Introduction

A leaf spot and blight disease of onion were first reported by Ajrekar (1923) from the Bombay state of India and it was considered that disease caused by a species of Macrosporium sp., the Macrosporium porri was first described as blight causing pathogen of Allium species (Cooke and Ellis 1879). Thus, the taxonomy of Alternaria species on Allium crops causing leaf spot and blight disease was confused, it was first described as the pathogen M. porri, later it was classified as the taxonomy of Alternaria species by A. allii Nolla, based on the symptomatology of pathogen (Nolla 1927), the pathogen maintains its identity with M. porri similarity, thus suggested the appropriate name as purple blotch for the disease due to the presence of large size lesions on leaves and seed stalks (Angell 1929) and the further name changed to A. porri (Cifferi 1930). The name A. allii was resurrected by Simmons (2007) in his identification manual, where it is described as five large spored and long-beaked species from Allium, and thus spores could distinguish based on morphology, the number of beaks and branches, the A. porri and A. allii are closely related and form two distinct clades differ by 8 nucleotides in their RPB2 sequences (Simmons 2007; Woudenberg et al. 2014). Taxonomy of purple blotch disease-causing pathogen has been classified in the Kingdom: Fungi, Division: Ascomycota, Class: Dothideomycetes, Order: Pleosporales, Family: Pleosporaceae, Genus: Alternaria, the Alternaria sect. Porri contained 82 Alternaria species, sect. Porri of A. porri and A. allii cause purple blotch disease in onion had characterized by broadly ovoid, or obovoid, ellipsoid, sub-cylindrical, or obclavate medium to large conidia, and were disto and euseptate, single or in small chains with a simple or branched extended filamentous beak. Conidia enclose multiple transverse, slightly constricted and longitudinal septa and secondary conidiophores can form apically, or laterally (Lawrence et al. 2013). The A. porri isolates are unable to differentiate on conidia, conidiophores shape and size, beak and septa of conidia. Whereas, the isolates are differentiated based on colony color on Sabouraud's medium and Brown's medium, and were differ significantly for aggressiveness, incubation period, disease incidence, and disease severity. The Czapek's medium was found best for growth of isolates, the isolates do not sporulate on culture media and sporulate poorly on the host plant (Gupta et al. 1987).

Epidemiology of Alternia species causing purple blotch disease

An epidemic attack by A. porri on onions was occurred at Baringo, Kenya occurred in the year 1961, and it causes distinct lesions on plant leaf blades, a) purple or brown blotches (Fig. 2) white, irregular spots or flecks with varying proportion (Bock 1964).

Sources of inoculums

Diseased debris containing pathogen fruiting body with conidia in the farm field, or nearby farm is the primary source of inoculum for succeeding bulb crop followed by seed crop of onion (Pandotra 1964), fungus remain as mycelium in onion leaf debris from diseased plants (Muimba-Kankolongo 2018) and spread by the punctures made by the thrips, opening of stomata pores and epidermal layers. The fruiting body with conidia of leaf lesions fabricated as principal source of inoculum for infections and for disseminating the disease. The frequent wind blows, and rain splashes disperse the conidia and mycelium among the stalks and leaves of plants are the secondary source of inoculums (Fig. 1). The pathogen prolong their existence on leaves, seed stalks and on soil surface, hence burring of diseased debris in the soil about 2 to 6 inches deep below the soil surface, thus lead to the complete knock out of pathogen by breaking on existence of primary source of inoculum (Pandotra 1964, 1965).

