Seasonal response of okra ( Abelmoschus esculentus L . Moench ) genotypes for okra yellow vein mosaic virus incidence

One of the major limiting factors of okra is the incidence of okra yellow vein mosaic virus (OYVMV), its vector being whitefly. Infection of 100% plants in a field is very common and yield losses range from 50 to 94% depending on the stage of crop growth at which infection occurs. A total of 91 okra genotypes were evaluated in summer and rainy season of 2012 to 2013, out of which only 6 genotypes (IIHR123, IC90381, CI140982, IC141065, IIHR1, and Kavya) were found to have highly resistant disease reaction during both the season. The highest fruit yield in rainy and summer season was observed in the genotypes IC32855 (308 g/ plant) and IC99646 (212.3 g/plant), respectively. These genotypes could be used for further hybrid breeding programme due to their high yield and lower disease incidence for OYVMV.


INTRODUCTION
Okra is one of the most popular vegetable crops cultivated throughout India for its tender green fruits.Okra requires a long and warm growing season for optimum growth and development.India is the largest producer of okra in the world with total area of 0.52 million hectares and production 6.26 million tones green pods, whereas productivity of the crop is 12.1 MT/ ha (Anonymous, 2012).Okra can be grown twice a year in the Indian plains (rainy season and summer season).The major limiting factor for its cultivation is the incidence of okra yellow vein mosaic virus (OYVMV) which is transmitted by whitefly (Bemisia tabaci Gen.) (Rana et al., 2006).This disease is caused by a complex, consisting of the monopartite begomovirus, okra yellow vein mosaic virus (family: Geminiviridae) and a small satellite DNA β component (Jose and Usha, 2003).This disease and its insect vector cause heavy losses to okra by affecting the quality and yield of the fruits.Infection of 100% plants in a field is quite common and yield losses range from 50 to 94% depending on the stage of crop growth at which infection occurs (Sastry and Singh, 1974).The initial symptom on young leaves is a diffuse, mottled appearance.Older leaves have irregular yellow interveinal areas.Clearing of the small veins starts near the leaf margins, at various points, about 15 to 20 days after infection.Thereafter, the vein clearing develops into a vein chlorosis.The newly developed leaves exhibit an interwoven network of yellow vein, which enclose the green patches of the leaf.

Symptom
Severity grade
Fruits developing on infected plants have irregular yellow areas which follow a longitudinal alignment.Due to heavy infestation the fruits become malformed and reduced in size.The fruits are mostly yellow, small, tough and fibrous (Brunt et al., 1996).If plants are infected within 20 days after germination, their growth is retarded; few leaves and fruits are formed (Sastry and Singh, 1974).
The extent of damage declines with delay in infection of the pathogens.Plants infected 50 and 65 days after germination suffer a loss of 84 and 49%, respectively (Sastry and Singh, 1974;Khan et al., 2005).With this severe production constraint in purview, the objectives of this study were to identify okra genotypes for resistance/ tolerance to OYVMV based on a percent disease incidence and coefficient of infection in okra under field conditions.The outcome of the study could open avenues for utilization of these genotypes in further hybrid breeding programme for OYVMV tolerance/resistance in okra.

MATERIALS AND METHODS
The present investigation was carried out using 91 diverse okra genotypes including 4 checks (Arka Anamika, Arka Abhay, VRO-6 and Pusa Sawani) collected from NBPGR, New Delhi, India and IIHR, Bengaluru, India, screened and evaluated at the research farm of the Department of Horticulture (Vegetable and Floriculture), Bihar Agricultural College, Bihar Agricultural University, Sabour, Bhagalpur (Bihar) in two different seasons, Summer and Rainy, 2012 to 2013.The recommended agronomic practices for raising a good okra crop were adopted.Observations were recorded on 12 economically important traits viz., days to first flowering, days to 50% flowering, first fruit picking, first flowering node, fruit length (cm), number of branch/ plant, plant height (cm), plant canopy width (cm), number of fruits/ plant, average fruit weight (g), fruit yield/ plant (g) and percent infestation of OYVMV.The soil of the plot was sandy loam in texture having good fertility, properly leveled and well drained.Scoring of OYVMV disease incidence was done on a scale of 0 to 4 (Table 1) at 15 days intervals (30, 45, 60 and 75 days) after sowing and PDI and CI value were calculated by the procedure coined by Banerjee and Kalloo (1987).

