Disease reaction studies of maize ( Zea mays L . ) against turcicum leaf blight involving indigenously identified cytosterile source

Among biotic stresses affecting maize, the turcicum leaf blight caused by Exserohilum turcicum is one of the most important diseases in India. Disease reaction studies against turcicum leaf blight were done with two crosses viz., 15C (A) x I-318 (R) and I-401(A) x I-318(R) for all six generations with P1, P2 and F1 having 30 plants each and F2 (300 plants), BC1 (180 plants) and BC2 (180 plants). Analysis of variance of arc sin transformed data for leaf blight in the present study revealed significant variability has been exhibited by fungus to infect different generations of a particular cross. In I-15C(A) x I-318(R) cross, F1 was moderately resistant to turcicum leaf blight but F1 of I-401(A) x I-318(R) cross was moderately susceptible to the disease. Disease screening of both crosses indicated that the latent period was longer, suggesting presence of resistant genes in both the crosses which further can be exploited in the production of successful commercial hybridsby using these CMS sources as parents to develop turcicum leaf blight (TLB) resistant, cost effective and stable hybrids.


INTRODUCTION
Maize (Zea mays L.) holds a unique position in world agriculture as food, feed and source of diverse Industrially important products. Maize is cultivated on nearly 100 million hectares in developing countries and about 70% of the total maize production in developing world is from low and lower middle income countries (Faostat, 2010). In sub-Saharan Africa, it provides food and income to over 300 million households (Tefera et al., 2011).
Turcicum or northern corn leaf blight (NCLB) is a serious foliar wilt disease of maize in many tropical and temperate environments. NCLB is a severe fungal disease causing yield losses worldwide, is most effectively controlled by resistant varieties. Genomic prediction could greatly aid resistance breeding efforts (Frank et al., 2013). It is caused by the ascomycete fungus Setosphaeriaturcica (Luttrell) Leonard and Suggs, with its conidial state Exserohilum turcicum (Passerini) Leonard and Suggs. Symptoms can range from small cigar shaped lesions to complete destruction of the foliage (Welz and Reiger, 2000). Turcicum leaf blight causes extensive defoliation during grain filling period, reduce succulence of leaves and stalk necrosis resulting in grain yield losses (Perkins and Pederson, 1987). The disease was reported as early as 1923 in India and assumed as an epiphytotic form in Kashmir valley (Koul, 1957). The disease is favoured by high humidity with moderate to high temperatures from three leaf stages to grain development of crop (Palaversic et al., 2012).Turcicum leaf blight disease in maize is particularly prevalent during Kharief (rainy) season in the Zones I, II and IV as delineated by the All India Coordinated Reseach Project (Maize), namely Peninsular, North eastern and Northern hill regions. Yield losses due to TLB worldwide can range from 27 to 90% in addition to predisposing plant to stalk rots and reducing forage value (Chenulu and Hora, 1962).
Turcicum leaf blight (TLB) is characterized by long elliptical, greyish green or tan leaf lesions that first appear on the lower leaves and increase in size and number until very little living tissue is left. Yield is reduced due to lack of carbohydrates for grain filling (Paliwal et al., 2000). Eight three-way and four commercial maize hybrids for yield and resistance to maize streak virus using controlled leaf hopper infestation and turcicum leaf blight under artificial inoculation was studied. The hybrid 053WH54 had multiple resistances to turcicum leaf blight and maize streak virus. The hybrids 043WH61 and 043WH07 were high-yielding even at high disease pressure while 043WH41 and 013WH03 were relatively low yielding at low disease pressure. This showed the inherent genetic diversity of the hybrids. The hybrids ZS 225, 043WH61 and 043WH07 are recommended for production in areas with high prevalence of both diseases (Karavina et al., 2014). The use of resistant cultivars is most effective, economical and environmental friendly means to control epidemics of turcicum leaf blight. Thus, in the present study, screening for TLB was carried out in all the six generations of both crosses viz., 15C (A) x I-318 (R) and I-401 (A) x I-318 (R).