Etiology

The conidium germinate to form pre-penetration structure and penetrate into the leaf surface through stomata as well as through epidermis, the maximum conidia germinate at 25°C within 24 h (h) of inoculation. The conidium produce several germ tubes and nurture across the leaf surface, further form bulbous appressorium on epidermal cell (52.4% of appressoria) and on stomata (48.6% of appressoria). The bulbous primary hyphae developed under the appressoria, it could lead to for secondary hyphae development within 48 h after inoculation, hyphae grew and penetrate into the intercellular spaces of mesophyll cells (Aveling et al. 1994), then turned to reddish brown septate non-sporulating mycelium (Datar 1994) and cause purple blotch lesions favored by high relative humidity (70% RH), while low RH (40%) resulted in white flecking (often sterile) after prolonged periods of infection. The appressoria and lesions are form in broad optimum temperatures range from 21 to 30°C (Bock 1964).

Alternaria porri is a potentially important pathogen in winter grown Allium crops. The conidia of A. porri germinate within 2 h (4°C), advances for production of terminal and intercalary appressoria (10°C). The maximum counts of appressoria produces after 24 h at 25°C. Pathogens penetrate via epidermis and stomata of leaves with high frequency of stomata penetration. Prominently, infection occur after 16 h of leaf wetness at 15°C and 8 h of leaf wetness at 10–25°C, and severity of infection increases with increasing leaf wetness over 24 h at all temperatures, and cause lesions (Suheri and Price 2000). The moist weather causes lesions to cover with brown mold sporangia. The suitable periods of rain, or heavy dew and favorable environmental condition promote the disease development (Muimba-Kankolongo 2018).

Symptomatology

An epidemic of purple blotch at Baringo, Kenya was revealed by purple or brown blotches, and white, irregular spots or flecks with varying proportion (Bock 1964). Symptomatic expression of purple blotch disease first appears on leaves with 2–3 mm in diameter of whitish water soaked lesions, these lesions enlarge, coalesce, zonate and lesions turn brown to purplish color under favorable conditions (Fig. 2). Seldom, lesion surface covered by black fruiting bodies under humid conditions (Verma and Sharma 1999). Onion plants showing purple blotch symptoms mainly due to the colonization of A. porri and Stemphylium vesicarium (onion leaf blight disease), consequently as on disease advancement the pathogens manly infect on leaves and floral stalks shows typical purple blotch lesion symptoms, the pathogens A. porri, S. vesicarium or mixtures of both accounted for 2.6, 39.8 and 57.6%, respectively. Hence, purple blotch disease is a complex disease caused by two pathogens by the synergistic association, S. vesicarium initiate the infection and facilitate the task of A. porri for causing purple blotch symptoms (Abdel-Rahim et al. 2017). The older plant tissues more susceptible than younger plants leaf blades to the fungus infestation, infection cause small elliptic tan colored water-soaked lesions that soon turn brownish color and later form purplish lesions with darker margin covered by yellow zone of necrotic tissue, as on disease advances the lesions enlarged to form concentric rings, girdling of leaf and stem cause down fall of plant shoot (Muimba-Kankolongo 2018).

Effect of purple blotch disease incidence on plant growth and bulb yield

Onion is susceptible to numerous foliar diseases, those reduces bulb yield and quality (Cramer 2000) and purple blotch is an important disease of onion across the world (Chaput 1995; Cramer 2000; Schwartz et al. 2005) especially in warm and humid environments (Suheri and Price 2001). The fungus infestation cause on both leaves and flower stalks (Bock 1964) and reduce onion tops production by 62–92% (Suheri and Price 2001), cause bulb yield loss of 30% (Everts and Lacy 1990) and 10% losses in s

Fig. 1: Life cycle of Alternaria porri Cifferi causing purple blotch disease in onion, a) conidia mycelia present in debris, weeds and alternate host plants as act as primary source of inoculums, b). Diseased plants in field act as Secondary source of inoculums cause repeated infection and lead to the disease outbreak in the crop

