Status and distribution of purple blotch disease in major onion growing districts of karnataka

Introduction

Onion (Allium cepa L.) popularly known as the "Queen of kitchen" is regarded as the poor man's staple spice. It is one of the five most important fresh market vegetable crops grown worldwide belonging to the family Alliaceae (Cramer, 2000) [3] . The primary centre of origin lies in Central Asia, extensively cultivated across diverse agroclimatic regions and the major onion growing countries in the world are China, India, United States, Turkey, Iran, Egypt and Pakistan. Onion is growing throughout the year in different ecological conditions and consumed by almost all classes of people on account of its medicinal and dietary value. It also exhibits a number of therapeutic properties such as antibacterial, antifungal, anthelminthic, anti-inflammatory, antiseptic and antispasmodic etc. (Dar et al., 2020) [4] . It also owns potent medicinal value in ayurvedic and homeopathic therapy. It is rich source of minerals, vitamins and dietary fibers. Allylpropyldisulphide is the main ingredient responsible for pungency in bulbs which helps to prevent cancer. Due to their enormous commercial and medicinal value, onion is cultivated in almost all countries of the world and consumed across the globe (Kim et al., 2022) [7] . In India, the production of onion is 31.00 billion tones with an area of 1.79 billion ha. And in Karnataka, it occupies an area of 231.84 mha, production of 2.78 billion tones and productivity of 11.99 mt/ha (Anon, 2022). The major onion producing states in India are Maharashtra, Madhya Pradesh, Karnataka, Rajasthan and Gujarat. The maximum compound growth rate in onion export of India to all countries was reduced in (2018-19) compared to (1996-97) in quantity and value in Indian rupee (Jadhav et al., 2022) [6] . Hence, in the present investigation various aspects on Purple blotch was undertaken during the period 2023 with reference to survey and surveillance of purple blotch disease in major onion growing areas of the northern Karnataka.

Material and methods

A roving survey was conducted to know the status of the purple blotch in major onion growing regions of Karnataka viz., Bagalkote, Belagavi, Ballari, Bidar, Chitradurga, Gadag, Haveri, Dharwad, Kalaburagi, Kolar, Koppal, Raichur, Tumkur, Vijayapura and Yadgiri districts (Fig. 1). During the survey, the observation of disease severity was recorded by randomly selecting 25 onion plants per plot. Based on this observation the per cent disease index was calculated. Scoring of the disease was done in the field by using 0 -5 scale given by Mayee and Datar (1986) [8] ; Muthaiah et al. (2021) [9] ; Chauhan et al. (2024). The per cent disease index (PDI) was calculated by using formula given by Wheeler (1969) [14] .

