Barley spot blotch intensity, damage, and control response to foliar fungicide application in southern Brazil

Barley spot blotch (SB) is a frequent foliar disease of the brewing barley crop in southern Brazil. At present, there are few reports regarding the efficacy of fungicide spray programs in relation to the severity of SB damage to barley crop yield. The goals of this study were as follows: 1) to determine the crop growth stage at which severity of SB was most strongly related to grain yield (GY), thousand kernel weight (TKW), and granulometry (G) damage in brewing barley and 2) to evaluate the efficacy of fungicides in controlling SB and the effect of fungicide application timing on disease intensity and yield. The experiments were performed using the BRS Cauê barley cultivar at Muitos Capões municipality, Rio Grande do Sul State, southern Brazil, during the 2009 and 2010 growing seasons. The experiments involved a randomized complete block design with five treatments and four replicates. The treatments varied by the number of fungicide applications. The control received no fungicide application. SB severity assessments were performed at the following growth stages: main stem and one tiller developed, main stem and four tillers developed, swelling 2nd node detectable, flag leaf ligule just visible, full boot, and 50% of spikes visible. SB severity data were integrated as a function of time to determine the area under the disease progress curve (AUDPC). SB resulted in damage of up to 49.2% and 34.6% in GY, 19.9% and 19.7% in TKW, and 26.0% and 11.2% in G in the 2009 and 2010 growing seasons, respectively. A frequency of three and four foliar fungicide applications controlled 70.1% and 78.3% of SB in the 2009 growing season and 85.8% and 88.6% in the 2010 growing season, respectively. GY and SB severity showed a significant negative relationship in both the 2009 and 2010 growing seasons and at all growth stages evaluated. The greatest GY damage related to SB severity occurred at the final growth stages at full boot and 50% of spikes visible. There was a significant negative relationship between AUDPC and GY, with damage coefficients of 17.19 and 22.94 in the 2009 and 2010 growing seasons, respectively. There was a positive net return on barley GY, TKW, and G when four fungicide applications were used for SB control.