CHEMICAL CONTROL OF PSEUDOCERCOSPORA LEAF SPOT (PSEUDOCERCOSPORA ABELMOSCHI) OF OKRA SEED CROP
G. Kibria and I. H. Mian. 2012. Chemical control of Pseudocercospora leaf spot (Pseudocercospora. abelmoschi) of okra seed crop. Bangladesh J. Plant Pathol. 28 (1&2): 53-58.
A field experiment was conducted to evaluate the efficacy of five fungicides against Pseudocercospora (P. abelmoschi) leaf spot or black mould of okra seed crops showed that foliar spray with Aimcozim 200 EW (Carbendazim) @ 0.10%, Emivit 50 WP (Copper Oxychloride) @ 0.35%, Indofil M-45 (Mancozeb) @ 0.20%, Tilt 250 EC (Propiconazole) @ 0.05% and Folicur 250 EC (Tebuconazole) @ 0.10% decreased percent disease index (PDI) by 485.74-2465.77, 240.74-970.68, 124.39-537.50, 92.81-366.50 and 72.80-209.40% reduction in PDI, respectively over control recorded at 65, 72, 79, 86 days after sowing. On the other hand, the fungicides increased plant height, fruit yield, seed yield, seed germination and seed vigor index by 7.50-30.78% in 18.88-46.70% in 29.02-94.02% in 15.03-19.79% in 21.71-58.07%, respectively over control. The maximum reduction in disease severity and increase in plant growth, yield and yield attributes were achieved with Emivit followed by Aimcozim, Folicur and Tilt. Based on findings of the study Emivit 50 WP and Folicur 250 EC may be recommended to control PLS of okra seed crop.
Okra also known as lady’s finger (Abelmoschus esculentus) is widely cultivated and popular vegetable crop in Bangladesh. It can be grown round the year in the country except few cool months (Mid December to late February). However, summer (Kharif) is the best season for growing okra for good fruit. In Kharif season, humidity and temperature are high and okra plants are vulnerable to many fungal pathogens causing various diseases. In Bangladesh, Pseudocercospora leaf spot (PLS) or black mould caused by Pseudocercospora abelmoschi (Cercospora abelmoschi) is a common disease of okra especially okra seed crop grown during late winter and early summer (Pabitra 2009, Anon. 2011, Jiskani 2011). The disease causes tremendous yield loss of okra seed crop (Pabitra 2009).
The most effective and easily available method to control the disease is application of chemical fungicides (Pant and Mukhopadhayay 2001, Singh et al. 2003). Report from India reveals that Cercospora causes sooty black, angular spots which are responsible for severe defoliation. Spraying Mancozeb or Zineb 2 g or Carbendazim 1 g per liter of water is recommended to control the disease (Anon. 2011).
To grow healthy seed crop of okra it is necessary to develop suitable methods of PLS control. Generally, fungicides are recommended to control foliar okra diseases including PLS (Marium et al. 2006). A study was conducted in Bangladesh to evaluate the efficacy of Thiovit (0.45%) and Bavistin (0.1%) to control PLS (P. abelmoschi). Bavistin applied alone at 15 days interval showed the lowest disease incidence followed by Thiovit+Bavistin at 30 days interval. The number of fruits/plant, number of seeds/fruits, and seed yield/plot increased due to those treatment. Bavistin and Thiovit were recommended to control PLS (Rahman et al. 2000). However, it is not wise to depend on only two fungicides to such an economically important disease of okra. Considering the above facts, the present study was undertaken to evaluate the efficacy of five fungicides to control PLS of okra seed crop.
MATERIALS AND METHODS
Five fungicides namely Aimcozim 200 EW (Carbendazim), Emivit 50 WP (Copper Oxychloride), Indofil M-45 (Mancozeb), Tilt 250 EC (Propiconazole) and Folicur 250 EC (Tebuconazole) were tested against PLS of okra seed crop under natural field conditions. The fungicides were selected based on their effectiveness against foliar diseases of okra as reported by many other investigators (Pant and Mukhopadhayay 2001, Singh et al. 2003).