Disease rating scale

Results and discussion

The per cent disease severity (PDI) across all the surveyed regions ranged from 11.60 to 52.80 per cent. The minimum disease intensity was observed in Hosakere village (Madhugiri taluk, Tumakur district) with 11.60 per cent while the maximum disease intensity was recorded in Raichur district, Raichur taluk and Signodi village (52.80%) during Kharif 2023 and rabi 2023-24 (Table 1). This variation in per cent disease severity indicates that the intensity of purple blotch is seriously influenced by regional agroclimatic conditions, crop management strategies and varietal response. Among the districts surveyed, Dharwad exhibited the maximum mean PDI of 31.47 per cent followed by Raichur (30.40%) and Yadgir (28.40%). In contrast, Belagavi recorded 17.87 per cent and Tumakur 19.33 per cent showed comparatively minimum disease intensity during Kharif 2023 and rabi 2023-24. The overall district mean values highlight that northern dry zones and irrigated belts favoured maximum disease intensity compared to transitional zones. In Raichur district, PDI values ranged from 23.20 to 52.80 per cent, with a mean disease intensity of 30.40 per cent. The maximum disease intensity was recorded in Singnodi village (52.80%) of Raichur taluk, where the variety Nasik red was grown under irrigated conditions. The minimum disease intensity was observed in Margatnal (23.20%) of Lingasugur taluk and Jutmardi (23.20%) of Devadurga taluk. Other locations such as Neeralkere village of Lingasugur taluk recorded comparatively maximum disease intensity (40.80%), while most rainfed areas exhibited moderate disease levels. In Ballari district, purple blotch intensity varied from 20.80 to 39.20 per cent, with an overall mean PDI of 26.84 per cent. The maximum disease intensity was observed at Sandur village (39.20%) of Sandur taluk, where Hybrid red (Ellora) was cultivated. In contrast, the minimum disease intensity was recorded at Bhujanganagar village (20.80%) of Sandur taluk. Villages under Siruguppa and Bellari taluks showed relatively uniform and moderate disease intensity, ranging between 24.80 and 27.20 per cent. In Yadgiri district, PDI values ranged from 25.60 to 32.00 per cent with a mean disease intensity of 28.40 per cent. The maximum disease intensity was recorded in Chamnal village (32.00%) of Shahapur taluk whereas the minimum disease intensity was observed at Mailapur village (25.60%) of Yadgiri taluk. Other villages in the district recorded moderate levels of purple blotch intensity under irrigated conditions. In Kalaburagi district, PDI values ranged from 15.20 to 25.60 per cent, with a mean disease intensity of 20.72 per cent. In Kalaburagi taluk, the minimum disease intensity was recorded at Keribhosga village (15.20%) where Nifad selection (Panchganga) was cultivated under irrigated conditions while the maximum PDI was observed at Nagoora village (21.60%) under rainfed conditions with Hybrid red (Ellora). In Chitapur taluk, comparatively maximum disease pressure was noticed, with the maximum PDI recorded at Watwati village (25.60%) followed by Herur village (23.20%) both under rainfed conditions with Nasik red, whereas the minimum disease intensity in this taluk was observed at Shellagi village (20.80%). Similarly, in Aland taluk, where onion cultivation was predominantly under irrigated conditions purple blotch intensity ranged from 18.40 to 23.20 per cent with the minimum PDI at Kadaganchi village (18.20%) and the maximum at Telekuni village (23.20%) both grown with Nifad selection (Panchganga). In Bidar district, the intensity of purple blotch disease varied from 18.40 to 29.60 per cent, with a mean PDI of 23.87 per cent. The maximum disease intensity was recorded at Kalwaadi village (29.60%) of Bhalki taluk, whereas the minimum disease intensity was observed at Wadankera village (18.40%) of Humnabad taluk. Among the surveyed locations, Talmadgi (26.40%) and Dhanura (27.20%) also recorded comparatively higher disease intensity. Most of the surveyed fields cultivated Nifad selection (Panchganga) under irrigated flat-bed conditions. In Koppal district, purple blotch intensity ranged between 20.80 and 29.60 per cent, with a mean PDI of 24.53 per cent. The maximum disease intensity was recorded at Katarki village (29.60%), while the minimum disease intensity was observed at Kidadhal village (20.80%) of Koppal taluk. Hiresindogi village recorded a disease intensity of 23.20 per cent. The surveyed onion fields were predominantly irrigated with cultivation practices involving both flat-bed and ridges and furrows methods. In Belagavi district, comparatively minimum disease pressure was recorded, with PDI values ranging from 15.20 to 20.00 per cent and a mean disease intensity of 17.87 per cent. The maximum disease intensity occurred at Budigoppa village (20.00%), while the minimum disease intensity was observed in Yaragatti village (15.20%) of Savadatti taluk. In Dharwad district, purple blotch intensity varied from 18.40 to 52.00 per cent, with a mean PDI of 31.47 per cent. The maximum disease intensity was recorded at Amminabhavi village (52.00%) of Dharwad taluk, followed by Gabbur village (34.40%) of Hubballi taluk. The minimum disease intensity was observed at Kusugal village (18.40%). In Vijayapura district, PDI values ranged from 13.60 to 48.80 per cent, with an overall mean disease intensity of 25.44 per cent. In Vijayapura taluk, disease intensity ranged from 16.80 to 23.20 per cent, with the minimum PDI recorded at Hittinahalli village (16.80%) and the maximum at Ankalagi village (23.20%), where Nifad selection (Panchganga) was cultivated under rainfed conditions. In Basavana Bagevadi taluk, the minimum disease intensity in the district (13.60%) was observed at Mattinal village with Nifad selection (Panchganga), whereas the maximum PDI (48.80%) was recorded at Yarnal village under Hybrid red (Ellora) followed by Shankarnagar village (34.40%) with Pusa fursangi. In Kolhar taluk, purple blotch intensity ranged from 18.40 to 28.80 per cent with the minimum at Kolhar village (18.40%) under Bheema super and the maximum at Mattinal village (28.80%) with Hybrid red (Ellora). In Gadag district, purple blotch intensity varied between 21.60 and 33.60 per cent, with a mean PDI of 25.87 per cent. The maximum disease intensity was recorded at Naragund village (33.60%), whereas the minimum disease intensity was observed at Arban village (21.60%) of Naragund taluk. In Haveri district, PDI values ranged from 22.40 to 26.40 per cent, with a mean intensity of 24.20 per cent. The maximum disease intensity was observed in Devaragudda village (26.40%) of Ranebennur taluk, while the minimum disease intensity was recorded at Hanumanamatti village (24.20%). In Bagalkote district, purple blotch intensity ranged from 20.80 to 25.60 per cent, recording a mean PDI of 23.20 per cent. The maximum disease intensity was observed at Mamatageri village (25.60%) of Bagalkote taluk, whereas the minimum disease intensity was recorded at Tulsigeri village (20.80%). In Chitradurga district, disease intensity varied from 23.20 to 28.80 per cent, with a mean disease intensity of 25.13 per cent. The maximum disease intensity was recorded at Giriyapura village (28.80%) of Challakere taluk, while the minimum disease intensity was observed at Rangenahalli village (23.20%). In Tumakur district, comparatively minimum disease intensity was recorded, with PDI values ranging from 11.60 to 25.60 per cent and a mean disease intensity of 19.73 per cent. The maximum disease intensity was observed at Thippagondanahalli village (25.60%), whereas the minimum disease intensity was recorded at Hosakere village (11.60%) of Madhugiri taluk. In Kolar district, the intensity of purple blotch ranged from 18.40 to 20.80 per cent with a mean PDI of 19.73 per cent. The maximum disease intensity was recorded at Gajalbavi village (20.80%) of Mulbagal taluk, whereas the minimum disease intensity was observed at Jangamahosahalli village (18.40%). Achampalli village recorded a disease intensity of 20.00 per cent. Purple blotch PDI varied depending on the type of onion cultivars. The local variety and Nasik Red variety recorded maximum disease intensity levels. Similarly, Local susceptible varieties and Nasik Red exhibited maximum level of PDI values (>40%) whereas Bellary Red and improved lines such as Arka Kalyan, Arka Bheema and Bhima Super showed moderate to maximum levels of resistance with PDI values ranging from 5-10 per cent (Ravichandran et al. 2017) [11] . The disease intensity was more pronounced at physiological maturity crop stage coincided with canopy closure and favourable microclimatic conditions for disease development. Cultivar susceptibility played a decisive role in disease distribution. Fields with red soils showed relatively maximum disease intensity compared to black soils possibly due to differences in soil moisture retention. Irrigated conditions favoured maximum level of disease intensity (mean PDI >30%) compared to rainfed conditions where disease intensity remained relatively moderate to minimum. Continuous soil moisture under irrigated conditions might have created a favorable environment for conidial germination and disease progression. Flatbed planting exhibited maximum level of PDI compared to ridge planting suggesting that better drainage and aeration in ridge planting suppress the disease to some extent. This supports earlier reports that field microclimate plays a critical role in purple blotch disease development (Angadi et al., 2018) [1] . The observed variation in disease intensity among districts can be attributed to differences in microclimate, varietal susceptibility and irrigation practices. Disease development was severe in fields with continuous irrigation, possibly due to sustained leaf wetness and high relative humidity which promote conidial germination and infection. In contrast, fields under rainfed conditions exhibited relatively moderate to minimum disease intensity (mean PDI <25%) likely due to intermittent moisture and limited canopy humidity. Similarly, fields established on flat beds showed maximum level of disease intensity than those under ridge and furrow planting supporting the idea that improved drainage and aeration reduce microclimatic humidity conducive to infection (Ravichandran et al. 2017) [11] . Soil type also influenced disease dynamics. Fields with red soils known for better drainage but higher surface humidity under irrigation recorded maximum disease intensity than black soils where moisture retention was more consistent but surface evaporation limited spore survival. These findings align with the reports of Pandey et al. (2020) [10] and Sharma et al. (2021) [12] observed that soil moisture gradients significantly modulate foliar disease microenvironments. The survey results clearly indicated that purple blotch disease is widely prevalent throughout Karnataka though its disease intensity differed significantly across the districts, taluks and villages. The maximum disease intensity in Raichur, Dharwad and Yadgiri districts can be attributed to irrigated conditions, conducive weather and cultivation of susceptible varieties. On the other hand, districts like Belagavi, Haveri and Tumakuru with relatively minimum level of disease intensity may benefit from less favourable environmental circumstances for the pathogen or adoption of tolerant varieties (Vijaykumar et al., 2022) [13] . The observed range of disease intensity aligns with earlier findings that purple blotch intensity is heavily reliant on crop growth stage, cultivars and weather factors such as relative humidity and temperature. The survey also underlines the role of planting method and irrigation in disease management. Thus, integrated management practices including varietal resistance, crop rotation and appropriate cultural practices are essential to minimize purple blotch intensity in onion growing regions of Karnataka (Pawar et al., 2023), (Singh et al., 2024) and (Hariprasad and Palakshappa, 2021) [5] . Collectively, these findings indicate that the distribution and severity of purple blotch disease in onion are strongly governed by interactions among regional climatic factors, soil type, crop establishment methods and varietal susceptibility. The consistently maximum disease intensity observed in humid and irrigated regions underscores the need for integrated disease management strategies including the selection of resistant cultivars, improved drainage, crop rotation and region specific fungicidal applications. Tailoring these strategies to the distinct agroecological zones of Karnataka will be crucial for minimizing disease impact and enhancing sustainable onion production.

