Field Assessment of Onion Genotypes for Resistance against Purple Blotch Complex ( Alternaria porri and Stemphylium vesicarium ) under Artificial Epiphytotic Conditions in Indian Punjab

Forty-one genotypes of onion were evaluated for their relative response to purple blotch complex caused by Alternaria porri and Stemphylium vesicarium under artificial epiphytotic condition in field during Rabi 2014-2015 and 2015-2016. Significant variation in disease severity among the onion genotypes was found over two years but the pooled analysis revealed that most of the variations were due to genotype x year interaction. None of the genotypes exhibited resistance to the disease complex. Of the test genotypes, eighteen were found moderately susceptible while twenty one were found susceptible to the disease complex. Two genotypes viz., Punjab Naroya and OSR-1364 exhibited highly susceptible reaction. However, six genotypes viz., P-96, P-30, P-121, OSR-1359, P-98 and OSR-1370 showed superior yield performance. Purple blotch complex exhibited highly significant negative correlation of 0.71 and 0.67 with fresh bulb yield and bulb size, respectively. Cluster analysis of onion genotypes was performed based on disease and yield variables and the genotypes were furthered grouped into five clusters based on average values of disease and yield variables.


Introduction
Onion (Allium cepa L.), also known as "queen of kitchen" is one of the oldest known and important high value spice cum bulbous vegetable crop cultivated throughout India.. Onion is highly susceptible to many foliar, bulb and root pathogens, which reduce its yield and quality (Cramer, 2000).Among the diseases, purple leaf blotch (PLB) caused by Alternaria porri (Ellis) Cif. and Stemphylium leaf blight Simmons, are the major diseases of onion world-wide affecting the foliage severely resulting in crop loss ranging from 30 to 100 per cent both in seed and bulb crop from year to year (Awad et al., 1978;Everts and Lacy, 1990a, b;Brar et al., 1990;Aveling et al., 1993Aveling et al., , 1994;;Chaput, 1995;Cramer, 2000) and are more prevalent in warm and humid environment (Suheri and Price 2000a, b).The typical oval to elliptical lesions are colonised by either Alternaria porri, Stemphylium vesicarium or mixtures of both pathogens and

Research Article
thus, the symptoms are generally impossible to be distinguished (Suheri and Price, 2001).The PLB as well as SLB occur synchronously on the same umbel bearing stalk.The weather preferences for both being similar, the loss is additive.Since PLB + SLB are progressing together on the same host, it is considered to be a complex problem (Suheri and Price 2000 a, b).Uddin et al. (2006) also reported that the SLB pathogen (S. vesicarium) is first to initiate infection, which is followed by subsequent infection by the pathogen of PLB (A. porri) and hence, the disease is designated as purple blotch complex (PBC).
Resistant cultivars, if available, are the most effective, practical, eco-friendly and economical choice.However, at present, no such resistant varieties are available in India (Shashikanth et al., 2007).Although management of disease using fungicides is an efficient option, indiscriminate use of fungicides lead to development of resistance in pathogen population besides being costly.In addition, the chemical control is debatable on account of health, ecological and economic reasons.The present investigation was thus, carried out to determine the source of resistance among onion genotypes against PBC under artificial epiphytotic conditions in the field.
Observation on bulb size (diameter in cm) was recorded on 25 randomly selected plants for all replications with the help of digital Vernier caliper and fresh bulb yield per plot was recorded after harvesting during both the years.The plot yield was recorded in kilogram for all the genotypes per replication and fresh bulb yield per hectare was extrapolated in t/ha.

Statistical Analysis
The two years data on disease severity and yield parameters (fresh bulb yield and size) were subjected to combined analysis of variance under randomized block design (RBD) using Statistical software SPSS 22.0 (SPSS Inc., USA) and the significance of differences between the means were compared using Least Significant Difference (LSD) at 5 percent level of significance.Prior to that, Levene's test for homogeneity of variances was performed.In all cases, the P-value was greater than 0.05, indicating that the variation in the two years of the study was not significantly different.The linear correlation between the pooled diseased severity and yield parameters were computed.Further, cluster analysis of the onion genotypes was done on the basis of Euclidean distance using agglomerative method (complete linkage) in statistical software, Statistical Tool for Agricultural Research (STAR version 2.0.1)developed by International Rice Research Institute (IRRI) .

