Whitefly and weather
The first occurrence of whitefly on soybean was detected 13 days after sowing, on the 27
th SMW (standard meteorological week), which was July 8
th, 2021 (Table 2). The Whitefly population was varied in four varieties (Table 2). The highest whitefly counts, 3.87 per trifoliate (visual) and 8.53 pe plant (cage), were recorded in NRC 86 at 29
th SMW (July 16-22). Simultaneously, the highest whitefly counts, 2.80 per plant (visual) and 7.00 per plant (cage) were recorded in JS 335 at 29
th SMW. The mean initial whitefly population was 1.80 per trifoliate (visual) and 3.50 per plant (cage) at 27
th SMW,
i.e. July 8
th. After that, a sudden increase in the whitefly population was observed and attained its peak at 29
th SMW (July 16-22), which was 3.29 per trifoliate (by visual) and 7.73 per plant (by cage). At the time of the peak of the whitefly, maximum and minimum temperatures were 33.4 and 25.4°C respectively, whereas sunshine, wind speed, rainfall, morning Rh and evening Rh were 5.5 hrs, 3.9 km/hr., 35.4 mm, 84.10 and 70.0, respectively (Fig 1). Whitefly remained active till the crop’s maturity, but a sudden decline in the whitefly population was seen at the time of maturity. The dynamics of the whitefly were nearly uniform in all varieties. Present findings corroborate the findings of
Raghuvanshi et al., (2014),
Ahirwar et al., (2015) and
Marabi et al., (2017a), as they also reported that the first appearance of whitefly on soybean ranged from 14-30 DAS (28
th-33
rd SMW) to till maturity. Previous workers also reported similar weather parameters during the peak of white flies in the present investigation (
Raghuvanshi et al., 2014).
In Pearson correlation, maximum and minimum temperature and sunshine hours exhibited positive relation [r= 0.535, 0.497 and 0.582 (visual) and 0.541, 0.426 and 0.659* (cage), respectively] with mean whitefly population (Table 3). Among varieties, whitefly counts in JS 335 (0.639* and 0.635*) and MACS 1520 (0.672* and 0.639*) significantly correlated with maximum temperature in both visual and cage counts. A similar significant positive relation was exhibited between white flies and sunshine hours. While rainfall, morning and evening humidity, rainy days, wind speed and crop age exhibited negative non-significant correlation with the mean white fly count in a cage of both methods. However, Wind speed exhibited a significant negative relation with the white fly count in a cage of JS 335 (-0.753*) and MACS 1520 (-0.764*). Present findings conform with the findings of many researchers (
Kalkal et al., 2015;
Marabi et al., 2017a;
Marabi et al., 2017b; Amrate et al., 2023a) about white fly, maximum temperature and sunshine hours. However, researchers also reported that max temp had a negative impact on the whitefly population (
Sharma and Kumar, 2014). Rainfall negatively correlated with the whitefly population (
Marabi et al., 2017a;
Amrate et al., 2023a).
Progression of YMD
Yellow mosaic disease (YMD) appeared on the 28
th SMW (Fig 1). At initiation, yellow mosaic disease severity was very low (Fig 1). During this period, high temperature (max 33.3°C and min 25.1°C), low rainfall (33.2 mm), less humid (evening 58.3-morning 85.1 %) and high whitefly population [2.65 per trifoliate (visual) and 5.78 per plant (cage)] prevailed. After that, there was a sharp increase in the disease severity between the 30
th and the 33
rd SMW. Maximum weekly disease progression was noticed in 32
th SMW (06/08/2021 to 12/08/2021). At the time of maximum disease progression, max temp and minimum temperature were 28.5 and 24.2°C, respectively, whereas sunshine hours, rainfall, morning RH and evening RH were 0.5 hrs, 11.1 mm, 90.0 and 79.0%, respectively. While the whitefly population was 1.70 per plant (visual) and 3.04 per plant observed (cage) and the mean whitefly population was 2.37 per plant (Table 2). After that, disease progression was slow and maximum mean severity (36.71%) was recorded at 36th SMW on 76 DAS. Among four varieties, maximum disease severity was recorded in JS 20-34 (48.16%), followed by MACS 1520 (38.86%), JS 335 (30.91%) and NRC 86 (28.92%) (Fig 1, Fig 3). Previous workers have also reported similar weather conditions for the development and progression of YMD (
Singh et al., 2009;
Silodia et al., 2018; Amrate et al., 2023a). Amrate et al., (2023a) indicated that low mean temp and high rainfall in July (cool) lead to low white fly and low YMD and conversely, high mean temperature and low rainfall in July (hot) lead to high white fly and high diseases in the overall season.
Relation of YMD, weather, crop age and whitefly
Pearson correlation matrix between corresponding week’s weather parameters, crop age and vector population with YMD severity revealed that max temp and sunshine hours (r = -0.771**and -0.792**, respectively) were significant negatively and evening RH and wind speed (r = 0.717* and 0.708*, respectively) were significant positively correlated with mean YMD progression (Table 4). A similar significant relation was exhibited in the case of all four varieties for a max temperature, sunshine hour, evening RH and wind speed. While rainfall, Morning RH, rainy days and crop age (r = 0.315, 0.597, 0.522 and 0.024, respectively) had a positive, non-significant relation with YMD progress. Meanwhile, in JS 20-34, crop age showed a negative correlation (r = -0.055) with disease progression. Meanwhile, minimum temp and mean vector population (r = -0.613 and -0.341, respectively) were negatively correlated with YMD progression.
Relation between disease progression and previous week parameters (
viz., weather parameters, crop age and vector population) revealed that evening RH, rainy days and wind speed (r = 0.831**, 0.806** and 0.713*, respectively) had a significant positive correlation with disease severity (Table 4). Similar to the current week’s pattern, max temp and sunshine hours (r = -0.763* and -0.714*, respectively) were negatively correlated with the progression of disease severity. While rainfall, morning RH and crop age had a positive and minimum temp, the mean vector population had a negative, non-significant relation with the disease. A similar pattern, as revealed with the current week’s relation, was exhibited except on rainy days wherein disease progression in all the varieties and their mean were positively significantly related. Previously,
Marabi et al., (2017b) reported that maximum temp and sunshine hours significantly positively impacted YMD incidence. Researchers also supported our findings regarding the negative association of YMD with rainfall and other parameters in blackgram and soybean (
Marabi et al., 2017b;
Srivastava et al., 2021). Amrate et al., (2023a) reported that the coefficient of infection of YMD was significantly correlated with the whitefly population.