while minimum relative humidity (44.9 to 60.2% RH), wind velocity (during morning) (0.55 km to 1.04 km/h.) and total rainfall (5.74 mm) had positive significant correlation with disease progression upto 59%. Whereas in 20 th November sowing minimum temperature (9.69 to 15.74°C, minimum(39 to 88%) and maximum (96 to 99%) relative humidity, wind velocity (during morning)(0.4 to 1.29 km/h), bright sunshine hours (3.8 to 9.0 h) and total rainfall (69.6 mm) had positive significant correlation and vapour pressure (noon, morning)(9.7-29.2 mb and 9.2-14.2 mb respectively), wind velocity (evening) (0.4 to 1.3 km/h) had negative significant correlation on disease progression upto 62%. In 5 th December sowing, vapour pressure (noon)(9.7 to 17.5 mb), bright sunshine hours (3.8 to 9.2 h), total rainfall (73.7 mm) and minimum temperature(9.7-18.2°C) had positive significant correlation on disease progression upto 73%. In the West Bengal condition the suitable sowing date of mustard will be last week of October with tolerant variety Bagirathi could be cultivated to avoid loss from Alterania blight and increase the seed yield.


INTRODUCTION
Indian mustard (Brassica juncea) is widely cultivated in Indian sub-continent because of its relative tolerance to biotic and abiotic stresses and inherent high yield potential.Amongst the major constraints in releasing higher yields, the disease likes Alternaria blight incited by Alternaria brassicae (Berk & Sacc.) is one of the most widespread and destructive disease throughout India causing 35 to 40% yield losses (Kolte et al., 1987).The disease also adversely affects quality by reducing seed size, imparting seed colours and oil content (Kaushik et al., 1984).A number of fungicides have been recommended to control this disease but the spraying of fungicides in standing crop in practically difficult, uneconomical and non eco-friendly to the environment.
However, the need for repeated application of fungicides to attain desirable level of disease control discourage the extensive adoption of chemical control by most marginal and resource poor farmers.Because of the present day public perception on pesticide contamination of foods specially the edible oils, there is need for development of alternative economical and ecofriendly approaches for leaf blight disease management (Chattopadhyay et. al., 2005).As Alternaria leaf blight severity requires high humidity and moderate temperature for rapid development, screening for potential resistance should be carried out when optimal levels of these two factors prevail (Sinha et. el., 1992).In order to determine the most effective dates of sowing that permits high or low level of leaf blight for screening cultivars against Alternaria leaf blight for crop production.Three cultivars were sown on four different dates over two years which is aimed at utilizing sowing date to reduce the devastation cause by the disease and the study of weather parameters particularly help the disease severity.

MATERIALS AND METHODS
The field experiment was conducted during rabi season in two consecutive years in 2008-2009and 2009-2010 at Jaguli University Instructional Farm of Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, West Bengal (22.93° N, 88.33° E, 9.75 m above sea level).The climate of the region was humid sub-tropical, with a mean annual rainfall of 1500 mm of which 800 mm was received between June and September.The soil was sandy loam in texture having pH between 6.5 and 6.6 with good water holding capacity and organic carbon content of 0.7%.
The 3 cultivars like Seeta (susceptible), Binoy (moderately susceptible) and Bhagirathi (moderately resistant) and four dates of sowing 20 October, 5 November, 20 November, and 5 December were employed in a split plot design with three replicates.Within a replicate, the four dates of sowing were considered as main plots and within such a main plot the three cultivars were treated as sub plots.Dates of sowing were randomized first followed by randomization of cultivars within a date of sowing.Thus each replicate had represented by four dates of sowing Colums sub plots represented by three cultivars, providing a total of 36 plots.The dimension of each plot measured 5 x 5 m.Within a plot, one cultivar was sown in rows 30 cm apart and there was 15 cm spacing between plants in a row.Treatment wise thinning was done after 20 days of sowing to maintain the required plant population.Recommended fertilizers of N : P : K at 80 : 40 : 40 was applied in the form of Urea, single super phosphate and muriate of potash uniformly as basal dose at sowing and cultural practices were followed uniformly throughout the experimental periods.Although different foliar pathogens and insect pest may occur on mustard, during both years only, Alternaria leaf blight appeared which favoured the natural development of epidemics.Following the first appearance of leaf blight, plants were scored at weakly intervals.From each replicate 5 plants per plot and five leaflets on each plant were selected randomly and rated for blight severity and number of leaves infection.The blight severity was monitored throughout the life of each cultivar till siliqua maturity.During harvest percent siliqua infection was counted from five plants of each replication and numbers of spots per siliqua were counted from randomly selected 10 siliqua of each replication.
Disease severity records were averaged over three replicates for each cultivar and disease progress curves were plotted.For each plot, the area under the disease progress curve (AUDPC) was calculated as per Campbell and Madden (1990).
The severity Index (SI) was then calculated as: Sum of all numerical ratings Percent Disease Index (PDI) = x 100 Total number of leaf observed x maximum rating The yield attributes like number of siliqua per plant and yield kg/hectare was assessed after harvest.All the characters percent of leaf infection, AUDPC, percent of siliqua infection, number of spots per siliqua and yield and yield attributes were computed as analysis of variance of a split plot design (Syndecor and Cochran, 1967) to partition the variance due to cultivars, dates of sowing and their interactions.The significance was tested at 5% level.

