Keywords: Cochliobolus sativus, Pyrenophora tritici-repentis, host resistance, rice-wheat system

Helminthosporium leaf blight (HLB), a complex caused by Cochliobolus sativus and Pyrenophora tritici-repentis, is a major wheat disease in South Asia. There is lack of epidemiological study in rice wheat system documenting the incidence of both pathogens and the factors associated with their development. The experiment was conducted to observe symptom initiation and foliar blight progress in contrasting wheat genotypes, monitor airborne conidia of Cochliobolus sativus and P. tritici-repentis through out the wheat growing season, and compare disease epidemics and levels of foliar blight damage on grain yield and thousand-kernel weight of wheat grown under optimum, intermediate, and late seeding conditions. Replicated field experiments were conducted at the Institute of Agriculture and Animal Science, Rampur, Nepal in a group balanced strip split plot design during 2004 and 2005 wheat growing seasons. Four genetically diverse genotypes differing in maturity and level of resistance to HLB were sown on three dates; November 26(optimum), December 11 and December 26 (late). HLB pathogens were isolated from infected leaves at weekly interval starting from 1 week after germination till maturity. Pathogen infection by Cochliobolus sativus started on lower leaves near ground level within a week after emergence in both years. Cochliobolus sativus infection increased slowly throughout January and became prominent after third week of February. However, infection by P. tritici-repentis started after second to third week of February and became more prominent by first to second week of March. Aerial incidence was also monitored at weekly interval during wheat growing season. Aerial incidence of conidia was increased after second week of February. The number of air borne conidia and infected leaves by the two pathogens remained lower for several weeks under lower temperatures (below 16⁰C), followed by a sharp rise as temperature increased (above 20⁰C). The number of airborne conidia and incidence of infection on leaf samples both showed that Cochliobolus sativus were higher compared with P. tritici-repentis. The maximum infections by both the pathogens were found during third and fourth week of March.