PURPLE BLOTCH DISEASE OF ONION IN PUNJAB* I. Its Occurrence, Pathogenicity and Host Range BY V, R. PANDOTRA (Regional Research Laboratory, Jammu) Received July 24, 1964 (Communicated by Dr. L. D. Kapoor, F.^.sc.) INTRODUCTION ONION (Allium cepa L.) is one of the most important commercial vegetable crops grown in India. Many diseases affect this crop, some of which are of great economic significance in many parts of Punjab. One of these is Blight, caused by Alternaria sp. This disease is by far the most important and it has been found to cause severe losses to onion crop in Punjab. Ajrekar (1920-21) reported a leaf-spot and blight disease of Onion from Bombay and considered it to be caused, by a species of Macrosporium. Anyangar (1928) noted a similar disease at Coimbatore caused by Alternaria sp. and named it as Alternaria palandui. Since the fungus-causing blight in Punjab was found to be different from A. palandui, and has not been studied earlier, it was considered necessary to investigate this disease. Review of previous work.--from a study of the available literature on this disease, there seems to exist some confusion about its name and also its causal organism. Early investigators identified the causal organism as Macrosporium parasiticum and Macrosporium porri, but, later on Angell (1929) ruled out M. parasiticum as the causal fungus of this disease. Nolla (1927) also identified the causal organism as a species of Alternaria and named it as Alternaria allii. Angell (I929)on the basis of the symptoms given by Nolla, maintained its identity with Macrosporium porri and suggested its name as "Purple Blotch" being more appropriate due to the large size of the lesions on the leaves and seed stalks. Neergaard (1953) considering the onion fungus morphologically indistinguishable from that of potato early blight fungu. ~, * A part of thesis submitted to the Panjab University (1958) in partial fulfilment for the de~re of Master of Science in Agriculture (Plant Pathology).
]aurple Blotch Disease of Onion in Punjab--1 337 reduced it to a variety of Alternaria porri (Ell.) Neergaard (Macrosporium porri Ell.); however, early record shows that Cooke and Ellis (1879) first noted Macrosporium porri (earlier name of the fungus) on the leaves of Leak in New Jersey, but it was Thaxter (1889) who gave more authentic description of this fungus. In India, Ajrekar (1920-21) reported an onion disease from Bombay attributing it to Macrosporium sp., but in the following year, he again reported the same disease, now attributing Alternaria sp. as the parasite,. causing lc af-spot and drying up of the upper portion of the leaves and infection development of the bulbs. He further stated that the attack by this fungus was probably favoured by the presence of thrips, and that the fungus was a wound parasite. Symptoms of the disease as observed in Punjab.--The disease manifests itself on leaves and seed stalks first small, sunken, whitish flecks with purplecoloured centres. At this stage, they are circular or irregular in outline, one millimeter in diameter. Later, the spots enlarge rapidly into purplish areas, further coalescing to form large dead patches covering several square centimetres of the leaf or shoot area. Their length may go up to 4-6 inches or even more. The purplish area of the spot is separated from the healthy green tissues by a narrow, light-coloured zone. Within a period of 15-21 days alternating light and dark zones become clearly differentiated over the whole purple surface of the leaf and seed stalk. The most characteristic symptom of this disease, as observed, is the appearance of dark purple colour on the spots, the dark shade being due to the dark brown colour of the fruiting mycelium. The purple olour has often been seen to fade away in the case of very old lesions, the centre of the spots, however, retaining a mild purple colour. Nature of damage.--in case of severe infection under the influence of environmental conditions favouring the disease, the lesions completely girdle the leaf and the seed stalks resulting in the death of the tissues, thereby causing severe reduction in the production of seed or healthy bulbs. Due to the girdling of the leaves and seed stalks, the tissues above suffer from a serious dislocation of the food and resulting into shrivelling of the seeds. The mycelium of the fungus grows on the surface of the spots as well as inside the hallows of the leaves and seed stalks where it forms a weft-like accumulation, sometimes completely pluggin~ them.
