Evaluation of Inbred Lines and Development of Turcicum Leaf Blight Resistant Single Cross Maize Hybrids

Turcicum leaf blight (TLB) is the most important disease of maize growing areas of India. To identify new resistance sources and establish durability of known resistance sources, 30 maize inbred lines were evaluated against TLB under artificial inoculation during kharif 2013 and kharif 2014. Ten inbred lines were found resistant against TLB. Based on per se performance and resistance to TLB nine inbred lines viz., DMIT 105, DMIT 106, DMIT 111, DMIT 113, DMIT 118, DMIT 121, DMIT 123, DMIT 124 and DMIT 125 were used to develop 36 single cross hybrids and were screened to identify resistant hybrids. Thirty two hybrids were found resistant with disease score 1 and 2. Based on percent disease index (PDI) and area under disease progress curve (AUDPC) values, the components of blighting, inbred lines viz., DMIT 105, DMIT 113, DMIT 118 and DMIT 126 and hybrids viz., DMIT 105 × DMIT 125, DMIT 106 × DMIT 111, DMIT 106 × DMIT 121, DMIT 106 × DMIT 125, DMIT 111 × DMIT 113, DMIT 111 × DMIT 121, DMIT 113 × DMIT 118, DMIT 113 × DMIT 121, DMIT 113 × DMIT 125, DMIT 118 × DMIT 121, DMIT 118 × DMIT 124 and DMIT 121 × DMIT 123 were identified as slow blighters. These hybrids should be involved in the crop improvement programmes.

Maize (Zea mays L.) is one of the world's three most important cereal crops along with rice and wheat. India is rich in maize germplasm particularly of tropical and subtropical types with maximum variability and adaptability. Due to moderate low temperature and high humidity during the maize growing period, turcicum leaf blight of maize (syn. Northern leaf blight) caused by Exserohilum turcicum (Pass.) Leonard and Suggs is recurrent problem in most maize growing regions of India. Most of the cultivated genotypes are more or less susceptible to this disease and the loss in yield has been reported to vary from 28 to 91 per cent (Harlapur et al., 2000). Various options are available to control maize leaf blight such as the use of host plant resistance, cultural practices and fungicides. Host plant resistance is the cheapest and most effective way to control leaf blight disease because chemical treatments are expensive and often ineffective.
Slow blighting is a form of resistance, where despite a susceptible host reaction, the rate of disease development is very slow. Slow blighting is expressed in the reduced infection of a plant by a blight fungus, late appearance of blight in the life cycle of the host and retarded development of the fungus. Partial resistance is a form of incomplete resistance, characterized by a reduced rate of epidemic development (Parlevliet, 1979). The phenomenon of slow blighting was observed in Helminthosporium leaf blight of wheat (Patil, 2000) and partial resistance to TLB of maize (Mallikarjuna, 1998). The utilization of resistance source in breeding programme requires detailed information on various components of resistance under field conditions. Hence a field study was undertaken to determine the turcicum leaf blight response of 30 maize inbred lines to development of resistant single cross maize hybrids and to evaluate the resultant hybrids against TLB to identify resistant hybrids and blighting reaction.

MATERIALS AND METHODS
The experiments were conducted at Main Agricultural Research Station, maize Scheme, University of Agricultural Sciences, Dharwad, Karnataka (15° 25' N latitude, 70° 25' East longitude) with an altitude of 678 m above mean sea level. Each test line was sown in 2 rows of 4 m length and rows were spaced at 60 cm in randomized block design with two replications. The test genotypes were inoculated with E. turcicum multiplied on sorghum grain culture by whorl drop method. Inoculation was done twice at 35 and 45 days after sowing (DAS) followed by water spray so as to maintain required humidity for successful infection. Spreader rows of highly susceptible inbred CM 202 were planted at the border and at regular interval as a source of secondary inoculum for disease development. Observations on blight severity was recorded at the time of tasselling, 20 days after tasselling and at maturity using 1-5 scale (Payak and Sharma, 1983). Based on this scale, the genotypes were classified into three groups viz., resistant (disease score ≤2), moderately resistant (disease score 3) and susceptible (disease score >4). Further, PDI (Wheeler, 1969) and AUDPC (Wilcoxson et al., 1975) were calculated by using the following formula.
Where, K= Number of successive observations S i = Severity of disease at i th period S i-1 = Severity of disease proceeding to i th period T i -T 1 = time intervals between two observations

Screening of inbred lines
The  Table 1.

RESULT AND DISCUSSION
The inbred lines and hybrids of maize in present investigation were evaluated under artificial epiphytotic conditions during kharif 2014 and kharif 2015 to identify resistance source and to develop resistant maize hybrids against turcicum leaf blight (Helminthosporium turcicum).

Evaluation of hybrids
The disease score at maturity ranged from 1 to 3 in hybrids and resistant check compared to susceptible check (score 5). Out of 36 hybrids, fourteen hybrids possessed a disease score of 1 which were found to be highly resistant. Eighteen hybrids scored disease score of 2 indicating that they were resistant to the disease while, remaining four hybrids recorded disease score 3, which were found to be moderately resistant to the disease (Table 3). Similar results were reported by Kumar and Salgotra (2015).

Per cent disease index (PDI)
Significant differences in disease severity were observed among the inbred lines and hybrids between days to tasselling and at maturity. The difference of genotypes in disease severity was due to diversity in their genetic makeup as reported by Williams and Hallauer (2000) and Kraja et al. (2000). Per cent disease index (PDI) and area under disease progress curve (AUDPC), the components of blighting, were calculated to identify slow blighting genotypes.
Per cent disease index shows the cumulative value of disease in the target genotypes. Its value is based on disease severity.  (Table  4 and 5). In these genotypes, blight development was more or less the same during different periods of observation. Result indicates the resistance is oligogenic or polygenic with partial and race-nonspecific resistance. Hossain (1987) reported that maize genotypes Thaltzapam-8146 was found partially resistant to TLB. Sharma and Payak (1990) recorded durable resistance in two maize inbred lines CM-104 and CM-105 against Exserohilum turcicum and observed that these inbred lines were potential in transmitting resistance in hybrid combinations through additive gene action. Durability of their resistance was associated with polygenic control.

Area Under Disease Progress Curve (AUDPC)
The AUDPC estimates the area under the actual infection curve. It is expressed as accumulation of daily percent infection values and interpreted directly without transformation. The higher the AUDPC, the more susceptible is the clone or variety. The AUDPC is calculated from all the three ratings at different time thus leading to a more accurate phenotypic evaluation. The loss of active leaf area results in less photosynthetic available region during the grain filling stage which eventually results in producing smaller kernels. This reduction may eventually contribute to the overall yield losses.
Area under disease progress curve was calculated for inbred lines, 36 hybrids and resistant and susceptible checks and it is presented in  (Mallikarjuna, 1998). In general, AUDPC values took care of initial and terminal severity and also rate of infection. Hence, genotypes with lower AUDPC values can  DMIT 124 and DMIT 121 × DMIT 123 possessed slow blighting characters. Thus, the slow blight resistant character is very important and can be used in selection process for developing the hybrid or used as such for cultivation. If slow blighting genotypes are widely used in a disease control strategy, the rate of leaf blight development will not only be reduced during the rainy season, but also during the subsequent rabi / summer seasons when the resistance of slow blighting genotype in adult plant stage of growth is operating. Therefore, deployment of the identified slow turcicum leaf blighting genotypes could be an important TLB management strategy in maize.