Fig. 2: a) Conidium, b) Hyphae with conidiospores and c) Purple blotch infestion on onion crop stand

eed crop under congenial environmental conditions (Daljeet et al. 1992; Schwartz 2004). Purple blotch disease could cause heavy yield losses ranging from 2.5 to 87.8 per cent during kharif season (Srivastava et al. 1994), with maximum percentage in Karnataka (60%) and Maharashtra (90%) states of India in kharif and rabi seasons respectively (Gupta et al. 1994). A. porri spores present in the air are responsible for increase the disease incidence in onion crop, maximum incidence does occur with adequate leaf wetness duration at 5°C for 16 h and 8 h at 10–25°C. The numbers of lesions are increases with increasing leaf wetness duration and temperature (Suheri and Price 2001). The older leaves are more susceptible than younger leaves for purple blotch disease, infestation reduce the photosynthetic activity of leaves, thus lead to reduction in plant growth, bulb yield and seed yield (Verma and Sharma 1999).

Management and control of purple blotch disease – chemical and biological agents

The losses of bulb and seed yield of onion cause by purple blotch disease could prevent either by protective sprays fungicides or biological agents which are antogonistic effects on A. porri during crop cultivation. Despite several limitations in the field conditions like frequent or unexpected rainfall, weather modulation mainly humidity could favor to the pathogen outbreak and thus pathogen cause severe damage on the standing crop in the field under favorable condition. Under those circumstances the fungicide sprays are an effective method to control the disease to maintain the crop stand, several researchers standardized the optimum dose of sprays for the control of disease are presented in Table 1, however it adds additional cost to the cost of cultivation.

Table 1: Effective chemicals to control purple botch disease under field conditions

Brand name	Chemical composition	Concentration	References
Dithane M-45	Mancozeb 75% WP	0.2%	Wanggikar et al. (2014)
Rovral WP	Iprodione 41.6%	20 g per 10 liters of water (0.2%)	Akter et al. (2015)
Ridomil Gold WP	Mancozeb (64% W/W) + Metalaxyl-M (4% W/W)
Dithane M-45	Mancozeb 75% WP
AGEENT, Custom, DOZAN, Katyayani, DuoGuard	Cymoxanil 8% + Mancozeb 64% WP	2500 ppm	Rao et al. (2015)
Folicur	Tebuconazole 25 EC	0.1%	Yadav et al. (2017)
Score	Difenoconazol 25EC	0.1%	Yadav et al. (2017)
Score	Difenconazole 25 EC	0.1%	Kavitha et al. (2017)
Nativo 75 WG	Tebuconazole 50% + Trifloxystrobin 25%	0.05%	Kavitha et al. (2017)
Roshan plus	Hexaconazole 5% SC	0.1%	Nisha et al. (2020)
Real-mil	Mancozeb 64%WP +Cymoxanil 8% WP	0.3%	Nisha et al. (2020)

The bio-control agents are effective to control purple blotch disease, the bio agents namely Trichoderma spp., Penicillium spp., Aureobasidium pullulens, Sporobolomyces roseus and Cryptococcus luteolus were effective to control A. porri. The seed treatment with Trichoderma harzianum reduces the purple blotch disease incidence thus increase the bulb yield of onion (Chethana et al. 2012; Mishra and Gupta 2012). The seed treatment, seedling dip and three foliar sprays of bacteria namely Pseudomonas fluorescens, P. aeruginosa, and Bacillus subtilis, and fungi namely T. viride and T. harzianum could control the purple blotch disease in field conditions (Yadav et al. 2013). The botanical like clove extract of Allium sativum (10%) Aloe vera (10%), neem oil (20%) and pongamia oil (20%) resulted in inhibition of A. porri (Chethana et al. 2012).

Development of resistance varieties to purple blotch

The development of purple blotch disease resistant cultivars, varieties or hybrids is another approach to control the disease, and it is an economical and environmentally friendly method as it reduces the ecological problems caused by the use chemicals to control purple blotch disease. However, the fungicides available in the market have low potential to manage onion purple blotch disease (Uddin et al. 2006; Abdel-Hafez et al. 2014). In this context, there is a need of hunting resistance source for purple blotch disease to improve the bulb production and productivity. Thus, at present there were no potential onion varieties or hybrids succeed in onion acreage showing resistance to purple blotch disease. Nevertheless, several researchers are hunting for purple blotch disease resistance source of resistance lines from past several years.

