Online publication date: 15 October 2010

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1 This article was downloaded by: [Rodrigues, Fabrício Ávila] On: 22 October 2010 Access details: Access Details: [subscription number ] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Journal of Plant Nutrition Publication details, including instructions for authors and subscription information: FOLIAR SPRAY OF POTASSIUM SILICATE ON THE CONTROL OF ANGULAR LEAF SPOT ON BEANS F. Á. Rodrigues a ; H. S. S. Duarte a ; D. C. Rezende a ; J. A. Wordell Filho b ; G. H. Korndörfer c ; L. Zambolim a a Department of Plant Pathology, Viçosa Federal University, Viçosa, Brazil b EPAGRI, Research Center for Familiar Agriculture, Chapecó, Brazil c Soil Science Department, Uberlândia Federal University, Uberlândia, Brazil Online publication date: 15 October 2010 To cite this Article Rodrigues, F. Á., Duarte, H. S. S., Rezende, D. C., Filho, J. A. Wordell, Korndörfer, G. H. and Zambolim, L.(2010) 'FOLIAR SPRAY OF POTASSIUM SILICATE ON THE CONTROL OF ANGULAR LEAF SPOT ON BEANS', Journal of Plant Nutrition, 33: 14, To link to this Article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Journal of Plant Nutrition, 33: , 2010 Copyright C Taylor & Francis Group, LLC ISSN: print / online DOI: / FOLIAR SPRAY OF POTASSIUM SILICATE ON THE CONTROL OF ANGULAR LEAF SPOT ON BEANS F. Á. Rodrigues, 1 H. S. S. Duarte, 1 D. C. Rezende, 1 J. A. Wordell Filho, 2 G. H. Korndörfer, 3 and L. Zambolim 1 1 Department of Plant Pathology, Viçosa Federal University, Viçosa, Brazil 2 EPAGRI, Research Center for Familiar Agriculture, Chapecó, Brazil 3 Soil Science Department, Uberlândia Federal University, Uberlândia, Brazil This study aimed to determine if potassium silicate (KSi) sprays could reduce the intensity of angular leaf spot. In field experiment 1, bean plants were sprayed with KSi (ph 10.5) at rates of 8, 20, 40, and 60 g L 1. In 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 1, ph 5.5), potassium hydroxide (KOH) (6.5 g L 1, ph 5.5), tebuconazole (0.5 L ha 1 ), and control. In experiment 4, the treatments were the same as in experiment 3, but the ph of the KSi and KOH solutions was Plants sprayed with water served as a control treatment for all field experiments. Plants were artificially inoculated with Pseudocercopora griseola before products application. For experiments 3 and 4, the treatment with KOH was included to equalize the amount of potassium (K) with the treatment corresponding to the application of KSi. Disease severity was evaluated using a scale with values ranging from 0.2 to 30.4% at 85 days after seedling emergence. Plant defoliation, Si, and K concentration in the plant tissues, and yield were also determined. There was no relationship between KSi rates and Si concentration in leaf tissues, but Si concentration increased by 58 and 57%, respectively, as the KSi rates increased from 0 to 60 g L 1 regardless of the ph. The K concentration in leaf tissues did not change among the treatments. Disease severity decreased by 42 and 30%, respectively, at the highest KSi rate with ph 5.5 and ph 10.5 over the control. Disease severity levels were similar between the KSi and KOH treatments, but they were significantly higher compared to the tebuconazole. Plant defoliation at the highest KSi rate with ph 5.5 and 10.5 was 17 and 33%, respectively, less than the control. Plant defoliation decreased with the application of KSi with ph 5.5 and 10.5 in 29 and 34%, respectively, compared to the control. Yield increased by 30 and 43%, respectively, as the KSi rates increased from 0 to 60 g L 1 with ph 5.5 and No statistical difference in yield was detected between the KSi and KOH treatments regardless of the ph used, but both were different from tebuconazole. Significant differences in yield were found only between tebuconazole and the control. Results from this study suggest that foliar application of KSi and KOH has the potential to reduce angular leaf spot severity. The KSi did not offer any Received 27 December 2008; accepted 9 November Address correspondence to F.Á. Rodrigues, Viçosa Federal University, Department of Plant Pathology, Laboratory of Host-Parasite Interaction, Viçosa, Minas Gerais State, Brazil. fabricio@ufv.br 2082

