Influence of weather parameters on powdery mildew disease of mustard under North Saurashtra agroclimatic zone

Mustard is one of the most important oilseed crop grown under a wide range of agroclimatic conditions in India. The crop is damaged by various biotic and abiotic stresses. The weather conditions during December and January are the most congenial for outbreak of powdery mildew, which has been a limiting factor for successful cultivation of mustard. Dange et al., (2002) reported 17 per cent yield loss due to this disease in mustard in Gujarat. Weather parameters such as temperature and relative humidity play an important role in disease development (Gadre et al., 2002; Desai et al., 2004 and Kohire et al., 2008). However, scanty information is available on the epidemiological aspects of this disease under north Saurashtra agroclimatic conditions where the crop is cultivated during rabi season with limited irrigation. The crop is mainly affected by powdery mildew causing considerable losses in terms of both quality and quantity. Hence, the present investigation was carried out to determine the influence of meteorological parameters on the development of disease.

Mustard is one of the most important oilseed crop grown under a wide range of agroclimatic conditions in India. The crop is damaged by various biotic and abiotic stresses. The weather conditions during December and January are the most congenial for outbreak of powdery mildew, which has been a limiting factor for successful cultivation of mustard. Dange et al., (2002) reported 17 per cent yield loss due to this disease in mustard in Gujarat. Weather parameters such as temperature and relative humidity play an important role in disease development (Gadre et al., 2002;Desai et al., 2004 andKohire et al., 2008). However, scanty information is available on the epidemiological aspects of this disease under north Saurashtra agroclimatic conditions where the crop is cultivated during rabi season with limited irrigation. The crop is mainly affected by powdery mildew causing considerable losses in terms of both quality and quantity. Hence, the present investigation was carried out to determine the influence of meteorological parameters on the development of disease.
Field experiments were conducted during four consecutive rabi seasons from 2007-08 to 2010-11 at Dry Farming Research Station, J.A.U., Jam-Khambhalia of north Saurashtra agroclimatic zone of Gujarat state. The mustard cultivar 'GM-2' was sown on 16 th October during 2007-08 to 2009-10 and 18 th October during 2010-11, adopting recommended package of practices in plot size 12.0 m x 9.0 m. The crop was protected against mustard sawfly (Athelia lugens proxima Klug) and mustard aphid (Lipaphis erysimi Kalt) by spraying endosulfan @ 0.07 and imidacloprid @ 0.005 per cent, respectively, during the years of experiment. Standard package of practices were followed to raise the crop.
The disease observations were recorded at four days interval from 20-tagged plants from initiation of the disease till the maturity of the crop and the severity was calculated on 0-12 point scale (Solanki, 1995). For each observation date, PDI was calculated by averaging 20tagged plants to give a single value using the formula given by Wheeler (1969).
Data on weather parameters such as maximum and minimum temperatures, morning and afternoon relative humidity (RH 1 and RH 2 ) were recorded from nearby agrometeorological observatory. Correlation and multiple regression analyses was carried out to determine the relationships between weather parameters and disease progress.

Weather factors and disease development
The PDI was recorded from the date of initiation of the disease to the crop harvest at an interval of four days. Correlation coefficient values based on the data of four consecutive years were calculated.
Mean weather parameters of the season and the disease intensity are presented in Table 1. The average maximum and minimum temperature during the four seasons varied from 27.2 to 28.9 °C and 10.9 to 12.6°C, respectively. Relative humidity during morning and afternoon ranged from 69 to 83 per cent and 27 to 42 per cent, respectively. The disease intensity ranged from as high as 91.     Saharan and Kaushik (1981); Dang et al., (1998) and Gadre et al. (2002). They reported that temperature had significant positive relationship with the powdery mildew disease of mustard. Present findings are also in close conformity with the results obtained by Desai et al. (2004) and Kohire et al. (2008). They reported importance of temperature and relative humidity in disease development by E. cruciferarum in mustard.

Prediction of powdery mildew disease
To formulate simple and effective model for powdery mildew prediction, weather factors were considered. Powdery mildew symptoms were observed in second fortnight of December with the per cent disease intensity from initiation of disease to crop harvest as 1.25 to 88.8; 2.50 to 91.7; 2.09 to 87.1 and 1.67 to 85.9 respectively, during the year 2007-08, 2008-09, 2009-10 and 2010-11. The prediction equation was developed during all the four years and pooled (Table 3). The R 2 values revealed that the weather parameters that were regressed on disease intensity could explain the variation to the extent of 59 to 81 per cent in different years. It may be concluded that maximum temperature (28.9ºC) coupled with afternoon RH (40%) favoured powdery mildew intensity under natural June 2013] Influence of weather on powdery mildew of mustard in Saurashtra conditions during the year 2010-11.
Regression model developed for predicting PDI showed that different weather variables influenced differently and responsible for the increase or decrease of PDI to a given extent. This may be due to different weather conditions in different years. The favourable environmental conditions identified for development and progress of powdery mildew disease was maximum temperature between 27.2 to 28.9ºC and afternoon RH from 27 to 42%. These observations are in agreement with the findings of Dang et al., (1998) at Hissar andSolanki et al, (1999) at Anand, Gujarat with regards to powdery mildew of mustard.

CONCLUSIONS
Powdery mildew initiated at the crop age of 66-74 days, which gradually increased till harvest. Out of four variables, the most contributing variables identified for the disease development under North Saurashtra agroclimatic conditions were maximum temperature and afternoon relative humidity.