Results and Discussion
The data presented in The perusal of Table 4 revealed that PBC exhibited highly significant negative correlation with fresh bulb yield and bulb size in both the seasons.The pooled analysis over two crop seasons revealed that that PBC exhibited highly significant negative correlation of 0.71 and 0.67 with fresh bulb yield and bulb size, respectively.
Further, the onion genotypes were grouped into five clusters (Fig. 1 and Table 5) based on Euclidean distance using complete agglomerative clustering method.The cluster III recorded the highest mean disease severity of 61.05 per cent followed by cluster V (59.56%) cluster II (52.21%) and cluster V (39.83%) while cluster I recorded the lowest mean disease severity of 35.51 per cent.The highest mean fresh yield (21.53 t/ha) and bulb size (5.28 cm) was recorded in cluster I followed by cluster III (19.18 t/ha and 4.76 cm) and cluster IV (16.70 t/ha and 4.44 cm) while the lowest mean fresh yield (10.27 t/ha) and bulb size (3.60 cm) was recorded in cluster V.The genotypes in cluster III viz., Punjab Naroya and CT-1402 were found to exhibit tolerant reaction to PBC as it recorded superior yield performance despite the higher disease severity.18 onion genotypes to evaluate resistance against PLB and reported none of the genotypes as resistant.The genotype Arka Kalyan was found MR while the genotypes viz., Rampur Rose, Agrifound Rose, Arka Pragati, Arka Niketan, Arka Pitamber and Arka Bindu were found MS.Dhiman et al. (1986) reported that none of the 18 genotypes raised for bulb crop were resistant against PLB.Pathak et al. (1986) found only one genotype IR-56-1 as resistant and five genotypes (viz., IHR-25, IHR-44, IHR-499, IHR-500 and Arka Kalyan) as MR.Sugha et al. (1992) screened 94 onion genotypes against PLB under natural epiphytotic conditions out of which only two genotypes viz., IC39178 and IC49371 were found resistant.Sharma (1997) reported that out of 86 onion genotypes, five genotypes viz., IC48059, IC48179, IC39887, IC48025 and ALR exhibited R reaction to PLB.

Fig. 1 :
Fig. 1: Cluster analysis using agglomerative method (complete linkage) of onion genotypes based on disease severity and yield variables Behera et al. (2013) reported that among 22 onion varieties screened against PLB and SLB of onion, only one variety VG-18 performed best by exhibiting R and MR reaction to PLB and SLB, respectively.Chethana et al. (2011) screened18 onion genotypes to evaluate resistance against PLB and reported none of the genotypes as resistant.The genotype Arka Kalyan was found MR while the genotypes viz., Rampur Rose, Agrifound Rose, Arka Pragati, Arka Niketan, Arka Pitamber and Arka Bindu were found MS.Dhiman et al. (1986) reported that none of the 18 genotypes raised for bulb crop were resistant against PLB.Pathak et al. (1986) found only one genotype IR-56-1 as resistant and five

Table 3 :
Comparative yield performance of onion genotypes screened under artificial epiphytotic conditions in the field against purple blotch complex duringRabi 2014Rabi  -2015Rabi   and 2015Rabi  -2016 S.N.

Table 4 :
Correlation matrix for purple blotch complex and yield parameters of onion **The correlation coefficient is significant at 1 per cent level of significance

Table 5 :
Average values ± standard deviation of disease and yield variables of different clusters of onion genotypes screened against onion purple blotch complex under artificial epiphytotic condition in field * Range of variables within each cluster; Cophenetic correlation coefficient= 0.67