3 Potassium Silicate and Angular Leaf Spot on Beans 2083 advantage over the KOH spray, suggesting a lack of a direct effect of Si accumulated in the leaf tissue on disease control. Keywords: silicon, disease control, Phaseolus vulgaris L., foliar disease INTRODUCTION Angular leaf spot, caused by the fungus Pseudocercospora griseola (Sacc.) Ferraris, is one of the most severe disease of common bean (Phaseolus vulgaris L.) worldwide. In Brazil, the world s largest producer and consumer of common bean, this disease is economically important particularly in the state of Minas Gerais (Rodrigues et al., 1999). The pathogen attacks the aerial parts of the plant especially pods, seeds, leaf petioles, and lower surfaces of leaflets causing premature leaf drop, foliar and stem necrosis that culminate in poorly filled seeds and reduced seed quality (Liebenberg and Pretorius, 1997). Yield losses of susceptible cultivars can be as high as 80% in bean fields located in areas with environmental conditions favorable for disease development and with a high pressure of pathogen inoculum as a result of monoculture (Schwartz et al., 1981). According to Sartorato and Rava (1992), there is a reduction of 7.9% in yield for every 10% increase in angular leaf spot severity. In Brazil, it has been reported that in the absence of adequate methods to control angular leaf spot such as crop rotation and use of fungicides, reductions in yield can be up to 70% (Mora-Brenes et al., 1983). For the small farmers, the use of bean cultivars resistant to angular leaf spot is the most economical method to avoid yield losses. However, the presence of several races of the pathogen (Sartorato, 2000) complicates the issue for the farmers. Even though bean cultivars resistant to some pathogenic races are available to the growers, genotypes resistant to all prevalent races are not available (Schwartz et al., 1982; Pastor-Corrales et al., 1998; Ferreira et al., 2000). Fungicide sprays can reduce the severity of angular leaf spot and increase yield (Rodrigues et al., 1987, 1999), but their use is limited due to environmental problems, expense, and the potential emergence of resistant pathogen populations. Therefore, alternative environment-friendly methods of control need to be investigated. Until recently, studies aiming to investigate the effect of silicon (Si) on the control of foliar diseases in monocotyledons and dicotyledons have involved the amendment of calcium silicate to the soil to supply Si to the roots or the culture of plants in hydroponic solutions containing soluble Si (Datnoff et al., 2007). Foliar application of Si may offer an important and viable alternative for angular leaf spot control or as a supplement to the use of fungicides. According to Barber and Shone (1966), bean plants grown in hydroponic cultures absorbed Si to the extent that the concentration of it in the xylem sap greatly exceeded the external

4 2084 F. Á. Rodrigues et al. solution of 0.07 mm. This indicates that the plants absorbed Si against the concentration gradient. Takahashi et al. (1976) reported that the concentration of Si in tissues of bean plants grown in hydroponic culture was of 1.24%. Voogt and Sonneveld (2001) found that bean plants were able to accumulate 224 mmol Si per kg of dry tissues, which was 1.5 times less compared to what was reported by Horst and Marschner (1978). The beneficial effects of Si, whether direct or indirect, to plants under biotic and or abiotic stresses have been reported to occur in a wide variety of crops such as barley, cucumber, oat, rice, rye, sugarcane, and wheat (Datnoff et al., 2007). In dicotyledons such as cucumber, muskmelon, and zucchini squash, foliar sprays of potassium silicate at and above 17 mm effectively reduced the number of powdery mildew colonies on leaves (Menzies et al., 1992). Bowen et al. (1992) also reported that application of potassium silicate (KSi) to soil at 1.7 mm of Si did not reduce the number of colonies of powdery mildew on grape leaves while foliar