Determination of optimum timing of fungicides application for the control of barley spot blotch

Spot blotch (Bipolaris sorokiniana)is one of the most destructive leaf diseases that causes significant damages to barley during epidemic periods. Fungicide application is a rapid control practice for this disease during epidemic periods. Several fungicides have been proposed for controlling of spot blotch, but these fungicides are effective if they reduce the disease pressure on important plant leaves, especially flag leaf. The results of fungicides application at different growth stages of barley are inconsistent, and also there is no comprehensive study on the timing of fungicides application for controlling of this disease in Iran. Therefore, the present study was designed to determine the appropriate timing of fungicides application at four growth stages of barley to decrease the disease pressure on the upper leaves of the plant, the timing of spraying on reduction of disease damage, and to increase yield and economic profit. A field experiment was conducted in a split plot design with four replications. The main and sub-main factors of the experiment were cultivars (Sahra and Yousef) and the spraying treatments, respectively, and the common fungicides (such as Tilte, Folicur, Falcon, and Rexduo) were sprayed at four timings including, T0 (early stem elongation to node forming 1), T1 (node formation 2-3), T2 (the full flag leaf emergence) and T3 (flowering). The spraying treatments were selected as once, twice, three and four times spraying that were Tr1 (once spraying at early stem elongation to node forming 1), Tr2 (once spraying at node formation 2-3), Tr3 (once spraying at full flag leaf emergence), Tr4 (once spraying at flowering), Tr5 (twice spraying at node formation 2-3 + full flag leaf emergence), Tr6 (twice spraying at full flag leaf emergence+ flowering), Tr7 (twice spraying at early stem elongation to node forming 1+ node formation 2-3), Tr8 (three times spraying at node formation 2-3+ full flag leaf emergence+ flowering), Tr9 (four times spraying at early stem elongation to node forming 1+ node formation 2-3+ full flag leaf emergence+ flowering) and Tr10 (check). The efficiency of spraying treatments was evaluated one week after the last spraying by determining the index of disease (e.g., incidence and severity, area under the disease progress curve, AUDPC) as well as comparing the yield and yield components. The results of this study showed that treatments with more than once spraying from the node formation 2-3 to flowering stages (Tr5, Tr6, Tr8 and Tr9 treatments) could decrease the values of AUDPC-I between 76.8-86.1% and 68.3-81.2% in Sahra and Yousef cultivars, respectively. Whereas the values of AUDPC-S decreased between 62-65.7% and 60.1-62.7% in Sahra and Yousef cultivars, respectively. The highest percentage of yield increase was observed in the treatments with more than once spraying from the node formation 2-3 to flowering stages, which was more than 37% (1092.8 to 1284.1 kg/ ha) and 41% (1167.8 to 1290 Kg/ ha) in Sahra and Yousef cultivars, respectively. In both cultivars, the comparison of economic profit showed that Tr5 treatment (node formation 2-3 to flag leaf emergence stage) not only decreased the disease severity, but also increased the yield. It also reduced costs and had the highest economic profit compared to other spraying treatments (Tr6, Tr8 and Tr9). According to obtained results, the treatment of Tr5 (twice spraying at T1+T2 timing) was the best timing of fungicide application for the reduction of the disease pressure and the increase of yield and economic profit.