The onion hybrids cross viz., Red Creole × Kaharda and Kaharda × Red Creole were resistant to purple blotch disease, these hybrids are performed better than their parents and other hybrids in terms of disease incidence and bulb yields, negative environmental correlation was noted between disease incidence and bulb yield significantly, higher disease incidence could lower bulb yield was due to environmental effects rather than the genotypes (Abubakar and Ado 2008).

Selves of second-generation mutant (M₂₎ onion plants under epiphytotic conditions, revealed the disease resistance against purple blotch with 7.60% M₂ plants (1–10 PDI), while 12.80% M₂ plants with moderately resistance (11–25 PDI), these disease resistance plants was associated with more than two times higher chlorophyll content (95.8–108.10 mg/100 g) with dark waxy leaves than the normal green foliage (Patil et al. 2008). The disease resistance (1–10 PDI) was noticed in M₄ white onions (15.80%) than the M₄ red onions (2.20%). Furthermore, majority of M₄ population (about 70–75%) were moderate resistance (11–25 PDI) against the purple blotch disease in both red and white onions. Therefore, further crop improvement is essential for incorporation of disease resistance by advancement of 3–4 selection cycles (Patil et al. 2009).

Purple blotch disease resistance attributed by cuticle thickness; thus resistance could break by wounding and it is naturally associated with sand storm blast (Bock 1964). The cultivars screened in search of resistance to purple blotch disease, thus the varieties VL-1, PBR-1, PBR-5 and PRR are found resistant to purple blotch disease (Daljeet et al. 1992). The onion accession CBT-Ac77 and the variety Arka Kalyan was found highly resistant to purple blotch resistance among 43 Allium genotypes screened under field conditions, thus suggested the newly identified resistance sources were the potential donors for purple blotch resistance breeding (Nanda et al. 2016).

Genetics of purple blotch resistance

The purple blotch disease resistance line PBR-287 was identified as a good source of resistance, and hence it was used in the cross with susceptible parent. Parents, F₁ and individual F₂ progenies were subjected for genetics of resistance and RAPD marker analysis. The results reveal that F₂ individuals segregated in 3:1 ratio for resistance (Ganesh and Veeregowda 2005). The molecular markers linked to purple blotch disease resistance was developed by using F₁, F₂, and BC₁populations, which were developed from resistant (R) parent Arka Kalyan and the susceptible (S) parent Agrifound Rose. The inheritance of purple blotch disease revealed that the F₁ was resistant, while 498 F₂ plants and 128 BC₁lines segregated in 3R:1S and 1R:1S ratio. Hence, A. porri resistance (ApR) is controlled by a single dominant gene and thus designated as ApR1 gene (Chand et al. 2018).