sprays of potassium silicate at 17 mm reduced the number of powdery mildew colonies by more than 60%. The authors explained that the reduced severity of powdery mildew by potassium silicate sprays may be partly due to a physical barrier to hyphal penetration and to a resistance response involving the lateral movement of Si and its deposition within the leaf at fungal penetration sites. Since little, if any, research is available on the influence of Si on bean diseases, the objective of this study was to determine the effectiveness of potassium silicate sprays as an alternative method to control angular leaf spot on beans. MATERIALS AND METHODS Experimental Design, Plant Growth, and Application of the Treatments Four experiments, under field conditions, were arranged in a randomized complete block design with three replications. Each experimental plot (6 m 2 ) consisted of four 3 m long rows spaced 0.5 m apart. There was 1.5 m between plots. In order to avoid cross contamination among sprayed plots, only the two central rows of each plot were used for disease, defoliation, and yield assessments and 0.5 m at the end of each row was omitted. Twelve seeds of the cultivar Carioquinha Talismã, susceptible to angular leaf spot, were sown to achieve 10 plants per linear meter of row. Plots were maintained using conventional commercial bean cultural practices. These included top dressing with fertilizer, insecticide sprays, weeding, and overhead irrigation when necessary. In experiment 1 the treatments were potassium silicate (KSi) (FertiSil R, PQ Silicas, São Paulo, Brazil; 26.7% SiO 2 and 13.1% K 2 O) at rates of 8, 20, 40, and 60 g L 1 with a ph of In experiment 2 the treatments were

5 Potassium Silicate and Angular Leaf Spot on Beans 2085 the same as listed above, but the ph of the KSi solutions was adjusted to 5.5 using 5 M phosphoric acid. Plants sprayed with water served as a control treatment in both experiments. The KSi was sprayed only to the two central rows in each treatment plot at 25, 35, and 45 days after inoculation which corresponded, respectively, to V4, V6, and V8 growth stages. In experiment 3 the treatments were KSi (40 g L 1, ph 5.5), potassium hydroxide (KOH) (6.5gL 1, ph 5.5), tebuconazole (0.5 L ha 1 ), and control (plants sprayed with water). In experiment 4 the treatments were KSi (40 g L 1, ph 10.5), KOH(6.5gL 1, ph 10.5), tebuconazole (0.5 L ha 1 ), and control (plants sprayed with water). The ph of the KSi and KOH solutions was adjusted to 5.5 using 5 M phosphoric acid. The ph of the fungicide solution was not changed for both experiments. For experiments 3 and 4 the treatment with KOH was included to equalize the amount of potassium with the treatment corresponding to the application of KSi. The KSi and KOH solutions as well as the tebuconazole were sprayed at 15, 35, and 45 days after inoculation, which corresponded, respectively, to V3, V6, and V8 growth stages. Only plants from the two central rows in each treatment plot were sprayed to run-off (approximately 4 L per plot) using a backpack sprayer (Jacto model HD-20, São Paulo, Brazil). Inoculation Procedure A pathogenic isolate of P. grisea obtained from symptomatic bean plants was used to inoculate the plants. This isolate was preserved on filter paper at 80 C. Pieces of filter paper containing the fungus were transferred to Petri dishes with potato dextrose agar (PDA) medium. After three days, PDA plugs containing fungal mycelia were transferred to new Petri dishes containing V8-juice agar. These Petri dishes were kept in a growth chamber at 25 C with a 12 h photoperiod for 12 days. On the day of inoculation, the sporulating cultures of the fungus were flooded with 5 ml of sterile water and scraped with a rubber policeman. The resulting suspension was then homogenized and adjusted to a concentration of conidia ml 1. Gelatin (1%, wt vol 1 ) was added to the suspension to facilitate conidial adhesion to the leaf surfaces. The two rows of plants adjacent to the two central ones in each plot were artificially inoculated, at nightfall, with a suspension of P. griseola ( 0.5 L of suspension per two rows of plants) using a backpack sprayer (Jacto model HD-20, São Paulo, Brazil) at 20 days after seedling emergence (V4 growth stage). Disease, Plant Defoliation, and Yield Assessments The severity of angular leaf spot was visually assessed on twenty plants in the center of each two row plot using a scale based on the percentage of diseased leaf area with values ranging from 0.2 to 30.4 (Godoy et al., 1997)

