Effect of Planting Time on Incidence of Bacterial Disease and Yield of Broccoli during Dry Season in Southern Thailand

The trial was aimed to select suitable planting times when broccoli varieties could resist well to soft rot and black rot diseases during dry season was studies at Prince of Songkla University, Songkhla, Thailand, from January to June, 2011. Split-plot in randomized complete block design was used in this experiment with four replications. The results showed that the lowest incidence of soft rot disease was observed in all broccoli varieties when they were planted in January (4.19-7.18%). After that, it would increase (22.59-45.58%) but the incidence of black rot disease was not increased when planting was done in January, February, March and June. The incidence of black rot disease was highest in April (84.24-92.64%). The Yok Kheo was produced the highest total yield when the planting took place in January and March (12.31 and 10.65 t/ha, respectively). The Yok Kheo is an interesting new hybrid variety. Its higher yield was than Top Green which is popular variety grown in southern Thailand. The yield of Green Queen was not significantly different from Top Green. It can be harvested at 11.67 and 9.38 days earlier than Top Green and Yok Kheo, respectively.


INTRODUCTION
Broccoli (Brassica oleracea var.italica Plenck) is the most important herbaceous biennial "Cole" crop of the Brassicaceae family, which is one of the leading vegetables in the world (Ahmed and Siddiuqe, 2004;Khatun et al., 2012).Other members of the family include cauliflower (Brassica oleracea L. var.botrytis L.), cabbage (Brassica oleracea L. var.capitata L.) and kale (Brassica oleracea L. var.acephala DC.).Its optimum growing temperature is in the range between 10 and 20°C (Dhillon et al., 2005).Broccoli has high nutritive value and many health benefits.The American Cancer Society (ACS) indicated that it has several anticancerogen effects (Diputado, 1989;Sermenli et al., 2011).
Soft rot and black rot are the serious diseases in broccoli (Cui and Harling, 2006;Dutta et al., 2011).Soft rot disease caused by Erwinia carotovora ssp.carotovora.This bacterium enters plant tissues primarily through wounds, often created by insect feeding or bruising at harvest.Insect and water are effective modes in spreading the bacteria (Bhat et al., 2010).The disease affects both floret and stem tissue.It appears first as a water-soaked lesion followed by maceration and soft rotting of tissue (Ludy and Powelson, 1997).Black rot disease in broccoli caused by Xanthomonas campestris pv.campestris.Bacterium colonizes in the vascular system and the characteristic black rot system is V-shaped lesion at leaf margins with black veins, chlorosis and necrosis (Kocks and Zadoks, 1996).The suitable environments for bacterium spreading consist of high temperature about 30-36°C for soft rot disease and 25-35°C for black rot disease and high humidity (Vodhivanich, 2006).
The weather of southern Thailand is quite humid tropics and suitable for the heat of tolerant hybrid varieties which are better adapted to the high temperature of humid tropics allowing growth and produce high yields during the dry season (Nooprom and Santipracha, 2011).However, due to suitable environment for pathogens, the spreading of soft rot and black rot diseases can be found in broccoli planting that caused yield losses of broccoli growth.The objective of the current study was to investigate the difference of planting times of broccoli varieties on incidence of soft rot and black rot diseases and varieties of broccoli which can be resisted these pathogens for commercial production during dry season in Songkhla, southern Thailand.

MATERIALS AND METHODS
This study was conducted at Prince of Songkla University, Hat Yai, Songkhla, Thailand from January to June, 2011.The design was a split-plot in a randomized complete block.The main plots were planting times: January, February, March, April, May and June with sub-plots being early varieties of broccoli: Top Green, Green Queen, Yok Kheo and Special.These varieties show good performance such as, early growth and yield when growing in Songkhla of southern Thailand (Nooprom and Santipracha, 2011).Broccoli seed was sown in plastic baskets (13×16×4 inches).When the first true leaf had emerged, the seedlings were transplanted into 2 inch pots.At the fourth leaf stage, the seedlings were transplanted into the field.The plot size was 1×5 m.The plant and row spacing were 0.30×0.60m and the edge spacing was 0.2 m.Each plot had two rows of 32 Plants.The broccoli was regularly watered with a sprinkler early in the morning and early in the afternoon except for rainy days.Fertilizer 21N-0P-0K was applied three times, at 2, 3 and 4 weeks after transplanting was done.Moreover, fertilizer 15N-6.5P-12.5Kwas applied twice, at 5 and 6 weeks after transplanting.All plots were weeded with a hand hoe, twice, at 2 and 4 weeks after transplanting.
The incidence rates (%) of soft rot and black rot diseases were calculated from infected plants in the plot.The yield data was measured from head diameter (cm), head weight (g/plant) and total yield (t/ha).Data of average daily rainfall, minimum and maximum temperatures from January to June, 2011 was obtained from the Kho Hong Agro meteorological Station, Hat Yai, Songkhla, Thailand.Comparison among means was done with the analysis of variance and means separated by Duncan's Multiple Range Test (DMRT) at the 5% level of significance.

