Differential effects of spot blotch on photosynthesis and grain yield in two barley cultivars

Barley spot blotch (SB), caused by Cochliobolus sativus, is an important barley disease which causes extensive grain yield losses. These losses may not always correlate directly with the amount of diseased leaf area. Two barley cultivars, Quebracho (susceptible to SB) and Carumbé (with intermediate susceptibility to SB), were compared in field experiments in 2003, 2004 and 2006. Plots of each cultivar were either inoculated with C. sativus or protected with fungicide under field conditions to generate contrasting treatments: i) diseased, and ii) free of disease, respectively. SB severity over the growing season, photosynthetic rate on leaves with no visible symptoms and grain yield were assessed for each treatment and year. There was no treatment effect on cv. Carumbé, while cv. Quebracho showed a significant yield reduction, even though SB severity during the grain filling period was <10 %. This yield reduction was associated with a reduced photosynthetic rate at the beginning of the grain filling period in cv. Quebracho. A similar experiment was conducted under greenhouse conditions, adding a treatment without inoculum or fungicide. There were no differences in photosynthetic rate or grain yield per plant among treatments. These results suggest a distinct physiological response to SB infection among cultivars affecting leaf photosynthetic rate, and SB severity may not be the best estimator of yield losses caused by SB.