6 2086 F. Á. Rodrigues et al. at 85 days after seedling emergency (R5 growth stage). Plant defoliation was also assessed on the same plants at 85 days after seedling emergency using a scale from 0 to 5 where: 1 = no plant defoliation, 2 = 25%, 3 = 50%, 4 = 75% and 5 = more than 75% of plant defoliation. Yield (g/plant) was determined in the two central rows of each plot for a total area of 2 m 2 with approximately 40 plants. Seeds were weighed at 12% moisture content. Leaf Tissue Analysis for Si and Potassium (K) Concentration The Si concentration in leaf tissue was determined by a colorimetric analysis on 0.1 g of dried and alkali digested leaf tissue (Korndörfer et al., 2004). Dried leaf tissue was digested with a nitric-perchloric solution (3:1, v/v), and the concentration of potassium was determined by atomic absorption spectrophotometry. Tissue was obtained by collecting approximately 80 leaves from plants in the two central rows of each replication of each treatment and experiment, omitting 0.5 m at each row end (total area of 2 m 2 ). Leaves were washed with distilled water, dried, and ground to pass through a 40-mesh screen with a Thomas-Wiley mill (Thomas Scientific, Swedesboro, NJ, USA) before analysis. Data Analysis Data from each variable were subjected to analysis of variance (ANOVA), and linear regression procedures (SAS Institute, Inc., Cary, NC, USA). Treatment mean comparisons were made using Fisher s protected least significant difference (LSD, P < 0.05) test. RESULTS There was no relationship between KSi rates and Si concentration in the leaf tissues (P 0.05) in experiments 1 and 2, but Si concentration increased by 58 and 57%, respectively, as the KSi rates increased from 0 to 60 g L 1. The Si concentration values were of 10.2, 12.7, 16.4, 15.8, and 16.1 g kg 1 for experiment 1 and 9.5, 12.3, 13.7, 14.2, and 14.9 g kg 1 for experiment 2, respectively, for the KSi rates of 0, 8, 20, 40, and 60 g L 1. There was no difference among the treatments for Si concentration on leaf tissues in experiments 3 and 4. The values were 9.5, 12.7, 10.2, and 7.1 g kg 1 for experiment 3 and 7.5, 11.6, 10.4, and 8.7 g kg 1 for experiment 4, respectively, for the control, KSi, KOH, and tebuconazole treatments. There was no significant difference for K concentration among treatments in experiments 3 and 4. The values ranged from 15.5 to 16.5 g kg 1 and from 10.3 to 12.8 g kg 1, respectively, for experiments 3 and 4. The relationship between the severity of angular leaf spot and KSi rates was linear (P < 0.05) regardless of the experiment (Figure 1). The severity

7 Potassium Silicate and Angular Leaf Spot on Beans Severity (%) Y = x R 2 = 0.69 (Exp. 1, ph 5.5) Y = x R 2 = 0.61 (Exp. 2, ph 10.5) Potassium silicate rates (g/l) FIGURE 1 Relationship between angular leaf spot severity and potassium silicate rates at two ph values. Error bars represent standard deviations of mean. of angular leaf spot decreased by 42 and 30%, respectively, in experiments 1 and 2 as the KSi rates increased from 0 to 60 g L 1. In experiments 3 and 4, the highest severity of angular leaf spot was observed on the control treatments which differed (P < 0.05) from other treatments (Figure 2). The application of KSi at 40 g L 1 in experiment 3 and experiment 4 decreased FIGURE 2 Severity of angular leaf spot on beans at different treatments. Bars with the same letter, upper-case for experiment 3 and lower-case for experiment 4, do not differ significantly at P > 0.05 as determined by Fisher s protected LSD. Error bars represent standard deviations of mean. KSi = potassium silicate, KOH = potassium hydroxide, and T = tebuconazole. The ph values of the KSi and KOH solutions were, respectively, 5.5 and 10.5 for experiments 3 and 4.