Conclusion

The present investigation clearly revealed that purple blotch disease caused by Alternaria porri is widely prevalent across all major onion growing districts of Karnataka exhibiting considerable spatial and temporal variation in disease severity during Kharif 2023 and rabi 2023-24. The per cent disease index (PDI) varied markedly among districts and villages ranging from 11.55 to 52.80 per cent during Kharif 2023 and rabi 2023-24 highlighting the extensive distribution and economic importance of the pathogen. The maximum per cent disease severity was consistently recorded in Dharwad and Raichur districts whereas comparatively minimum per cent disease severity was observed in Belagavi and Tumkur districts. The observed variation in disease severity across locations could be attributed to differences in agroclimatic conditions, soil type, irrigation methods, crop management practices and varietal susceptibility. Areas characterized by humid and irrigated environments showed maximum per cent disease severity emphasizing the role of prolonged leaf wetness and favourable microclimatic conditions in promoting infection and pathogen spread. Similarly, flatbed planting and cultivation of susceptible varieties such as Nasik Red were found to enhance disease severity, whereas ridge planting and tolerant cultivars like Bellary Red showed relatively reduced infection levels. The findings corroborate earlier reports that purple blotch continues to remain a major constraint to onion productivity in Karnataka. The persistence of the pathogen across multiple agroecological zones suggests that environmental conditions across the state are conducive for its survival and dissemination. Therefore, integrated management strategies that combine the use of resistant varieties, appropriate planting methods, efficient irrigation scheduling and timely fungicidal interventions are essential for minimizing disease impact. Overall, this survey provides a comprehensive understanding of the current status and distribution pattern of purple blotch disease in Karnataka and serves as a valuable baseline for designing region specific disease predicting and management programs aimed at sustaining onion productivity and profitability in Karnataka.

Acknowledgement

The author sincerely acknowledges the support and facilities provided by the Department of Plant Pathology, University of Agricultural Sciences, Raichur. The author also extends gratitude to the Chairman and advisory committee members for their guidance throughout the investigation.

Conflicts of interest

The author declares no conflicts of interest.

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