The experiment was conducted in the experimental farm of Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU), Gazipur, Bangladesh during March to June 2009. The experiment was laid out following a randomized complete block design with three replications. The unit plot size was 4.5 m × 1.2 m. Drains of 50 cm width and 30 cm depth were dug around each unit plot for facilitating irrigation and drainage of excess water. The experimental field was prepared properly for good tilth following standard practices (Razzaque et al. 2000). Fertilizers were applied @ 50-24-30-10-1-0.5 kg of N-P-K-S-Zn-B per hectare, respectively. Manure was applied as cowdung at the rate of 4 t/ha (Anon. 2005). The entire quantity of cowdung, P, K, S, Zn and B and half of N were applied at the time of final land preparation. Remaining N was applied around the base of the plant as top dress and incorporated with soil at 3rd and 5th week after sowing. Genotype BD 1911 was used in this experiment. It is susceptible to Pseudocercospora leaf spot. Seeds of the genotype were soaked in tap water for 12 hours and sown at the rate of 3-4 seeds per pit maintaining row to row and plant to plant spacing of 60 and 50 cm, respectively. After 6 to 7 days of germination, the seedlings were thinned and only one apparently healthy seedling was allowed to grow per pit. If any seedling died within three weeks of germination it was replaced by seedling of same age raised in polyethylene bags (9 x 15 cm). The growth medium used in polyethylene bags was prepared by mixing with decomposed cowdung and soil in equal proportion. Irrigation and other necessary intercultural operations were done throughout the cropping season for proper growth and fruit production. Irrigation was applied after each top dressing of urea and whenever necessary. Every time, irrigation was followed by mulching and weeding.
Suspensions of Aimcozim, Emivit 50 WP, Indofil M-45, Tilt 250 EC and Folicur 250 EC were prepared in water at 0.1, 0.35, 0.20, 0.05 and 0.10% concentration, respectively and were applied as foliar spray starting from the onset of PLS (78 days after sowing) and continued for four weeks with 7 days interval.
Data on severity of PLS was recorded at 65, 72, 79 and 86 days after sowing (DAS) based on a 0-8 scale (Rahman and Nahar 1990). Disease severity was expressed in Percent Disease Index (PDI) for the leaf spot, which was computed according to the following, a standard formula as described by Wheeler (1969) and Mian (1995):
*The numerical ratings were obtained by multiplying the number of leaves with their respective grades of indexing scale
Data on plant growth, seed yield, selected yield contributing parameters, and seed quality were also recorded. Quality of seeds in terms of germination, seedling growth and vigor index of okra seeds was determined following methods of International Seed Testing Association (ISTA) (Anon. 1996). Seedling vigour was determined using a standard formula (Baki and Anderson, 1972) viz. Vigour Index = (mean root length + mean shoot length) x percent emergence. Fruits were collected from each of the unit plot after their ripening and seeds were separated and processed. Recorded data were subjected to statistical analysis following standard procedure (Gomez and Gomez 1984) using MSTAT-C statistical software. ANOVA was performed and means were compared following DMRT. Whenever necessary, data were transformed using ArcSin method before performing ANOVA.
RESULTS AND DISCUSSION
Effect on PLS severity
Maximum PDI of 35.13, 42.31, 51.21 and 56.96 was observed under control at 65, 72, 79 and 86 DAS, respectively. The disease severity was reduced to 6.21- 20.33 at 65 DAS, 5.92-23.93 at 72 DAS, 3.01-21.20 at 79 DAS and 2.22-18.41 at 86 DAS due to application of Indofil M-45, Tilt 250 EC, Folicur 250 EC, Aimcozim 200 EW and Emivit 50 WP, respectively. The reduction was significant (P=0.05) compared to control (Table 1). The maximum reduction was achieved with Emivit 50 WP followed by Aimcozim, Folicur 250 EC, Tilt 250 EC and Indofil M-45. The fungicides gave 485.74-2465.77, 240.74-970.68, 124.39-537.50, 92.81-366.50 and 72.80-209.40% reduction in PDI, respectively (Fig. 2). Differences in rates of reduction of PLS severity under different fungicides was significant.
Relationship of PDI with Days after sowing
At all stages of data collection (65, 72, 79, 86 DAS), the PDI of PLS were maximal under control, which was followed by Indofil, Tilt, Folicur, Aimcozim and Emivit. Severity of PLS (PDI) was positively and linearly correlated with DAS and their relationship was highly significant under control. Under all fungicidal treatments the relationship between PDI and DAS was also linear and significant but negative (Fig. 1). The results indicate that application of the fungicides reduces severity of PLS gradually with the plant age due to decrease in inoculum potential after fungicidal sprays.
Effect on plant height
Foliar spray with all fungicides increased plant height significantly over control except Indofil M-45. The maximum increase was achieved with Emivit 50 WP followed by Aimcozim 200 EW, Folicur 200 EW and Tilt 250 EC. Their effectiveness to increase plant height was not significantly different with an exception (Table 2).