Molecular perspective

Three breeding lines PBR-287, MS-65-268 and Arka Kalyan-704 were identified as resistant to purple blotch disease, and these lines were subjected to random amplified polymorphic DNA (RAPD) analysis with 160 ten mer random primers for identification and estimation of genetic relationship among resistant and susceptible (Arka Niketan) varieties. Out of 160 primers 41 primers are exhibited polymorphism among the accessions, and DNA profile with 5 primers namely OPB02, OPB13, OPC09, OPC12 and OPF08 distinguished resistant lines from the susceptible line. The PCR products ranged from 300 bp to 2000 bp and were consistent, unambiguous and repeatable primer with an average of 2.65 polymorphic bands, 11.65 monomorphic bands were produced per primer (14.3 bands). Principle component analysis (PCA) confirmed the least genetic dissimilarity (40%) was recorded between PBR-287 and MS-65-268; whereas, the highest (65%) was found between lines Arka Niketan-709 and MS-65-268 (Ganesh and Veeregowda 2005). The nucleotide rbcL and matK were used and developed SSR markers to detect the purple leaf blotch (PLB) gene, and it does exist on shorter arm of eight chromosome at s1/s2 locus. The PLB gene conferred resistance to purple leaf blotch in onion mutant lines (BP2-75/2, BP2 -100/1, BP2-100/2) and mutant variety BARI Piaz-2 was successfully detected in the mutant lines using SSR markers (Chakraborty et al. 2015). The ApR1 gene was linked with seven markers namely AcISSR47₁₂₅₇, AcISSR68₁₆₀₀, AcISSR103₁₄₁₆, AcSSR7, AcSSR22, AcSSR31, and AcSSR33 showed polymorphism among resistant and susceptible bulks and were used in genotyping of mapping populations (F₂ and BC₁). The three inter simple sequence repeats (ISSR) were converted into sequence-tagged markers (STS), the single-copy status of resistant locus association confirmed by southern blotting. The markers linked closely at 1.3 centi Morgan (cM) distance of AcSSR7 (SSR) and ApR-450 at 1.1 cM (STS) to the ApR1 locus. These findings could be recommended for facilitating the introgression of ApR1 gene to desirable genotypic backgrounds (Chand et al. 2018).

Conclusion and Future Perspective

Purple blotch caused by A. porri is a serious disease, incur to heavy yield losses in the bulb and seed crop of onion. The yield losses can be controlled by efficient crop management practices, crop rotation, protective sprays with fungicides, use of biological agents in onion production could control the purple blotch disease, but these crop management activates add the additional cost to the production cost of onions. Thus, the identification of effective and stable resistance varieties are Arka Kalyan, VL Paiz-1 and breeding lines PBR-287, PBR-1, PBR-5 and PRR (Ganesh and Veeregowda 2005; Daljeet et al. 1992), hybrids are Red Creole × Kaharda and Kaharda × Red Creole (Abubakar and Ado 2008), accession namely CBT-Ac77 (Nanda et al. 2016) would be useful sources of resistance to purple blotch thus these are useful in breeding of onions resistance to purple blotch. Development of genomic SSR markers is a practical tool set for genetics studies in onion (Baldwin et al. 2012), thus ApR1 gene flanking markers could be applicable in MAS with high efficiency (Chand et al. 2018). The chromosomal location of the ApR1 gene is yet to be ascertain, thus fine mapping of resistant locus may be preceded by advanced DNA markers such as single nucleotide polymorphisms (SNPs) with more close linkage (Chand et al. 2018). The validation of ApR1 gene flanking SSR and ISSR markers in other genotypes need to be focused for the identification and isolation of potential purple blotch disease resistance source.

Conflict of Interest

We the authors declare that have no conflict of interest.

Ethical Approvals

The manuscript was not submitted anywhere else, and results were presented without fabrication, falsification, or inappropriate data manipulation. Research does not pose any threat to public health or national security.

References

Abdel-Hafez SII, KAM Abo-Elyousr, IR Abdel-Rahim (2014). Effectiveness of plant extracts to control purple blotch and Stemphylium blight diseases of onion (Allium cepa L.) in Assiut, Egypt. Arch Phytopathol Pflanzensch 47:377‒387

Abdel-Rahim IR, SII Abdel-Hafez, KAM Abo-Elyousr (2017). Onion purple blotch symptoms, at Assiut Governorate (Egypt), caused by synergistic association between Alternaria porri and Stemphylium vesicarium. J Plant Dis Prot 124:195‒200

Abubakar L, SG Ado (2008). Heterosis of purple blotch (Alternaria porri (Ellis) Cif.) resistance, yield and earliness in tropical onions (Allium cepa L.). Euphytica 164:63‒74

