Integrated disease management of purple blotch in onion (Allium cepa L.)

Introduction

Onion (Allium cepa L.) is one of the most widely cultivated vegetable crops and forms an essential component of human diets throughout the world. It is valued for its distinctive flavor, nutritional quality, and medicinal properties. Onion bulbs are rich in carbohydrates, vitamins, minerals, antioxidants, and sulfur-containing compounds that contribute to human health. India ranks among the leading producers of onion globally and plays a significant role in supplying domestic and international markets [1][2][3] . However, onion productivity is adversely affected by numerous biotic and abiotic stresses, among which fungal diseases constitute a major challenge. Purple blotch, caused by Alternaria porri (Ellis) Ciferri, is one of the most economically important foliar diseases of onion. The disease occurs in almost all onion-growing regions and can cause yield losses ranging from 30% to 80% under favorable environmental conditions. Initial symptoms appear as small water-soaked lesions on leaves and flower stalks, which subsequently enlarge into characteristic purple-centered spots surrounded by chlorotic halos. Severe infection results in leaf drying, premature senescence, reduction in photosynthetic activity, and poor bulb development [4][5][6][7] . The pathogen thrives under conditions of high humidity, moderate temperatures, and prolonged leaf wetness. Disease outbreaks are particularly severe during rainy seasons and in densely planted fields with poor air circulation. Traditional management practices rely heavily on repeated fungicide applications. Although fungicides provide effective disease suppression, indiscriminate use has led to environmental pollution, fungicide resistance, and concerns regarding food safety.

Consequently, sustainable and eco-friendly disease management approaches have gained increasing importance. Integrated Disease Management (IDM) combines biological, cultural, botanical, and chemical methods to provide effective and environmentally sound disease control. Biological agents such as Trichoderma harzianum and Pseudomonas fluorescens suppress pathogens through competition, antibiosis, and induction of systemic resistance. Botanical extracts derived from neem, garlic, and other medicinal plants possess antifungal properties and can reduce pathogen development. Integration of these components with judicious fungicide use offers a promising approach for sustainable management of purple blotch disease [8][9][10][11][12] . The present investigation was therefore undertaken to evaluate different IDM components and identify an effective management strategy for controlling purple blotch disease and improving onion productivity.

Objectives 1.

To evaluate different integrated disease management practices against purple blotch of onion. 2. To assess the effect of biological control agents and botanical extracts on disease incidence and severity. 3. To determine the influence of IDM treatments on growth and bulb yield. 4. To identify sustainable and economically viable disease management strategies for onion cultivation.

Materials and Methods

Experimental Site and Climatic Conditions:

The field experiment was conducted during the Rabi season of 2024-2025 at the KVK Jammu Experimental Farm under subtropical climatic conditions. The experimental site was situated at an altitude of approximately 250 m above mean sea level and experienced average temperatures ranging from 18 to 30 °C with relative humidity between 65 and 90%. The soil of the experimental field was sandy loam with good drainage and a pH of 6.8-7.2. Recommended agronomic practices for onion cultivation were followed throughout the crop-growing period.

Experimental Material

Healthy seedlings of onion cultivar 'N-53', known to be susceptible to purple blotch disease, were used in the experiment. The pathogen responsible for purple blotch, Alternaria porri (Ellis) Ciferri, was isolated from naturally infected onion leaves collected from diseased fields. Isolation was carried out on potato dextrose agar (PDA) medium and purified by the hyphal-tip technique. Identification of the pathogen was based on colony morphology and microscopic characteristics of conidia.

Experimental Design and Treatments

The experiment was laid out in a Randomized Block Design (RBD) with eight treatments and three replications. Each plot measured 3 × 2 m with a spacing of 15 × 10 cm between plants and rows. Seedlings were transplanted uniformly and maintained under standard crop management practices.

Treatment Code Treatment Description T1

Seedling treatment with Trichoderma harzianum (10 g L⁻¹) T2

Foliar spray of neem seed kernel extract (5%) T3

Seedling treatment with Pseudomonas fluorescens (10 g L⁻¹) T4

Foliar spray of garlic extract (10%) T5

Trichoderma harzianum + neem seed kernel extract T6

Pseudomonas fluorescens + garlic extract T7 IDM package: Trichoderma harzianum + neem extract + Mancozeb (0.25%) T8 Untreated control

Preparation of Botanical Extracts

Fresh neem seeds and garlic cloves were collected and thoroughly washed. The materials were crushed separately with distilled water and filtered through muslin cloth to obtain crude extracts. The filtrates were diluted to obtain the desired concentrations of 5% neem seed kernel extract and 10% garlic extract. Fresh solutions were prepared before each application to ensure maximum efficacy.