Effect of sowing dates
Severity of Alternaria blight was significantly influenced by date of sowing in all the three cultivars for both the years 2008-2009 and 2009-2010.Susceptibility of the Alternaria leaf blight varied with the dates of sowing.Significant effect of sowing dates in both the years of experimentation on Alternaria leaf blight severity and area under disease progress curve was noticed (Table 1).It was evident that disease severity increased gradually in crops with delay in sowing between 20 th November to 5 th December and decreased with early sowing 20 th October to 5 th November.Three varieties, Bhagirathi, Seeta and Binoy also showed increased disease severity with delay in sowing.Bhagirathi showed minimum disease severity (14.99%;AUDPC 8.27) on 20 th October sowing and maximum in Seeta (26.94%;AUDPC 19.56).With delay in sowing the disease severity also *Corresponding author.E-mail: sunitamahapatra@yahoo.co.in  1).Percent of siliqua infection and numbers spots per siliqua were significantly increased with delay in sowing and it was noted on all the three varieties.Two years pooled mean showed that percent of siliqua infection was minimum in Bhagirathi (10.36%) and maximum in Seeta (29.36%) when sown on 20 th October and with delay in sowing there was a significant increase in percent of siliqua infection.Minimum was noted on 20 th October sowing followed by 5 th November and maximum in 5 th December followed by 20 th November sowing, though minimum percent of siliqua infection was recorded in Bhagirathi (17.28%) and maximum in Seeta (72.39%) followed by Binoy (72.04%) when sown in 5 th December.Number of spots per siliqua showed different reaction on different dates of sowing and it was increased significantly with delay in sowing irrespective of three varieties.Two years pooled mean showed minimum number of spots per siliqua on Bhagirathi (6.59) at 20 th October sowing followed by 20 th November (7.09) and also 5 th December sowing of the same Bhagirathi variety (7.35).Maximum number of spots per siliqua was noticed on Seeta (14.13) at 5 th December sowing followed by 20 th November (13.58) and also 5 th November sowing (11.78) (Table 2).
The yield characters like number of siliqua per plant and grain yield (kgha -1 ) were also reflected among the varieties due to different dates of sowing and their differences were statistically significant.Different years showed different results.The two years pooled mean showed maximum number of siliqua per plant on Bhagirathi at 5 th November sowing (151.63)followed by Bhagirathi at 20 th October sowing (134.45).Minimum number of siliqua per plant was harvested on Seeta at 20 th November sowing (75.57) followed by 20 th October sowing (98.60) (Table 3).
Similarly seed yield of mustard was statistically significant in respect to dates of sowing, varieties and interaction between dates of sowing and variety for the two consecutive years.The seed yield was lowest in case of 5 th December sowing for all the three varieties in all the two years of experimentation.The two years pooled mean showed that the maximum seed yield (1835.67 kgha -1 ) was recorded on 5 th November sowing followed by 20 th October sowing (1687.17kgha -1 ).It was also observed that in every date of sowing upto 5th November, Bhagirathi produced maximum seed yield followed by Binoy and least in Seeta.Similar observation was also observed in 5 th December sowing where as on 20 th November, maximum seed yield was harvested on Binoy (1367.5 kgha -1 ) followed by Bhagirathi (1339 kgha - 1 ) and least on Seeta (1112.17kgha -1 ) and their differences were statistically significant (Table 3).So among the three cultivars tested Seeta sown on all dates showed significantly more disease severity of leaf, siliqua infection and number of spots per siliqua followed by Binoy and least in Bhagirathi.It was also noticed that disease severity increased with age of plant and highest disease severity was recorded on all plant parts at 90 DAS.These results also confirm the results of Ghosh and Chatterjee (1988) and Saran and Giri (1987) that delay in sowing of rapeseed mustard reduce the siliqua percentage 24 to 57%.These results also support the findings of Mian and Akanda (1989) who have recommended early sowing for minimum damage of crop from Alternaria leaf blight disease.So, the progress of disease was found to be highly significant among the date of sowing and cultivars (Rahman andBiswas, 1994, Singh andSingh, 1999).The interaction of these variables were also significant (P<0.05).Among the three cultivars tested Bhagirathi sown on all the dates showed significantly less disease followed by Binoy and Seeta.It was also noticed that disease severity increased with age of plant and highest disease severity was recorded at 90 DAS.The present findings are in agreement with the result of Sarkar and Sengupta (1978) that susceptibility of rapeseed-mustard to Alternaria brassicae increases with the age of plant.So, the Bhagirathi on 5 th November sowing produced maximum seed yield and minimum disease severity on leaf and siliqua.The result also confirm the result of Dasgupta et al. (1991) that delay in sowing of Varuna variety by one month increased the incidence of leaf blight on leaves by 37% and on siluqua by 31% which reduced the yield up to 38%.