Ajrekar SL (1923). Annual report of the work done under plant pathologist to the government of Bombay Poona for the year 1921–22. In: Bombay Department of Agriculture Annual Report 1921–22, pp:102–104, Bombay, India

Akter US, MHO Rashid, MA Rahman, MR Islam, MM Haque (2015). Effect of the treatments in controlling purple blotch complex of onion (Allium cepa L.). Acad J Plant Sci 7:14‒19

Angell HR (1929). Purple blotch of onion (Macrosporium porri Ell.). J Agric Res 38:467‒487

Aveling TAS, HG Snyman, FHJ Rijkenberg (1994). Morphology of infection of onion leaves by Alternaria porri. Can J Bot 72:1164‒1170

Baldwin S, M Pither-Joyce, K Wright, L Chen, J McCallum (2012). Development of robust genomic simple sequence repeat markers for estimation of genetic diversity within and among bulb onion (Allium cepa L.) populations. Mol Breed 30:1401‒1411

Bock KR (1964). Purple blotch (Alternaria porri) of onion in Kenya. Ann Appl Biol 54:303‒311

Chakraborty M, T Quddus, S Rahman, MAK Azad, MR Hossain, A Hoque, MK Saha, MA Haque (2015). Molecular characterization of selected mutant lines of onion (Allium cepa L.) against purple leaf blotch disease using SSR markers. J Exp Agric Intl 8:261‒267

Chand SK, S Nanda, RK Joshi (2018). Genetics and molecular mapping of a novel purple blotch-resistant gene ApR1 in onion (Allium cepa L.) using STS and SSR markers. Mol Breed 38:109–121

Chaput J (1995). Identification of diseases and disorders of onions, pp:1‒9. Queens Printers for Ontario, Ontario, Canada

Chethana BS, G Ganeshan, AS Rao, K Bellishree (2012). In-vitro evaluation of plant extracts, bioagents and fungicides against Alternaria porri (Ellis) Cif., causing purple blotch disease of onion. Pest Manage Hortic Ecosyst 18:194‒198

Cifferi R (1930). Phytopathological survey of santo domingo (1925–1929). J Agric Univ Porto Rico 14:5–44

Cooke MC, JB Ellis (1879). New Jersey fungi. Grevillea 8:11‒16

Cramer CS (2000). Breeding and genetics of Fusarium basal rot resistance in onion. Euphytica 115:159‒166

Daljeet S, JS Dhiman, AS Sidhu, H Singh (1992). Current status of onions in India: Strategies for disease resistance breeding for sustained production. Onion Newslett Trop 4:43‒44

Datar VV (1994). Investigation of purple blotch of onion in India. Acta Hortic 358:259‒263

Everts KL, ML Lacy (1990). Influence of environment on conidial concentration of Alternaria porri in air and on purple blotch incidence on onion. Phytopatology 80:1387‒1391

Ganesh CT, R Veeregowda (2005). Fingerprinting and genetic diversity of purple blotch disease resistant and susceptible genotypes of onion (Allium cepa L.) using RAPD markers. Rec Trends Hortic Biotechnol 2:599‒561

Gupta RBL, VN Pathak, OP Verma (1987). Morphological, cultural, and pathogenic variations in Alternaria porri (Ell.) Cif., the incident of purple blotch of onion. Zentr Mikrobiol 142:155‒162

Gupta RP, KJ Srivastava, UB Pandey (1994). Diseases and insect pests of onion in India. Acta Hortic 358:265‒269

Kavitha SV, BG Naik, TH Kavita (2017). Management of Purple blotch disease of onion under field condition. J Pharmacogn Phytochem 6:1768‒1769

Lawrence DP, PB Gannibal, TL Peever (2013). The sections of Alternaria: Formalizing species-groups concepts. Mycology 105:530‒546

Mishra RK, RP Gupta (2012). In-vitro evaluation of plant extracts, bio-agents and fungicides against Purple Blotch and Stemphylium blight of onion. J Med Plant Res 6:5840‒5843