Effect of season on growth and yield attributing traits
The effect of growing seasons (summer and rainy) was markedly observed through the mean performance of the crop itself.The summer crop was earlier than the rainy crop (the respective mean days to first flowering being 39.71 and 46.14 days after sowing, respective mean days to 50% flowering being 46.43 and 53.21 days after sowing, respective node number to first flower being 3.93 and 7.79 and respective days to first fruit picking being 49.21 and 53.36 days after sowing) (Table 2).IC33332 was the earliest genotype, whereas among checks Arka Anamika was the earliest.IC18536 was the latest genotype whereas Pusa Sawani among the checks was the most late.The effect of season was evident in yield and yield attributing characters also.In summer (Table 2) the mean yield per plant was recorded to be 154.57g whereas in the rainy season it was massively increased to 247.85 g.Similar trends were observed in the various yield attributing traits, viz., number of fruits per plant, plant height, plant canopy and number of branches per plant which were lesser in summer and magnified in rainy season.However, the average fruit weight did not differ much over the seasons (12.20 g in summer and 12.21 g in rainy).During rainy season ( cm), whereas in rainy season IC15036 had the greatest canopy width (106.0 cm).The number of branches per plant was highest in IC43750 (5.0) in summer, whereas in rainy season the highest number of branches per plant was recorded in the check Arka Anamika (5.0).On the basis of correlation analysis during both the seasons (Table 4), days to first flowering, days to 50% flowering, plant height and number of fruits per plant are significantly correlated.However, number of fruits per plant has significant negative correlation during rainy season.It was found that days to first  flowering, days to 50% flowering, days to first fruit picking via first flowering node; number of branches per plant via plant canopy width have positive significant correlation during summer season.Similar pattern of correlation was also accounted by (Dhankar and Dhankar, 2002;Das et al., 2013;Solankey et al., 2013).

Effect of season on OYVMV attributing traits
In the rainy season crop the intensity of OYVMV was very high due to the favourable environmental condition for the vector, that is, whitefly and the virus (Das et al., 2013).
The mean incidence of OYVMV in summer season (Figures 1 and 2) was low (22.25%) compared to the high incidence in rainy season (51.96%).Out of 91 okra genotypes, 45 genotypes were found to be highly resistant, 17 resistant, 10 moderately resistant, 08 moderately susceptible, 06 susceptible and rest 05 showed highly susceptible reactions during summer season (Table 5).However, only 6 genotypes were found to be highly resistant (IIHR123, IC90381, CI140982, IC141065, IIHR1, Kavya), 6 resistant, 5 moderately resistant, 14 moderately sus-  On the basis of two season research work, it was observed that the okra crop has more yield potential during rainy season but this season was not only favours growth and development of crop but also OYVMYV infestation.None of the genotypes showed higher yield potential coupled with high resistance to OYVMV during rainy season (Figures 1 and 2).The check varieties were also more prone to OYVMV during the rainy season than in summer.The incidence of the disease in a particular genotype also varied from season to season, probably due to the influence of the environmental conditions.It is worth mentioning that Singh (1990) reported that hot weather with little rainfall was favourable for development of OYVMV and also for multiplication of the vector Bemisia tabaci.Earlier, other reports indicated that the incidence of OYVMV disease was higher during the rainy season when relative humidity was very high which support our findings (Sangar, 1997;Bhagat et al., 2001;Chattopadhyay et al., 2011;Das et al., 2013).

Conclusion
It may be concluded that there was significant effect of season on yield and yield attributing traits as well as incidence of OYVMV in okra.While summer crop is early, higher values for yield attributing traits and yield were recorded in rainy crop.Rainy season favours incidence of OYVMV.However, this season also favours the morphological and reproductive phase of okra.Moreover, rainy season crop has more yield potential than summer season crop.From the large number of genotypes evaluated under this study, the genotypes IIHR123, IC90381, CI140982, IC141065, IIHR1, Kavya, IC32855 and IC99646 and check variety Arka Anamika had high potential and lower incidence for OYVMV.Hence, these genotypes could be utilized for further breeding programmes.

Figure 1 .
Figure 1.Performance of promising okra genotypes for yield and OYVMV infection during summer season.

Figure 2 .
Figure 2. Performance of promising okra genotypes for yield and OYVMV infection during rainy season.

Table 1 .
Scale for classifying disease reaction in okra to OYVMV disease.
HR = Highly resistant, R = Resistant, S = Susceptible, H S = Highly susceptible, MR = Moderately resistant, MS = Moderately susceptible.Correlations between variables were tested for significance Note:

Table 3
number of fruits per plant was highest inIC128035 and  IC31398 (19.6)in summer crop, whereas in rainy crop the highest was observed in IC18553 and IC18537A (25.4).Plant height was maximum in IC99646 in summer (110.0 cm) whereas in rainy season in the check VRO-6 (145.0 cm) closely followed by IC13995 (135.5 cm).Plant canopy in summer season was highest in IC99646 (90.0

Table 2 .
Mean performance of promising okra genotypes during summer season.

Table 3 .
Mean performance of promising okra genotypes during rainy season.

Table 4 .
Genotypic character association in okra during rainy and summer season.

Table 5 .
Reaction of okra genotypes against OYVMV during both seasons (summer and rainy).