Effect of weather factors
The disease assumed serious proportions in crop sown between 20 th November and 5 th December and minimum progression on 20 th October and 5 th November sowing.Correlation analysis of disease severity with ten weather factors at different growth stages are indicated that weather factors showed different correlation with disease severity at different sowing dates in two different years.In the year 2008-2009, different sowings dates showed different disease reaction and it was observed that at 20 th October sowing the maximum temperature (24.9 to 28.5°C), minimum temperature (9.7 to 18.4°C), minimum relative humidity (39.5 to 66.3%), vapour pressure (morning) (9.6 to 16.8 millibar), vapour pressure (noon) (9.7 to 16.9 millibar) and total rainfall (10.0 milimeter) had significant negative correlation with disease severity (0.001 to 53.13%) (Table 3a).whereas in the year 2009-2010, maximum temperature (22.8 to 29.7°C), minimum temperature (11.6 to 17.3°C), maximum relative humidity (97.1 to 98.8%), vapour pressure (morning) (11.0 to 15.5 millibar), vapour pressure (noon) (16.7 to 23.8 millibar) and bright sunshine hours (3.8 to 9.3 h) had negative significant correlation with disease severity while minimum relative humidity (74.4 to 88.3%), wind velocity (evening) (0.30 to 1.15 km/h), wind velocity morning (0.4 to 1.15 km/h) had significant positive correlation with disease severity (0.001 to 52.5%) at 90 DAS (Table 3b).Whereas, two years pooled mean showed that maximum temperature, minimum temperature and vapour pressure (morning) had negative significant correlation with disease severity and other seven factors were statistically non significant in the development of disease upto 90 DAS (Table 3c).

