Foliar application of potassium silicate reduces the intensity of soybean rust F. A. Rodrigues A,C , H. S. S. Duarte A , G. P. Domiciano A , C. A. Souza A , G. H. Korndörfer B and L. Zambolim A A Vi¸ cosa Federal University, Department of Plant Pathology, Vi¸ cosa, Minas Gerais State, 36570-000, Brazil. B Uberlândia Federal University, Agronomy Institute, PO Box 593, Uberlândia, Minas Gerais State, 38400-902, Brazil. C Corresponding author. Email: fabricio@ufv.br Abstract. This study aimed to determine if potassium silicate (KSi) sprays could reduce the intensity of soybean rust. In the field experiment 1, soybean plants were sprayed with KSi (pH 10.5) at rates of 8, 20, 40 and 60 g/L. In the field experiment 2, with the same treatments, the pH of the KSi solutions was 5.5. In experiment 3, the treatments were KSi (40 g/L, pH 10.5), potassium hydroxide (KOH) (6.5 g/L, pH 10.5), epoxiconazole + pyraclostrobin, and control. In experiment 4, the treatments were the same as in experiment 3, but the pH of the KSi and KOH solutions was adjusted to 5.5. Plants sprayed with water served as a control treatment for all field experiments. Plants were artificially inoculated with Phakopsora pachyrhizi before the application of products. Even though there was no relationship between silicon (Si) concentration in leaf tissue and KSi rates, Si concentration increased by 67% and 73% respectively above the control, when averaged across all KSi rates for experiments 1 and 2. The relationship between the severity of soybean rust and KSi rates at pH 10.5 and 5.5 was, respectively, linear and quadratic. Soybean rust severity at the highest KSi rate (pH 5.5) was 70% less than the control. In experiments 3 and 4, the highest disease severity was observed on the control treatment which differed from other treatments. The application of KSi at pH of 10.5 (Exp. 3) and 5.5 (Exp. 4) decreased soybean rust severity by 36% and 43%, respectively, over the control. No significant difference in disease severity was found between KSi and KOH treatments, but they were significantly different, regardless of pH used, from the epoxiconazole + pyraclostrobin treatment. In greenhouse experiments, Si concentration in leaf tissue was higher with the application of KSi, regardless of the pH of the solutions, as compared with water spray. Soybean rust severity and the number of pustules were higher on leaves of plants sprayed with water, as well as on those sprayed with phosphoric acid + NaOH, and phosphoric acid + KOH, when compared with the application of KSi. There was no difference between the KSi and KOH treatments, regardless of the pH, and epoxiconazole + pyraclostrobin sprays for disease severity and number of pustules. This is the first study where field and greenhouse experiments have demonstrated a reduction of soybean rust intensity with foliar application of KSi. This information may be valuable in areas where soybean is grown as a monoculture, and where high yielding but susceptible cultivars cannot be grown because of the occurrence of frequent severe epidemics. Additional keywords: Glycine max, Phakopsora pachyrhizi, Potassium salts, Silicon. Introduction Soybean rust, caused by the fungus Phakopsora pachyrhizi Syd., is the most devastating disease of soybean (Glycine max (L.) Merrill) in Brazil. In the 2001 growing season, when disease control measures had not yet been adopted, yield losses were estimated at 10%, an increase from the 5% yield loss reported for the previous growing season (Yorinori et al. 2005). On leaves, pustules first appear as small chlorotic and irregularly shaped spots, turning tan to brown or reddish (Sinclair and Hartman 1999). Lesions are found on petioles, pods and stems, but are most abundant on leaves. The fungus is easily spread, and disease causes premature defoliation, fewer seeds per pod, decreased number of filled pods per plant and early maturity. All the above contribute to severe yield losses (Yang et al. 1991; Sinclair and Hartman 1999). Conditions that promote full soybean canopy development are most suitable for disease development (Melching et al. 1989). Control of soybean rust is not easy, and there is no straightforward approach that would seem applicable to every situation in every country where disease becomes prevalent (Bromfield 1980). Currently used soybean cultivars are susceptible to the disease (Yorinori et al. 2005; Costamilan et al. 2008) and disease is managed mainly with fungicide sprays (Miles et al. 2003; Reis et al. 2007). Alternatives to the extensive use of fungicides to manage soybean rust are needed. Silicon (Si) may provide one alternative as its beneficial effects, whether direct or indirect, to plants under biotic and or abiotic stresses are reported to occur in a wide variety of crops such as barley, cucumber, oat, rice, rye, sugarcane, and wheat (Datnoff et al. 2007). Until recently, studies designed to investigate the CSIRO PUBLISHING Australasian Plant Pathology, 2009, 38, 366–372 www.publish.csiro.au/journals/app Ó Australasian Plant Pathology Society 2009 10.1071/AP09010 0815-3191/09/040366