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).

Screening of inbred lines
Out of thirty inbred lines, ten lines viz., DMIT 105, DMIT 106, DMIT 111, DMIT 113, DMIT 118, DMIT 121, DMIT 123, DMIT 124, DMIT 125 and DMIT 126 recorded less than or equal to score 2 and grouped them as resistant; twelve inbred lines viz., DMIT 101, DMIT 103, DMIT 104, DMIT 107, DMIT 108, DMIT 109, DMIT 110, DMIT 112, DMIT 119, DMIT 122, DMIT 127 and DMIT 129 possessing score 3.0 were categorized as moderate resistant and eight inbred lines viz., DMIT 102, DMIT 114, DMIT 117, DMIT 120, DMIT 130, DMIT 115, DMIT 116 and DMIT 128 exhibited TLB score 4 were categorized as susceptible and susceptible check (CM202) had score 5 was categorized as highly susceptible to TLB (Table 2). These are in agreement with results obtained by earlier workers (Chandrashekara et al., 2014 and Singh et al. 2014) while working with turcicum leaf blight of maize.
Table (2):
Categorization of maize inbred lines based on the reaction to E. turcicum under artificial epiphytotic condition.

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).
Table (3):
Categorization of maize hybrids based on the reaction to E. turcicum under artificial epiphytotic condition.

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. Four inbred lines viz., DMIT 105, DMIT 113, DMIT 118 and DMIT 126, fifteen hybrids which were developed from resistant inbreds viz., DMIT 106 × DMIT 111, DMIT 106 × DMIT 121, DMIT 106 × DMIT 125, DMIT 111 × DMIT 113, DMIT 111 × DMIT 118, DMIT 111 × DMIT 121, DMIT 111 × DMIT 124, DMIT 113 × DMIT 118, DMIT 113 × DMIT 121, DMIT 113 × DMIT 125, DMIT 118 × DMIT 121, DMIT 118 × DMIT 123, DMIT 118 × DMIT 124, DMIT 118 × DMIT 125 and DMIT 121 × DMIT 123 and resistant check P 3051 showed not only delayed onset of the disease but also ended up with significantly lower 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.
Table (4):
Per cent disease index (%) and area under disease progress curve of turcicum leaf blight in inbred lines of maize.

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 Table 4 and 5. The AUDPC values differed considerably for genotypes. The lowest AUDPC values were noticed in five inbred lines viz., DMIT 105 (424.47), DMIT 113 (427.03), DMIT 118 (366.65), DMIT 121 (493.64) and DMIT 126 (387.62). Among hybrids and checks, twelve hybrids viz., DMIT 105 × DMIT 125 (399.10), DMIT 106 × DMIT 111 (292.74), DMIT 106 × DMIT 121 (185.46), DMIT 106 × DMIT 125 (379.34), DMIT 111 × DMIT 113 (333.75), DMIT 111 × DMIT 121 (285.69), DMIT 113 × DMIT 118 (173.59), DMIT 113 × DMIT 121 (193.19), DMIT 113 × DMIT 125 (143.66), DMIT 118 × DMIT 121 (245.52), DMIT 118 × DMIT 124 (300.10) and DMIT 121 × DMIT 123 (289.25) and resistant check P 3051 (364.35) showed low AUDPC values. The high AUDPC values among hybrids were observed in the hybrid DMIT 113 × DMIT 123 (823.75) and in susceptible check CM 202 (2442.93). This is in accordance with the work of earlier reports (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 be considered as slow blighters. It indicated that such genotypes were more tolerant to pathogen without any economic damage.
Table (5):
Per cent disease index (%) and area under disease progress curve of turcicum leaf blight in maize hybrids.

Based on the results of the present study, it is concluded that the inbred lines viz., DMIT 105, DMIT 113, DMIT 118, DMIT 121 and DMIT 126 categorized as highly resistant and resistant can be well utilized successfully for developing hybrids and composites in future breeding programme.

In the cross combinations, the hybrids with disease score less than 2 for turcicum leaf blight along with desirable specific combining ability for grain yield and its component traits appear to be suitable for cultivation in blight predominant areas. In the present study, it is observed that the inbred lines viz., DMIT 105, DMIT 113, DMIT 118, DMIT 121 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 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.