RESULTS AND DISCUSSION
Soft rot incidence: The lowest incidence of soft rot disease was observed when planting was done in January (4.97%).In February, soft rot incidence was increased (45.58%).After that, it declined (22.59-39.57%)during March and June.The highest incidence (34.66%) was observed in the Green Queen followed by Top Green and Special while the Yok kheo had the lowest incidence (16.73%).The interactions between planting times and varieties were significantly influenced on the soft rot incidence.The four varieties planting in January were all observed with the lowest incidence (4.19-7.18%)as shown in Table 1.The highest disease incidence was obtained from the Green Queen and Top Green when planting occurred in February (84.72 and 60.05%, respectively).High incidence observed between February and June might be due to high temperature (31.99-33.80°C) in that period, as well as continuous raining during March and June (mean rainfall of 111.60-325.80 mm) as shown in Fig. 1.These factors affected increasing relative humidity.Kucharek and Bartz (2000) and Higashio and Yamada (2004) reported that high temperature and relative humidity had suitability on soft rot cultivation and spreading.This result was consistent with Vodhivanich (2006) that high temperature about 30-36°C and high relative humidity cause increasing soft rot incidence in Brassicaceae family.
Yield responses: Broccoli planting in January and March was obtained the high head diameter and weight (11.17-11.18 cm and 277.18-302.37 g/plant, respectively) as shown in Fig. 2 and 3 which were not significantly different (p≤0.05).In February, broccolis were obtained decreasing head diameter and weight (10.55 cm and 229.12 g/plant, respectively).The lowest head diameter and weight were obtained with planting between April and June.The highest head diameter (11.58 cm) and weight (299.18g/plant) was obtained in Yok Kheo, followed by the Top Green and Green Queen which were not significantly different (p≤0.05) while the lowest head diameter and weight were found in the Special.The interactions of planting times and varieties of broccoli showed that the Yok Kheo planted in January got the highest head diameter and weight (13.18 cm and 357.55 g/plant, respectively).The lowest head diameter and weight were observed in the Special when planted between April and May.Broccoli planting in January and March produced high total yield (7.57-9.36t/ha) as shown in Fig. 4 while the total yield was low when grew in other months (4.28-4.71t/h).The Yok Kheo gave the highest total yield when planted in January (12.31 t/ha).The lowest total yield was obtained with the Special when grew between April and June (1.82-2.92t/ha).Between April and June, broccoli had decreasing head diameter and weight due to high temperature (32.75-34.20°C)as shown in Fig. 1.The high temperature (30-40°C) caused decreasing broccoli vegetative growth that affected yield attribute losses (Kaluzewicz et al., 2009).This result confirmed (Bjorkman and Pearson, 1998) that over high temperature affected broccoli growth and yield losses.In addition, high temperature and relative humidity caused the high soft rot and black rot diseases and eventually decreasing growth and yield.Bhat et al. (2010) reported that soft rot caused greater total loss of vegetable crops than any other bacterial diseases while Krauthansen et al. (2011) reported that black rot caused yield losses of Brassicaceae family.

CONCLUSION
Our study showed that January, February, March and June had suitability for broccoli planting due to no black rot incidence in these months.Whiles, broccoli soft rot was observed with moderate incidence except in January when the lowest soft rot incidence (4.97%) was observed.High total yield (7.57-9.36t/ha) in broccoli planting was produced.The Yok Kheo, Green Queen and Top Green are the suitable varieties for commercial production in Songkhla during the dry season.Especially, the Yok Kheo can resist well to soft rot and black rot.

Table 1 :
Interactions between planting times and varieties on soft rot incidence (%) in broccoli

Table 2 :
Interactions between planting times and varieties on black rot incidence (%) in broccoli