Muimba-Kankolongo A (2018). Vegetable production. In: Food crop production by smallholder farmers in Southern Africa, pp: 205‒274. Academic Press, London

Nanda S, SK Chand, P Mandal, P Tripathi, RK Joshi (2016). Identification of novel source of resistance and differential response of Allium genotypes to purple blotch pathogen, Alternaria porri (Ellis) Ciferri. Plant Pathol J 32:519‒527

Nisha P, A Pragya, T Puja (2020). Management of purple blotch complex of onion (Allium Cepa cv. Red Creole) under field condition in Rukum-West, Nepal. Malays J Sustain Agric 4:71‒74

Nolla JAB (1927). A new Alternaria disease of onions (Allium cepa L.). Phytopathology 17:115‒132

Pandotra VR (1965). Purple blotch disease of onions in Punjab. Proc Ind Acad Sci 62:229‒234

Pandotra VR (1964). Purple blotch disease of onion in Punjab. Proc Ind Acad Sci B 60:336‒340

Patil RS, MA More, US Shinde, MN Bhalekar (2009). Screening of M4 bulb crop of red and white onions against purple blotch disease. Adv Plant Sci 22:299‒301

Patil RS, BV Pawar, SS Mehetre, US Shinde (2008). Evaluation of M2 bulb crop for blub development and disease resistance. Adv Plant Sci 21:671‒673

Rao AS, G Ganeshan, YL Ramachandra, BS Chethana (2015). Field evaluation of fungicides against Alternaria porri (Ellis) Cif., causing purple blotch of onion (Allium cepa L.). I J Agric Environ Biotechnol 8:89‒95

Schwartz HF (2004). Botrytis, downy mildew and purple blotch of onion. Crop Series. Diseases http://www.ext.colostate.edu

Schwartz HF, DH Gent, ME Bartolo (2005). Purple blotch http://www.highplainsipm.org/HpIPMSearch/Docs/Purple Blotch-Onion.htm

Simmons EG (2007). Fungal Biodiversity Centre; Utrecht, Netherlands. Alternaria: an Identification Manual. CBS Biodiversity Series 6, Utrecht, Netherlands

Srivastava PK, BS Bharadwaj, PP Gupta (1994). Status of field diseases and selected pest of onion in India. Nat Hortic Res Dev Found Newsl 14:11‒14

Suheri H, TV Price (2001). The epidemiology of purple leaf blotch on leeks in Victoria, Australia. Eur J Plant Pathol 107:503‒510

Suheri H, TV Price (2000). Infection of onion leaves by Alternaria porri and Stemphylium vesicarium and disease development in controlled environments. Plant Pathol 49:375‒382

Uddin M, M Islam, N Akhtar, A Faruq (2006). Evaluation of fungicides against purple blotch complex of onion (Alternaria porri and Stemphylium botryosum) for seed production. J Agric Sci Technol 9:83‒86

Verma LR, RC Sharma (1999). Diseases of horticultural crops: Vegetables, ornamentals and mushrooms, pp:353‒356. Indices Publishing Company, New Delhi, India

Wanggikar AA, SS Wagh, DP Kuldhar, DV Pawar (2014). Effect of fungicides, botanicals and bioagents against purple blotch of onion caused by Alternaria porri. Intl J Plant Prot 7:405‒410

Woudenberg JHC, M Truter, JZ Groenewald, PW Crous (2014). Large-spored Alternaria pathogens in section Porri disentangled. Stud Mycol 79:1‒47

Yadav PM, KB Rakholiya, DM Pawar (2013). Evaluation of bioagents for management of the onion purple blotch and bulb yield loss assessment under field conditions. Bioscan 8:1295‒1298

Yadav RK, A Singh, S Jain, AS Dhatt (2017). Management of purple blotch complex of onion in Indian Punjab. Intl J Appl Sci Biotechnol 5:454‒465