MATERIALS AND METHODS
The material for study was developed by attempting the crosses 15C (A) x I-318 (R) and I-401 (A) x I-318 (R) during kharif 2010 to generate F 1 generation at High Altitude Rice Research Sub-station, Larnoo. The F 2 and backcrosses generation (BC 1 and BC 2 ) were developed at Winter Nursery Centre (ICAR) Hyderabad, during rabi 2010-11. All the six basic set of generations P 1 , P 2 , F 1 , F 2 , BC 1 and BC 2 and 15C (B), I-401 (B) and restorer R-line I-318 (R) of the crosses thus obtained were raised and screened for turcicum leaf blight.
Six generations of each cross were evaluated in randomized complete block design with three replications at the Experimental Farm of Division of Plant Breeding and Genetics, Sher-e-Kashmir University of Agricultural Sciences and Technology-Kashmir, Shalimar during kharif 2011. The nonsegregating (P 1 , P 2 and F 1 )  and segregating generations (F 2 , BC 1 and BC 2 ) were raised in four and six rows with inter and intra row spacing of 60 and 25 cm, respectively. Screening for disease was carried out with 30 plants each of P 1 , P 2 and F 1 and 300 plants of F 2 , 180 plants each from BC 1 and BC 2 .The plants were inoculated artificially at 5-6 leaf stage with the conidial suspension of test fungus (E. turcicum) 4-5 x 10 4 conidia ml -1 (Figures 1, 2 and 4) in the evening hours and high humidity was maintained by spraying water 5-6 times in the next 3 days to ensure infection. All the leaves on infected plants were scored using 0-5 scale (Figure 3) adopted by maize pathology unit, CIMMYT as: 0 = no symptoms; 1 = one to few scattered lesions on leaves covering up to 10% leaf area; 2 = lesions on leaves covering 11-25% leaf area; 3 = lesions on leaves covering 26-50% leaf area; 4 = lesions abundant on leaves covering 51-75% leaf area; 5 = lesions abundant on almost all leaves, plants prematurely dried or killed with 76-100% leaf area covered.
The per cent disease incidence and severity were calculated in each observation as per the following formula:    d) e) f) (MS) in cross I-401 (A) x I-318 (R) suggesting inheritance of resistance was governed by recessive genes. A single dominant gene in dent inbred GE 440 and in pop corn variety ladyfinger was detected by Hooker (1963) whereas mutagenic and major gene resistances for leaf blight have been reported by Jenkins and Roberts (1952) and Ullstrup (1970). Cross I-401 (A) x I-318 (R)F 1 was moderately susceptible (MS), suggesting that disease resistance was governed by additive genes in this cross. Thus, presence of resistant genes in both crosses can be exploited in the production of successful commercial hybrids by using these CMS sources as parents to develop TLB resistant, cost effective and stable hybrids. Also, further testing of these crosses through molecular markers can be helpful in identifying resistant gene in commercial hybrids. Additive gene action was of major importance in all studies done on inheritance of corn leaf blight (Hughes and Hooker, 1975;Sigulas et al., 1988;Carson, 1998). Resistant lines had different set of genes controlling resistances operating at different levels of disease intensity across different generations as reported by Jenkins and Roberts (1952). Shankaralingam et al. (1989) also suggested that additive gene action and dominance x dominance type of epistasis with duplicate nature are important in controlling resistance to TLB. Thus, both dominance and additive gene effects seem to be governing resistance to leaf blight. Turcicum leaf blight, a ubiquitous foliar disease of maize for which diverse qualitative and quantitative sources are available need to be incorporated in the susceptible cultivars by back-crossing because of unstable nature of qualitative Ht genes as envisaged by Welz and Reiger (2000). Therefore, pedigree and recurrent selection methods should be used in development of high yielding and resistant cultivars.