Application of Bioagents and Fungicides

For seedling treatment, roots of onion seedlings were dipped in suspensions of Trichoderma harzianum and Pseudomonas fluorescens for 30 minutes before transplanting. Foliar applications of botanical extracts and fungicides were initiated at the appearance of initial disease symptoms and repeated at 15-day intervals. Mancozeb (0.25%) was applied as a need-based spray under the IDM package.

Disease Assessment

Disease incidence and severity were recorded at 60, 75, and 90 days after transplanting. Disease severity was assessed

Economic Analysis

The integrated disease management package (T 7 ) produced the highest gross return (₹4,29,000 ha⁻¹), net return (₹3,02,000 ha⁻¹), and benefit-cost ratio (2.38), whereas the untreated control recorded the lowest economic returns.

Statistical Significance

Analysis of variance indicated highly significant differences among treatments for disease severity, bulb yield, and growth parameters (p < 0.01), confirming the effectiveness of integrated disease management practices in suppressing purple blotch disease and enhancing onion productivity.

Discussion

Purple blotch caused by Alternaria porri is one of the most destructive foliar diseases of onion and poses a serious threat to onion production worldwide. Effective disease management is essential for maintaining bulb quality and maximizing yield. The present investigation demonstrated that integrated disease management (IDM) approaches involving biological control agents, botanical extracts, and need-based fungicide application significantly reduced disease incidence and severity while improving plant growth and bulb yield [13][14][15] . The superiority of integrated treatments over individual components highlights the importance of combining multiple disease management strategies for sustainable crop production. Among the evaluated treatments, the IDM package consisting of Trichoderma harzianum, neem seed kernel extract, and Mancozeb recorded the lowest disease severity and highest bulb yield. The enhanced efficacy of this treatment may be attributed to the synergistic action of biological, botanical, and chemical components. Trichoderma harzianum is a well-known antagonistic fungus that suppresses phytopathogens through mechanisms such as mycoparasitism, antibiosis, competition for nutrients and space, and induction of systemic resistance in host plants [16][17] . The production of hydrolytic enzymes including chitinases, glucanases, and proteases by Trichoderma contributes to degradation of fungal cell walls, thereby inhibiting the growth and development of Alternaria porri. Similar findings regarding the suppressive effect of Trichoderma species against fungal pathogens have been reported by Harman et al. (2004) and Dubey et al. (2007). The effectiveness of neem seed kernel extract observed in the present study may be associated with the presence of several biologically active compounds, including azadirachtin, nimbin, salannin, and gedunin, which possess antifungal and antimicrobial properties. These compounds interfere with spore germination, mycelial growth, and sporulation of fungal pathogens [18][19][20][21][22] . The use of botanical extracts offers an environmentally friendly alternative to synthetic fungicides and reduces the risk of chemical residues in agricultural produce [23][24][25][26] . Previous studies conducted by Patel and Patel (2017) and Rani et al. (2020) also demonstrated significant inhibition of fungal diseases through plant-derived extracts, particularly neem formulations. The lower disease incidence recorded under integrated treatments may further be attributed to the judicious use of Mancozeb, which provided rapid protection against secondary infection and disease spread during favorable environmental conditions. Fungicides remain an important component of disease management; however, their continuous and indiscriminate use may result in pathogen resistance, environmental contamination, and destruction of beneficial microorganisms [27][28][29][30][31][32][33][34][35] . Therefore, integrating fungicides with biological agents and botanicals minimizes dependence on chemicals while ensuring effective disease suppression. Similar conclusions