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5 th November sowing in the year 2008-2009 showed that maximum temperature (20.9 to 26.5°C) and bright sunshine hours (5.7 to 9.1 h) had negative significant correlation whereas minimum temperature (9.5 to 17.1°C), minimum relative humidity (40.1 to 62.2%), vapour pressure (morning) (9.4 to 12.7 millibar) and total rainfall) (5.4 to 19.4 millimeter) had positive significant correlation with disease severity at 90 DAS (0.12 to 58.57% ).Other weather factors were not statistically significant with disease severity (Table 3a).In the year 2009-10, it was observed that minimum temperature (10.3 to 16.3°C) had significant negative correlation with disease severity while wing velocity (evening) (0.56-1.12 km/h) and wind velocity (morning) (0.52 to 1.04 km/h) had significant positive correlation with disease severity (Table 3b).The two years pooled mean showed that only wing velocity (evening and morning) had positive significant correlation with disease severity at 90 DAS and others factors were not statistically significant (Table 3c).
In 20 th November sowing, in the years 2008-2009 it was observed that maximum temperature (23.1 to 26.4°C) had negative significant correlation with disease severity while minimum temperature (9.7 to 12.1°C), maximum relative humidity (96.2 to 97.6%), minimum relative humidity (39.5 to 64.4%), vapour pressure (morning) (9.2 to 12.7 millibar), vapour pressure (noon) (9.7 to 12.1 millibar) had positive significant correlation with disease severity at 90 DAS (61.98%) (Table 3a).In the year 2009-2010, it was observed that maximum temperature (23.2 to 26.4°C), wind velocity (evening (0.50 to 2.00 km/h), wind velocity (morning) (0.5 to 2.00 km/h) and bright sunshine hours (3.8 to 9.0 h) had positive significant correlation with disease severity, while maximum relative humidity (96.2 to 97.7%), minimum relative humidity (39.5 to 64.4%) had negative significant correlation with disease severity (Table 3b).The two years pooled mean showed that maximum relative humidity wind velocity (evening), and wind velocity (morning) had positive significant correlation with disease severity where as other factors were not statistically significant (Table 3c).
5 th December sowing in the year 2008-2009, it was noticed that minimum temperature (10.1 to 17.1°C), maximum relative humidity (97.1 to 98.7%), vapour pressure (morning) (9.4 to 16.4 millibar), vapour pressure (noon) (9.9 to 15.8 millibar) were significant positive correlation with disease severity to reach 66.97%, while other factors were not statistically significant.In the year 2008-2009, maximum temperature (25.2 to 32.2°C), minimum temperature (10.3 to 17.9°C), minimum relative humidity (32.7 to 87.6%), vapour pressure (morning) (9.7-16.4millibar) and bright sunshine hours (5.7 to 9.1 h) had positive significant correlation with disease severity while vapour pressure noon (10.3 to 21.8 millibar) had negative effect.The two years pooled mean showed that maximum temperature, minimum temperature, vapour  vapour pressure (morning and noon) played a major role in Alternaria blight development irrespective of dates of sowing.Awasthi and Kolte (1994) reported that these two meteorological factors (temperatures and humidity) were extended profound effect on the disease progession of Alternaria blight of mustard.Increase in vapour pressure within the crop canopy cause spore liberation and also vulnerable to host for disease infection.Late sowing after 20 th November increase in vapour pressure there was a increase in disease severity.Whereas, 20 th October's  sown crop showed the opposite result.It was also observed that different dates of sowing produced different disease severity within the same weather condition due to difference in crop age.In late sown crop with increase in bright sunshine hours there was an increase in disease severity where as early sowing (20 th October) showed the opposite result.Gadare et al. (2002) reported that sunshine hours and crop age has significant correlation with disease severity.So, maximum temperature 26 to 29°C maximum relative humidity (>80%) and vapour pressure (> 8.0 millibar) favoured the progression of Alternaria blight of mustard when the crop age was above 40 DAS.From this study it may be concluded that by manipulating the sowing date of mustard between 20 th October to 5 th November with Baghirathi variety, the crop can be saved from heavy infection of Alternaria blight.Severity of disease at 90 DAS was favoured by high relative humidity and minimum temperature and vapour pressure in morning hours.

Figure 3 .
Figure 3.Effect of vapour pressure on disease severity.

Table 1 .
Percent of leaf infection and AUDPC of mustard variety under different dates of sowing in the two consecutive years.

Table 2 .
Percent of siliqua infection and number of spots per siliqua of ALB infected mustard variety under different dates of sowing in the two consecutive years.

Table 3 .
Number of siliqua per plant and yield (kgha -1 ) of ALB infected mustard variety under different dates of sowing in the two consecutive years.