Effect of weather parameters on incidence of purple blotch in onion

Introduction

Onion is susceptible to numerous foliar diseases those reduces bulb yield and quality (Cramer 2000) [3] and purple blotch is an important disease of onion across the world (Chaput 1995; Schwartz et al. 2005) [2,8] especially in warm and humid environments (Suheri and Price 2001) [10] . The fungus infestation cause on both leaves and flower stalks (Bock 1964) [1] and reduce onion tops production by 62-92% (Suheri and Price 2001) [10] , cause bulb yield loss of 30% (Everts and Lacy 1990) [5] and 10% losses in seed crop under congenial environmental conditions (Daljeet et al. 1992; Schwartz 2004) [4,7] . Purple blotch disease could cause heavy yield losses ranging from 2.5 to 87.8 per cent during kharif season (Srivastava et al. 1994) [9] . As part of the All India Net Work Research Project on Onion and Garlic (AINRPOG) trials have been conducted at Vegetable Research station, Rajendranagar to identify the promising entries for Telangana state with high production potential, high tolerance to purple blotch and also to study the effect of weather conditions on the incidence and spread of purple blotch in onion cultivars.

Materials and Methods

Bhima Red, Bhima Super, Bhima Shwetha and Bhima Safed were evaluated during Kharif, 2023 Bhima Red, Bhima Super, Bhima Shakthi, Bhima Shwetha and Bhima Safed were evaluated during late Kharif, 2023-24 and Bhima Shakthi and Bhima Shwetha were evaluated during Rabi, 2023-24. The seeds of the cultivars were procured from ICAR-Directorate of Onion and Garlic (DOGR), Pune. The cultivars were evaluated at Vegetable Research Station (SKLTGHU), Rajendranagar, Hyderabad at an altitude of 545m above mean sea level and at 78 0 39'93" E longitude and 17 0 32'27" N latitude. The experimental soil's were clay loam in texture, neutral in reaction, low in available nitrogen and phosphorous, high in potassium and belongs to the order Alfisol of shallow to medium depth. The seedlings were transplanted on to a raised bed i.e. broad bed and furrow with a plot size of 4.8 sq. m (4.0m x 1.2m). A plant spacing of 15cm x 10cm was adopted. The crop was fertilized with recommended dose of 100:50:50:40 kg NPKS and 5 kg zinc per hectare as recommended by ICAR-Directorate of Onion and Garlic Research, Rajgurunagar, Pune. 50% N and 100% P2O5, K2O, S and ZnSO4 were added as basal dose and remaining 50% N in two equal splits at an interval of 30 and 45 days after transplanting. The crop was sown during 34 th , 42 nd and 47 th standard weeks and harvested during 52 nd , 8 th and 13 th Std. weeks during kharif, late Kharif and Rabi seasons. Recommended cultural practices were followed to raise the crops successfully. Meteorological data of weather parameters (precipitation, maximum and minimum temperatures, relative humidity (RH-I &II) were taken during crop growth stage (transplanting to harvest) standard week wise was from Meteorological Observatory, ARI farm, Rajendranagar, Hyderabad. The disease incidence and Percent Disease Index was calculated using the below formulae

The incidence of purple blotch (PDI) was recorded on a scale of 0-5 (0: No incidence, 1:1-10%, 2:11-20%, 3:21-30%, 4:31-50% and 5:51-100%).

Results

Effect of Precipitation on incidence and spread of Purple blotch

Good amount of precipitation (11.2, 162, 36, 27 and 45.8 mm) during 35 th -39 th standard weeks i.e from 2 nd -6 th week after transplanting of onion in kharif was received (Fig1). A meager amount of precipitation less than 10 mm in a week was received during late kharif and rabi seasons. All the tested entries showed purple blotch incidence during all the seasons. The red onion entries Bhima Red, Bhima Super and white onion entries Bhima Shwetha and Bhima Safed tested during kharif, 2023 showed incidence of purple blotch and its severity was moderate a PDI rating of 2 on a scale of 0-5 (Table -1). The infection occurs after 16 hours of leaf wetness at 15 0 C and 8 hours of leaf wetness at 10-25 0 C and severity of infection increases with increasing leaf wetness over 24 hours at all temperatures and causes lesions (Suheri and Price 2000) [10] , the number of lesions increases with increasing leaf wetness duration and temperature, similar conditions were noticed in kharif, 2023 due to good amount of precipitation during 35 th -39 th standard weeks (2 nd -6 th week after transplanting). The moist weather conditions have caused the lesions to cover with brown mold sporangia. This suitable period of rain and favourable environmental condition promoted the disease development (Muimba-Kankolongo, 2018) [6] . The infestation leads to reduction in plant growth, bulb yield and seed yield (Verma and Sharma, 1999) [11] .

Fig-1: Precipitation received during different seasons of onion

Effect of Relative Humidity (RH-I &II) on incidence and spread of purple blotch

The relative humidity (RH-I) ranged between 76.0-93.0% during Kharif, 76.0-89.7% during late Kharif and 65.0-89.7% during Rabi season (Fig. 2). The relative humidity (RH-II) ranged between 30-80% during kharif, 30-62 during late kharif and 21.1-62% during rabi season (Fig. 3). During kharif all the entries tested (Red onion entries Bhima Red and Bhima Super, white onion entries Bhima Shwetha and Bhima Safed) recorded a PDI of 2 on a rating scale of 0-5. Under late Kharif conditions Bhima Red and Bhima Safed recorded a rating of 1 (1-10% disease incidence), Bhima Super, Bhima Shakthi and Bhima Shwetha recorded a rating of 2 (11-20% disease incidence). Under Rabi conditions Bhima Shakti and Bhima Shweta recorded low incidence of purple blotch a rating of 1 (1-10% disease incidence) on a scale of 0-5. Higher values of relative humdities (RH-I &II) during kharif and late kharif conditions might be responsible for more severity of purple blotch than in rabi. Red onion variety Bhima Red and white onion variety Bhima Safed owing to have high tolerance under high humid conditions might have shown low rating of 1 on a scale of 0-5.

Effect of Maximum and Minimum Temperatures on incidence and spread of purple blotch

The Maximum temperatures ranged between 27.

Yield performance of Onion varieties during kharif, late kharif and rabi seasons

Among the four varieties tested during kharif, Bhima Red recorded the maximum marketable bulb yield (274.1 q/ha) with moderate incidence of purple blotch. Under late kharif conditions Bhima Shakthi (292.5 q/ha) followed by Bhima Red (227.2 q/ha) recorded higher yields with moderate and low incidence of purple blotch respectively. In rabi both the entries tested (Bhima Safed, Bhima Shwetha) recorded good marketable bulb yields (385.8 and 382.7 q/ha) and also recorded low incidence of purple blotch.

Conclusion

Purple blotch disease severity in onion is more during Kharif and late Kharif than in Rabi. High amount of precipitation, higher relative humidity coupled with warm temperatures during Kharif and late Kharif seasons might have resulted in the greater incidence with more severity of purple blotch than in Rabi season. Bhima Red and Bhima Safed are more tolerant to purple blotch than other tested varieties.

Acknowledgement

AcknowledgementWe gratefully acknowledge to AII India Network Research Project on Onion and Garlic for funding the experiment and Director of Research, Sri Konda Laxman Telangana Horticultural University, Telangana, India for the research facilities provided to carry out the trials.

References

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