8 2088 F. Á. Rodrigues et al. 100 Y = x x 2 R 2 = 0.88 (Exp. 1, ph 5.5) Y = x x 2 R 2 = 0.89 (Exp. 2, ph 10.5) Defoliation (%) Potassium silicate rates (g/l) FIGURE 3 Relationship between plant defoliation and potassium silicate rates at two ph values. Error bars represent standard deviations of mean. (P < 0.05) the severity of angular leaf spot by 28.8 and 29.5%, respectively, over the control. No significant difference in disease severity was found between the treatments with KSi and KOH (P > 0.05), but these treatments were significantly different (P < 0.05) from the treatment with tebuconazole regardless of the ph used (Figure 2). A second order regression curve best described the effect of KSi rates on the percentage of plant defoliation regardless of the experiment (Figure 3). For experiments 1 and 2, the percentage of plant defoliation at the highest KSi rate was, respectively, 17 and 33% less than on the control, respectively. Percentage of plant defoliation decreased markedly (P < 0.05) with the application of KSi (29 and 34%, respectively, in experiments. 3 and 4) as compared to the control treatment (Figure 4). There was no statistical difference between the treatments KSi and KOH regardless of the ph used, but these treatments were different (P < 0.05) from the tebuconazole treatment. A quadratic model best described the relationship of KSi rates and yield (Figure 5). Yield increased by 30 and 43% at ph values of 5.5 (experiment 1) and 10.5 (experiment 2), respectively, as the KSi rates increased from 0 to 60 g L 1. Yield increased relative to the control by 58, 53, and 116% in experiment 3 (ph 5.5) for KSi, KOH, and tebuconazole treatments and by 47, 39, and 94% in experiment 4 (ph 10.5) for these same treatments (Figure 6). Significant difference in yield (P < 0.05) was found only between the treatment with tebuconazole and the control.

9 Potassium Silicate and Angular Leaf Spot on Beans 2089 FIGURE 4 Percentage of plant defoliation at different treatments. Bars with the same letter, upper-case for experiment 3 and lower-case for experiment 4, do not differ significantly at P > 0.05 as determined by Fisher s protected LSD. Error bars represent standard deviations of mean. KSi = potassium silicate, KOH = potassium hydroxide, and T = tebuconazole. The ph values of the KSi and PH solutions were, respectively, 5.5 and 10.5 for experiments 3 and 4. DISCUSSION This study was designed to determine the efficiency of foliar application of soluble Si, using KSi as its source, to control angular leaf spot considered to be the most serious disease in fields of beans in Brazil. Results provide the first evidence that spraying KSi on the beans canopy can decrease both Yield (g/plant) a Y = x x 2 R 2 = 0.82 (Exp. 1, ph 5.5) Y = x x 2 R 2 = 0.58 (Exp. 2, ph 10.5) Potassium silicate rates (g/l) FIGURE 5 Relationship between yield and potassium silicate rates at two ph values. Error bars represent standard deviations of mean.