have been drawn by Agrios (2005) and Nene and Thapliyal (1993), who emphasized the importance of integrated approaches in sustainable plant disease management. Significant improvements in vegetative growth parameters, including plant height, number of leaves, and bulb diameter, were observed in plants receiving integrated treatments. These findings may be explained by the plant growth-promoting activities of beneficial microorganisms. Species of Trichoderma and Pseudomonas fluorescens are known to enhance nutrient availability, improve root development, produce phytohormones, and stimulate systemic resistance, resulting in increased vigor and better tolerance to biotic stress. Reduced disease pressure also enables plants to maintain higher photosynthetic efficiency and allocate more assimilates towards bulb development. Similar observations have been reported by Prasad et al. (2002) and Singh and Singh (2004), who documented growth-promoting effects of microbial antagonists in several vegetable crops. The increase in bulb weight and marketable yield under integrated treatments indicates that effective disease suppression directly contributes to improved crop productivity. Purple blotch causes extensive foliar damage, leading to reduced photosynthetic activity and poor translocation of assimilates to developing bulbs. Consequently, severe infections result in smaller bulbs and lower yields. The reduction in disease severity achieved through IDM treatments maintained healthy foliage and enhanced carbohydrate accumulation, ultimately leading to higher bulb production. The present findings are in agreement with earlier reports demonstrating that integrated disease management practices significantly improve bulb yield and quality in onion cultivation. Economic analysis revealed that the integrated treatment involving Trichoderma harzianum, neem extract, and Mancozeb produced the highest net return and benefit-cost ratio. Although the cost of implementing integrated practices was slightly higher than that of individual treatments, the increased bulb yield and improved marketable produce resulted in superior economic returns. This finding highlights the practical feasibility and profitability of adopting integrated disease management strategies under field conditions. Sustainable disease management practices that combine biological and botanical components not only reduce input costs associated with repeated fungicide applications but also contribute to environmental protection and long-term agricultural sustainability. The present study underscores the significance of integrated disease management as an effective alternative to sole reliance on chemical fungicides. Increasing concerns regarding fungicide resistance, environmental pollution, and food safety necessitate the development of eco-friendly and sustainable approaches for disease management. Integration of microbial antagonists, botanical extracts, and need-based fungicide application provides a balanced approach that ensures disease control while preserving soil health and biodiversity. Such practices are consistent with the principles of integrated pest management and sustainable agriculture advocated globally. Nevertheless, the present investigation was conducted under a specific set of agro-climatic conditions, and the performance of the evaluated treatments may vary across locations and seasons. Therefore, further studies involving multi-location trials, optimization of application schedules, formulation development, and assessment of compatibility among bioagents and botanicals are necessary to validate their effectiveness under diverse environmental conditions. Additionally, molecular investigations on host-pathogen interactions and mechanisms of induced resistance may provide valuable insights for developing more efficient disease management strategies, the findings of the present study demonstrate that integrated disease management practices represent a promising, environmentally sound, and economically viable approach for managing purple blotch disease of onion. Adoption of such strategies can contribute significantly to sustainable onion production, reduced dependence on synthetic fungicides, and improved profitability for growers.