Effect on Yield number per plant and fruit size
Application of each fungicide increased the fruit number per plant and fruit length over control. The highest number of fruits was achieved with Emivit 50 WP, which was statistically similar to Aimcozim 200 EW and Tilt 250 EC. Significant increase in fruit length was achieved with only Emivit 50 WP, Aimcozim 200 EW and Folicur 250 EC. Differences in diameter of fruits under different treatments including control were not significant at any stage of data collection (Table 2).
Effect on seed yield and seed size
Number of seeds per fruit ranged 40.38-47.74 under different treatments including control. Their differences were not significant. All fungicides gave significant increase in seed yield per plant over control. The maximum seed yield was obtained with Emivit 50 WP followed by Aimcozim 200 EW, Folicur 250 EC and Tilt 250 WP. Seed size in terms of 1000-seed weight varied from 56.32-63.00 g. The highest 1000-seed weight was observed under the treatment with Emivit 50 WP and the lowest under control. However, the differences were not significant (Table 3).
Quality of seeds harvested from seed crop sprayed with different fungicides
Germination: Germination of seeds harvested from the seed crop increased significantly over control due to spray with five fungicides. The highest increase was achieved with Emivit 50 WP followed by Aimcozim 200 EW and Tilt 250 EC. Efficacy of those three fungicides to increase germination was not significantly different (Table 4).
Shoot and root length of seedlings: Seeds harvest from okra seed crop received foliar sprays with five fungicides against PLS gave considerable increase in shoot length of seedling compared to control. The increase was significant when the seed crop was sprayed with Folicur 250 EC and Emivit 50 WP. The root length of seedling was the highest where seeds were harvested from plants sprayed with Emivit 50 WP followed the Aimcozim 200 EW. The efficacy of the two fungicides to increase seedling root length was significant compared to control. Other three fungicides also increase root length of seedling but the increase was not significant compared to control (Table 4).
Seed vigor index: The seed vigor index was 900.00 under control. It was increased to 1492.88, 1422.60, 1342.00, 1205.43 and 1095.42 when the okra plants were sprayed with Emivit 50 WP, Folicur 250 EC, Aimcozim 200 EW, Tilt 250 EC and Indofil M-45, respectively. The increase was significant under all fungicide compared to control (Table 4).
Increase in plant growth, fruit and seed yield, seed germination and seed vigor
Spray of okra seed crop with Emivit 50 WP, Aimcozim 200 EW, Folicur 250 EC, Tlt 250 WP and Indofil M-45 increased plant height, fruit yield, seed yield, germination and seed vigor index by 7.50-30.78%, 18.88-46.70%, 29.02-94.02%, 15.03-19.79% and 21.71-58.07%, respectively over control (Table 4 and Fig. 2).
Results of the present experiment showed that PDI values of PLS was maximal under control. Foliar spray of okra seed crops with five fungicides namely Emivit 50 WP, Aimcozim 200 EW, Folicur 250 EC, Tilt 250 EC and Indofil caused considerable reduction in PDI of the disease. Maximum reduction in PDI was achieved with Emivit followed by Aimcozim, Folicur, Tilt and Indofil. The rate of reduction increased gradually with the progress of age of okra plants. The treatments with Emivit 50 WP, Aimcozim 200 EW, Folicur 250 EC, Tlt 250 WP and Indofil M-45 increased 7.50-30.78% in plant height, 18.88-46.70% in fruit yield, 29.02-94.02% in seed yield, 15.03-19.79% in germination and 21.71-58.07% in seed vigor index over control, respectively.
Findings of the present study are in agreement with the findings of many other investigators from Bangladesh and abroad. They also achieved satisfactory control of PLS using foliar spray with fungicides. Srivastava et al. (1992) found best control of Pseudocercospora on okra with Copper oxychloride followed by Carbendazim. Anon. (2011) mentioned the recommendation of spraying Mancozeb or Zineb 2 g or Carbendazim 1 g per liter of water to control the disease.
In the Philippines, Benlate 50 WP (Benomyl), Daconil 75 WP (Chlorothanil) and Funguran-OH (Copper hydroxide) are recommended to control PLS (Anon 2011). In Bangladesh, Rahman et al. (2000) evaluated the efficacy of Thiovit (0.45%) and Bavistin (0.1%) to control of Pseudocercospora leaf spot of okra. They found that Bavistin applied alone at 15 days interval showed the lowest incidence of the disease followed by Thiovit+Bavistin applied at 30 days interval. The yield and yield attributes of okra increased due to those treatment compared to control. They recommended Bavistin and Thiovit against PLS. Pabitra (2009) reported that Bavistin and Proud are effective to control PLS of okra and to improve plant growth, yield and seed quality and seed health. Based on findings of the present investigation Emivit 50 WP and Folicur 250 EC may be recommended to control PLS of okra.
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