10 2090 F. Á. Rodrigues et al. FIGURE 6 Yield of beans plants at different treatments. Bars with the same letter, upper-case for experiment 3 and lower-case for experiment 4, do not differ significantly at P > 0.05 as determined by Fisher s protected LSD. Error bars represent standard deviations of mean. KSi = potassium silicate, KOH = potassium hydroxide, and T = tebuconazole. The ph values of the KSi and KOH solutions were, respectively, 5.5 and 10.5 for experiments 3 and 4. angular leaf spot severity and plant defoliation. This is a valuable option that may be used in an integrated disease management strategy, especially when the cost of fungicide applications needs to be reduced. The major Si sources to plants are slags, KSi, or sodium silicate. They are applied either to soil, soil-less media or in hydroponic solutions to obtain a high content of Si in plants for disease control (Bélanger et al., 1995; Rodrigues et al., 2001; Datnoff et al., 2007). Little information is available regarding the use of foliar applications of Si to control plant diseases, especially on beans. Many important non-hydroponically grown crops, or those with an inefficient ability to uptake monosilicic acid from soil solution and in translocating it to shoots, cannot benefit from the many positive effects that Si brings to plants, such as reducing the intensity of several diseases and alleviating some abiotic types of stress (Datnoff et al., 2007). Therefore, foliar application of soluble Si may be an alternative to control diseases on some crops. Even though bean plants can absorb Si from soil-less media or hydroponic culture (Takahashi et al., 1976; Voogt and Sonneveld, 2001; Horst and Marschner, 1978; Barber and Shone, 1966), they are not as efficient as rice (Rodrigues et al., 2001). The KSi solution has a ph of 10.5, and the original idea behind decreasing ph to 5.5 was to maximize Si uptake by leaves. However, Si content in leaf tissue of plants sprayed with KSi solutions, regardless of ph, was very similar. The level of K between the KSi and KOH treatments was equalized to prevent K from confusing the effect of Si in disease control. Since K concentration in leaf tissue did not change, it can be concluded that the slight increase in the concentration of Si in the leaf tissue of bean plants

11 Potassium Silicate and Angular Leaf Spot on Beans 2091 with the KSi sprays accounted for differences in the level of disease response observed in this study. The spray of KOH to plants was as effective as KSi on reduce disease severity and plant defoliation. Plausible explanations for that is the osmotic effect of the KOH solution that may negatively affects conidia viability or the germtube growth or the formation of potassium phosphate upon the reaction of KOH with the phosphoric acid used to reduce the ph. Kettlewell et al. (2000) found a relationship between conidia germination and leaf area of wheat with symptoms of powdery mildew with the osmotic effect of potassium chloride (KCl) assessed by leaf water potential. Since the main effect of KCl in reducing powdery mildew development appears to be through osmosis, any other salt should have similar effects. Liang et al. (2005) showed that root application of Si was efficient in reducing powdery mildew on cucumber besides enhancing the activity of peroxidase, polyphenoloxidase, and chitinase. The foliar application of Si gave a satisfactory disease control, probably through a physical barrier of Si deposited on leaf surfaces or an osmotic effect of the silicate applied, but was not able to enhance the activity of enzymes involved in host defense response against pathogen attack. Guével et al. (2007) showed that foliar application of Si was less effective in controlling powdery mildew on wheat than root application. The reduction on disease severity by foliar Si application was due to a direct effect of the Si salt on the pathogen rather than one mediated by the plant, as in the case of root amendments that resulted in an optimal prophylactic effect. According to Yoshida et al. (1962), there is a thick layer of silica beneath the cuticle of rice leaves and sheaths. This cuticle-si double layer may be responsible for impeding pathogen penetration and, consequently, decrease the number of lesions on leaf blades as demonstrated in the rice-pyricularia grisea pathosystem (Seebold et al., 2001). However, this physical barrier is only efficient when Si is absorbed by the roots. In the case of KSi sprays, the contact of conidia with the KSi solution may affect conidial viability or the germtube growth and the physical impedance when it dries out may affect pathogen ingress. Indeed, pathogen secondary cycles can decrease and, therefore, the epidemic rate is slowed. Non-homogeneous whitish spots of dried solution of KSi on leaf surfaces of bean plants sprayed with high rates at ph 10.5 were often observed. Bowen et al. (1992) reported that the thick KSi deposits that coated a significant portion of the grape leaf cuticle prevented the penetration by germinating ascospores of Uncinula necator.in the areas of the leaf surface that had not been coated, fungal development was more extensive. Menzies et al. (1992) also showed that foliar applications of KSi to cucumber, muskmelon, and zucchini squash reduced the number Podosphaeria xanthii colonies and decreased powdery mildew severity. The gain in yield can be attributed to a decrease in disease severity as well as to a reduced defoliation caused by the pathogen. According to Jesus et al. (2001), the effect of angular leaf spot on bean yield was mainly the result of defoliation.