338 V.R. PANDOTRA EXIPE~MENtAL lsolahon of the fungus.--for the purpose of isolating the fungus, diseased shoots and leaves affected with "Purple Blotch" of onion were collected from different places in the Ludhiana, Jullundur and Patiala Districts. Over four hnrtdred isolations were made. Out of them about 80~o were those ofialternaria porri and about 10~ were those of Alternaria tenuis, the remaining being other SalarcLphytes. The isolates obtained from the Ludhiana and Julltmdur material were apparently similar, but those obtained from the Patiala material differed somewhat in colour and growth f6rm of mycelium. The Jullundur isolate was, therefore, discarded. The Ludhiana isolate was designated A and the Patiala isolate as B. As the fungus did not sporulate in culture, it was purified by the single hyphal-tip method Pathogenicity tests.--onion leaves and seed stalks, healthy and slightly injured with a needle, were inoculated with the fungus by placing mycelial bits of two isolates A and B. Proper humidity was provided for 48 hours. The control plants received the same treatment excepting inoculation with the fungus. The first symptom of the disease appeared after about three days in the form of whitish Sunken spots with purple centres both on the leaves and seed stalks mechanically injured. Such lesions further developed and girdled the woundett eaves and seed stalks completely within a period of about 10 days and tl~ee ~eeks respectively. No marked difference was observed be, tween the two isolatas in regard to their virulence. The fungus was isolai~ from ~he a~iifici~lly infected plants. It is apparent from Table I, fhat both the isolates of Alternaria porri are patlaogemc, on. onion plants and that the fungus cannot infect the plants in the absence of some mechanical injury, although, according to Nolla (19273. the wounds are not necessary for the attack by this fungus. Artificial inoculations of onion bulbs with this fungus was also attempted. The fungus produced a semi-watery decay of the bulbs within about 4 days at temperature of ~5-28 C. "rhe rotting bee~m complete in a period of 12-21 days and bulbs reduced"to a pulp. AYdfrcial inoeulatioa~ were also attempted on onion without may guce~s, inflorescence
Puq~h' Blotch Dise~,~e Of On~on in Punjab--I 339 T,ABYE" I ~ 5/:owm~z the re.suit ~71 inoculations with i~o[[ttes A aml B of Alternaria ~orri on onion leaves aml seed stalks oj the local variety lsolate Leaves Seed stalks Method of No. of No. of Percent- No. of No. of Percentinoculation planls plants age ptams plants age inoculated infected infection inoculated infected infection A. lnoculum placed on uninjured surface Six 0 0 Six 0 Inoculum placed on injured surface.. Six Six 100 Six Six I00 B. lnoculum placed on uninjured surface Six I) 0 Six 0 0 Inoculum placed on injured surface.. Six Sik ~00 Six- SiX 100 Control lnoculum not placed on uninjured surface.... Six 0 0 Six 0 0 lnoculum not placed on injured surface Six 0 0 Six 0 Wound parasitism of the hmgus.--in nature, according to Ajrekar (1920-21), the injury to onion ;planls probably has beery'seen to be caused by. thrips... Thrips tabaci and Hetiothripe.imlieus. To investigate tile :role of thrips in disease spread, three lot.~ eadb, containing two onion plants, were kcpt in the humidity chambers during [hc month of April. Thrips previously caught from omon fields were released on each plant to be inoculated, in control series thrips were not released. Now all the fo~ series were inoculated simply placing the mycelial bits of the fungus at several places on seed stalks. Thereafter more onion ihrips were let loose in the chambers excepting in the control series. Proper humidity was maintained for ~bur days. After the 4th day of inoculation, infection appeared on the plants where thrips had been put to feed. No infection appeared on the control plants indicating that mechanical injury is essential to infection by the "Purple Blotch " fungus,:as i~occurs in Punjab. The success of inoculation experiments, only in the case of tissues mechanically injured, also substzmtiates the above results.
340 V.R. ~AtqDOTRA Host range of the fungus.--for host range studies the following plants were tested in addition to onion: Allium sativum, Asphodelus fistulosus, Brassica oleracea var. Bulleta capitata, B. oleracea var. votrytis, Daucus carota, Dianthus sp., Beta benghalensis, B. vulgaris, Lactuca satlva, Narcissus sp., Solanum melongena, S. tuberosum, Raphanus sativum. Out of these plants, only Allium sativum and Asphodelus fistulosus took infection and that too when slightly injured. On these plartts, small sunken spots with purple centre appeared, which later enlarged to a maximum length of about one inch. SUMMAR "Purple Blotch" disease, which has recently been observed in Punjab, is caused by Alternaria porri (Ell.) Neergaard. It causes heavy damage to onion crop. Both the leaves and seed stalks are attacked. The symptoms of the disease under Punjab conditions have been described. Pathogenicity to onion leaves, seed stalks and bulbs has been established. It has also been proved that fungus causing "Purple Blotch" of onion is a wound parasite and has got very limited host range. ACKNOWLEDGEMENTS The author wishes to express his indebtedness to Dr. K. S. Bedi, former Profcssor and Plant Pathologist, Government Agricultural College, Ludhiana, under whose guidance this work was done at Ludhiana. He is highly grateful to Dr. Jr. C. F. Hopkins, Director, Commonwealth Mycological Institute, Kew, for his kind help in the identification of the fungus. Pd/FERENCES Ajrekar, S. L. Angell, H.R... Anyangar, C.R... Cooke, M. C. and EOis, J.B.*.. Nolla, J. A.B... Neergaard, P... Thaxter, g.*.. * Originals not seen. Ann. Rept. Bombay Dept. Agri., 1920-2f, pp. 121-22; 1921-22, pp. I02-04. Jour. Age. Res., 1929, 38, 467-87. Bull. Agr. Res. Pusa (Behar), 1928, p. 179. Grevillea, 1879, 8, 11-16. Phytopath., 1927, 17, 115-32. Plant Diseases, Oxford University Press, 1953, p. 391. Conn. State Agr. Expt. Sta. Ann. RepL, 1889, pp. 127-77.