Among the food crops, wheat is one of the most abundant sources of energy for the world population. Among wheat rusts, leaf rust is a major threat to wheat production. Present investigations were undertaken to examine the field response for assessing slow rusting to leaf rust (Puccinia recondita) of adult plant under field conditions in spring wheat genotypes. The studies were composed of four parts that are described briefly as below.
Fifteen wheat varieties (slow rusting and fast rusting) were evaluated for response of slow rusting to leaf rust epidemic in replicated trials (normal and late planting) for two years under artificially created leaf rust epidemics with and without fungicide protection. Data were recorded on grain yield, plant height, number of spikes, grains per spike, grain weight, grain fill rate and leaf rust. The yield was compared by area under disease progress curve as predictor of losses in yield. The wheat varieties named Inqilab-91, Iqbal-2000 and Parula were resistant to slow rusting group, Auqab-2000, Chenab-2000, Parwaz-94, Pavon-76, MH-97 and LU-26 proved to be slow rusting while, the other varieties Punjab-76, SA-42, Chenab-70 and WL-711 were found fast rusting varieties. Two varieties Punjab-96 and Pak-81 were classified intermediate (slow) rusting. Average loss in grain yields of the wheat cultivars ranged from 1.81 to 46.31 % that was significantly correlated with the grain weight loss ranged from 0.54 to 58.23 %. Average loss in grain number per spike ranged from 1.06 to 20.39 % and slight reduction was observed in spikes/m2 and plant height under rusted conditions. Negative phenotypic correlations of grain yield, spikes/m2, number of grains per spike, 1000-grain weight and grain fill rate were found with relative area under disease progress curve (RAUDPC), final disease severity and yield loss in leaf rust epidemic conditions. Average annual genetic progress in grain yield potential achieved through breeding averaged over four trials was computed to be 0.86% (R2= 0.682) for protected and 2.23% (R2= 0.787) for rusted conditions.
F1 diallel crosses were developed from eight susceptible vs. resistant wheat varieties in all possible combinations and the diallel analysis was performed to study gene action for yield, leaf rust and other traits. Highly significant additive and dominance affects with directional dominance effects, asymmetrical gene distribution and important role of specific genes were indicated for all the traits except harvest index where directional dominance effects were absent. Maternal and reciprocal effects were non-significant. Plant height, leaf rust incidence, days to heading, days to maturity, days between heading and maturity, spikelets per spike, peduncle length and protein content were controlled by additive with partial dominance. Harvest index was controlled by complete dominance whereas over dominance gene action predominated for other traits. Gene action was simple with absence of non-allelic gene interaction for all traits. The maximum dominant genes possessed by varieties were MH97, Nacozari, Crow, Parula and Inqilab91 and Chenab70 and SA42 displayed maximum recessive genes for leaf rust incidence.
Significant and positive heterosis and hetrobeltiosis were observed for all traits studied except heterobeltiosis in traits like days to maturity and spikelets per spike. Greater than half of 56 F1 crosses showed significant positive heterosis in plant height, days to heading, tillers per plant, grains per spike, spike length, 100 grain weight, grain yield per plant, straw yield per plant and biological yield per plant, while heterobeltiosis was estimated in more than half crosses out of 56 F1 crosses in leaf rust (AUDPC), tillers per plant, grains per spike, spike length, grain yield per plant, straw yield per plant and biological yield per plant. Maximum heterotic effects were manifested as 61.19, 30.67, 29.95, 51.89, 126.64, 123.14, 111.71 and 45.91 tillers per plant, grains per spike, spike length, 100 grain weight, grain yield per plant, straw yield, biological yield and harvest index from crosses; MH 97x Crow, Parula x Chenab 70, MH 97x SA 42, Crow x Chenab 70, SA 42 x Nacozari, MH 97 x SA 42, MH 97x SA 42 and Nacozari x Chenab 70, respectively. While, heterosis for leaf rust (AUDPC) ranged from -1.96 to -99.00. Positive association of leaf rust (AUDPC) was determined with plant height and days from heading to maturity but negative correlation were estimated with days to heading, days to maturity, grain yield, straw yield and biological yield. Thus in rusted conditions, leaf rust epidemic associated the yield reduction, reduced fertile tillers, lower grain weight, lower biological yield, lower straw yield, and lower harvest index in early heading and early maturing genotypes.
Genetics of field resistance to leaf rust was studied using three crosses of susceptible vs. resistant wheat varieties derived from a diallel cross. From each cross, developed six generations (P1, P2, F1, F2, BC1 and BC2) were grown in the field conditions and artificially inoculated with isolates of mixture of prevalent races to leaf rust. Leaf rust intensity and reaction type were observed and coefficient of infection was computed from six populations. Generation means analysis showed that additive, dominance and interactions prevailed in the inheritance of leaf rust. However, generation variance analysis indicated the absence of interactions. Narrow sense heritability estimates were low to leaf rust resistance.
For the study of genetic control for slow rust resistance to leaf rust, population of 200 F6 and 40 doubled haploids were developed from each of three crosses involving one fast rusting; SA42 and three slow rusting; Inqilab91, MH97 and Parula wheat varieties. Parents and progenies were planted in two replications in the field under artificial epidemic leaf rust to find out number of genes for slow rusting to leaf rust using method described by Wright (1968) and confirmed the results with frequency distribution. Highest broad sense heritability (99%) was estimated. Two to three numbers of genes were estimated for controlling slow rusting to leaf rust depending on the parents used in the cross. Association of slow rusting with plant height and days to heading was negative and positive correlation with grain filling period was found in the present material studied.
The resultant desirable wheat lines developed would be exploited as resistant germplasm for leaf rust.