12 2092 F. Á. Rodrigues et al. Results from this study indicate that foliar application of KSi and KOH solutions has great potential for reducing angular leaf spot severity and plant defoliation. This is the first study to demonstrate a reduction of angular leaf spot severity with foliar application of KSi. This information may be invaluable in areas where beans is grown as a monoculture, and where high yielding, but susceptible, cultivars cannot be grown because the occurrence of frequent severe epidemics. ACKNOWLEDGMENTS FÁR, GHK, and LZ thank CNPq for the fellowship. DC Rezende was supported by the FUNARBIC Program from Fundação Arthur Bernardes (FUNARBE). This work was supported by grants from FAPEMIG and PQ Silicas Brazil Ltda. We are indebted to Mr. Nivaldo S. Milagres for his technical assistance during the field experiments. REFERENCES Barber, D. A., and M. G. T. Shone The absorption of silica from aqueous solutions by plants. Journal of Experimental Botany 17: Bélanger, R. R., P. A. Bowen, D. L. Ehret, and J. G. Menzies Soluble silicon: Its role in crop and disease management of greenhouse crops. Plant Disease 79: Bowen, P., J. Menzies, J., and D. Ehret Soluble silicon sprays inhibit powdery mildew development on grape leaves. Journal of the American Society for Horticultural Science 117: Datnoff, L. E., F. Á. Rodrigues, and K. W. Seebold Silicon and plant disease. In: Mineral Nutrition and Plant Disease, eds. L. E. Datnoff, W. H. Elmer and D. M. Huber, pp St Paul, MN: The American Phytopathological Society Press. Ferreira, C. F., G. A. Carvalho, S. Nietsche, T. J. Paula Jr, E. G. Barros, and M. A. Moreira Inheritance of angular leaf spot resistance in common bean and identification of a RAPD marker linked to a resistance gene. Crop Science 40: Godoy, C. V., S. M. T. P. G. Carneiro, M. T. Iamauti, M. Dalla Pria, L. Amorim, R. D. Berger, and A. Bergamin Filho Diagrammatic scales for bean diseases: Development and validation. Journal of Plant Diseases and Protection 104: Guével, M. H., J. G. Menzies, and R. R. Bélanger Effect of root and foliar applications of soluble silicon on powdery mildew control and growth of wheat plants. European Journal of Plant Pathology 119: Horst, W. J., and H. Marschner Effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L.). Plant and Soil 50: Jesus, W. C., Jr., F. X. R. Vale, R. R. Coelho, B. Hau, L. Zambolim, L. C. Costa, and A. Bergamin Filho Effects of angular leaf spot and rust on yield loss of Phaseolus vulgaris. Phytopathology 91: Kettlewell, P. S., J. W. Cook, and D. W. Parry Evidence for an osmotic mechanism in the control of powdery mildew disease of wheat by foliar-applied potassium chloride. European Journal of Plant Pathology 106: Korndörfer, G. H., H. S. Pereira, and A. Nola Análise de silício: solo, planta e fertilizante. Boletim Técnico 1, Grupo de Pesquisa em Silício [Silicon analysis: Soil, Plant, and Fertilizers]. Uberlândia, Brazil: ICIAG-Universidade Federal de Uberlândia, Liang, Y. C., W. C. Sun, and J. Si Effects of foliar- and root-applied silicon on the enhancement of induced resistance to powdery mildew in Cucumis sativus. Plant Pathology 54: Liebenberg, M. M., and Z. A. Pretorius A review of angular leaf spot of common bean (Phaseolus vulgaris L.). African Plant Protection 3:

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