Effect of helminthosporium leaf blight on performance of timely and late-seeded wheat under optimal and stressed levels of soil fertility and moisture

Abstract

Helminthosporium leaf blight (HLB) is the most serious disease constraint to wheat ( Triticum aestivum L.) yields in the warm plains of South Asia. It occurs either singly or as a complex of spot blotch (caused by Cochliobolus sativus) and tan spot (caused by Pyrenophora tritici-repentis). Several abiotic stresses in the region affect the development and severity of HLB. A field study was conducted using three wheat cultivars (Sonalika=RR21, Bhrikuti and BL1473) during the 2001 and 2002 wheat seasons at two sites (farmer’s field and research station) in Nepal, where HLB is a serious problem every year. The objective was to determine the effect of low soil moisture and poor soil fertility in combination with foliar blight severity under natural inoculum pressure on the agronomic performance of timely and late-seeded wheat. The experiment was conducted in a multiple-strip layout in a randomized block with three replicates. Soil fertility stress was evaluated by using the recommended doses of N, P and K as opposed to no chemical fertilizer. Three irrigations and no irrigation were used to assess the effect of soil moisture stress. The experiment included normal and late seeding dates to take into account the average air temperature which increases as the wheat season progresses. Multiple HLB scores were recorded to calculate the area under the disease progress curve (AUDPC) and AUDPC per day was determined. Soil fertility stress increased AUDPC by 28.4%. Overall, soil fertility stress had a more pronounced effect on disease levels than reduced moisture stress, probably because rain had diminished the difference between moisture stress. HLB induced reductions of 20.5 and 15.5% in grain yield and thousand-kernel weight (TKW), respectively, under late seeding, compared to 13 and 7.8% under timely sown conditions. Under low soil fertility, reductions in grain yield and TKW due to HLB were 52.1 and 15.5%, respectively, compared to 26.1 and 8.0% under optimum soil fertility. HLB caused yield reduction of 22.1% under low soil moisture compared to 11.8% under optimum soil moisture. Compared to timely sown wheat, AUDPC per day and reductions in grain yield due to HLB were always higher under late seeding suggesting that AUDPC per day explained yield losses better than AUDPC. These findings demonstrate the joint effects of crop management factors on HLB severity and associated grain yield losses both under farmer’s field and research station conditions. The results imply that optimum crop husbandry must be adopted to minimize wheat yield losses due to HLB.