Conclusion

The present study demonstrated that integrated disease management practices effectively reduced purple blotch disease and improved onion yield. Among the evaluated treatments, the combination of Trichoderma harzianum, neem seed kernel extract, and need-based fungicide application provided the highest level of disease control and maximum bulb yield. Adoption of such integrated approaches can reduce dependence on synthetic fungicides, minimize environmental risks, and contribute to sustainable onion production. Further multi-location studies are recommended to validate these findings under diverse agroclimatic conditions.

References

1. Agrios GN. Plant pathology. 5th ed. Elsevier Academic Press; 2005.
2. Aveling TAS, Snyman HG, Naude SP. Evaluation of seed treatments for reducing Alternaria porri and Stemphylium vesicarium on onion seed. Plant Dis. 1993;77(10):1009-1011.
3. Bisht IS, Rana JC, Bhandari DC. Characterization and genetic diversity of indigenous onion cultivars. Genet Resour Crop Evol. 1998;45(6):531-540.
4. Brewster JL. Onions and other vegetable alliums. 2nd ed. CABI Publishing; 2008.
5. Cook RJ, Baker KF. The nature and practice of biological control of plant pathogens. American Phytopathological Society; 1983.
6. Dhingra OD, Sinclair JB. Basic plant pathology methods. 2nd ed. CRC Press; 1995.
7. Dubey SC, Suresh M, Singh B. Evaluation of Trichoderma species against Fusarium wilt pathogens. Biol Control. 2007;40(2):157-162.
8. FAO. FAOSTAT statistical database. Food and Agriculture Organization of the United Nations; 2023.
9. Gupta RP, Srivastava KJ, Pandey UB. Management of purple blotch and stemphylium blight diseases of onion. Indian Phytopathol. 1994;47(2):173-181.
10. Gupta RP, Sharma RC, Srivastava KJ. Diseases of onion and garlic. In: Chadha KL, Kalloo R, editors. Advances in horticulture. Vol. 13. Malhotra Publishing House; 1996. p. 459-468.
11. Gupta RP, Singh BP. Diseases and insect pests of onion and garlic. Indian J Agric Sci. 2013;83(3):245-253.
12. Harman GE. Overview of mechanisms and uses of Trichoderma spp. Phytopathology. 2006;96(2):190-194.
13. Harman GE, Howell CR, Viterbo A, Chet I, Lorito M. Trichoderma species-Opportunistic, avirulent plant symbionts. Nat Rev Microbiol. 2004;2(1):43-56.
14. Jhala RC, Momin BC. Integrated management of purple blotch disease of onion caused by Alternaria porri. J Plant Dis Sci. 2014;9(2):184-187.
15. Miller ME, Lacy ML. Purple blotch. In: Schwartz HF, Mohan SK, editors. Compendium of onion and garlic diseases. APS Press; 1995. p. 23-24.
16. Nene YL, Thapliyal PN. Fungicides in plant disease control. 3rd ed. Oxford and IBH Publishing; 1993.
17. Patel DS, Patel PR. Evaluation of plant extracts and bioagents against Alternaria porri causing purple blotch of onion. Int J Curr Microbiol Appl Sci. 2017;6(7):1436-1443.
18. Prasad RD, Rangeshwaran R, Anuroop CP, Rashmi HJ. Biological control of plant diseases through fungal antagonists. Curr Sci. 2002;83(12):1460-1468.
19. Rangaswami G, Mahadevan A. Diseases of crop plants in India. 4th ed. Prentice Hall of India; 2008.
20. Rani I, Sharma A, Devi P. Antifungal activity of neem and garlic extracts against fungal pathogens of vegetable crops. Int J Chem Stud. 2020;8(3):1825-1830.
21. Sharma SR. Effect of fungicidal sprays on purple blotch and bulb yield of onion. Indian Phytopathol. 1986;39(1):78-82.
22. Singh D, Singh HB. Biological control of plant pathogens through Trichoderma species. Indian J Mycol Plant Pathol. 2004;34(2):153-157.
23. Debnath A. Hormonal crosstalk and signal transduction mechanisms regulating plant reproductive development. Agric Rev Int J. 2022;1.
24. Singh RS. Introduction to principles of plant pathology. 4th ed. Oxford and IBH Publishing; 2006.
25. Shetti P, Singh S, Kumbar V, Dodamani S, Paranjape R, Kubade M. Understanding the therapeutic versatility of Apigenin and Quercetin: Broad applications in health and wellness. Acta Pharma Rep. 2025.
26. Islam S, Ali M, Howlader P, Mehedi MNH, Bose SK. Enhancing garlic productivity: Unveiling the potential of Alginate Oligosaccharides (AOS). Agric Arch Int J. 2025.
27. Srivastava KJ, Gupta RP, Pandey UB. Management of foliar diseases of onion and garlic. Indian J Plant Prot. 1994;22(2):125-129.
28. Tripathi P, Dubey NK, Shukla AK. Use of some essential oils as post-harvest botanical fungicides in the management of grey mould of grapes caused by Botrytis cinerea. World J Microbiol Biotechnol. 2008;24(1):39-46.
29. Sultana N, Saini PK, Kiran SR, Kanaka S. Exploring the antioxidant potential of medicinal plant species: A comprehensive review. J Plant Biota. 2023;2(2):9-18.
30. Agida OD, Chukwugozie ES, Chinwendu OB, Chigozie DB, Sojay TT, Abah MA, et al. Sustainable crop management strategies for climate-resilient agriculture. Agric Rev Int J. 2025.
31. Ampa T, Mandal RK. Agricultural productivity and its trends in India: An empirical study. Agric Arch Int J. 2025.
32. Oum IJA, Benoit-Contant Likeng LN, Nkoulou LFM, Djeuani CA, Ngalle BH, Bell JM. Seasonal variations and flowering stages of cassava (Manihot esculenta Crantz) in agroecological zone V of Cameroon. Environ Rep Int J. 2026.
33. Kumar KA, Chaitanya K, Neelima TL, Madhukar B. Advances in smart irrigation technologies for enhancing water use efficiency and crop productivity under changing climate conditions. Agric Rev Int J. 2024.
34. Whitehead MD, Tyler BM. Studies on the epidemiology of purple blotch disease of onion caused by Alternaria porri. Ann Appl Biol. 1962;50(3):389401.
35. Jeet I. Strengthening farmer producer systems and market linkages for enhancing agricultural profitability. Agric Rev Int J. 2022.