Evaluation of Silicon for Managing Powdery Mildew on Gerbera Daisy

Journal of Plant Nutrition, 31: 2131 2144, 2008 Copyright Taylor & Francis Group, LLC ISSN: 0190-4167 print / 1532-4087 online DOI: 10.1080/01904160802459641 Evaluation of Silicon for Managing Powdery Mildew on Gerbera Daisy C. Moyer, 1 N. A. Peres, 1 L. E. Datnoff, 2 E. H. Simonne, 3 and Z. Deng 1 1 Gulf Coast Research and Education Center, University of Florida-IFAS, Wimauma, Florida 2 Plant Pathology Department and Horticulture Department, University of Florida 3 Department of Horticulture, University of Florida-IFAS, Gainesville, Florida ABSTRACT Powdery mildew, caused by Erysiphe cichoracearum or Podosphaera fusca, is a common disease of gerbera daisy (Gerbera jamesonii) grown in Florida. Previous studies demonstrated that silicon reduces powdery mildew in Arabidopsis, cucumber, grape, strawberry, and wheat. In this study, two silicon (Si) sources, calcium silicate and potassium silicate, were evaluated for their ability to reduce powdery mildew in gerbera Snow White. The effect of calcium silicate in flower quality and the silicon uptake was also determined. Calcium silicate was not effective in reducing powdery mildew or in improving flower quality. The silicon content of gerberas treated with potassium silicate was slightly greater than that in plants treated with calcium silicate. However, the severity of powdery mildew was not reduced by potassium silicate. The results suggest that silicon may not be useful for managing this disease of gerbera daisy, possibly due to the low accumulation of silicon by gerbera leaves. Keywords: potassium silicate, calcium silicate, gerbera daisy, powdery mildew INTRODUCTION In Florida, gerbera daisy (Gerbera jamesonii Bolus ex. Hook f.) is produced mainly under greenhouse or shade house conditions as potted and bedding plants (USDA, 2006). Powdery mildew is an important fungal disease in gerbera that may be caused by either Erysiphe cichoracearum DC. or Podosphaera Received 27 November 2007; accepted 3 January 2008. Address correspondence to N. A. Peres, Gulf Coast Research and Education Center, University of Florida-IFAS 14625 CR 672 Wimauma, Florida 33598. E-mail: nperes@ufl.edu 2131

2132 C. Moyer et al. (Syn. Sphaerotheca) fusca (Fr.) S. Blumer (Daughtrey et al., 1995). This disease affects all plant parts and reductions in growth and in quality are the most important components of economic loss. Most nurseries in Florida use repeated applications of fungicides for managing powdery mildew (Larson and Nesheim, 2000). However, alternative methods for disease management are expected by the public due to the increasing concern that fungicides may negatively impact the environment and human health (Gullino et al, 1999). Silicon (Si) has been reported as a beneficial element that may enhance plant growth and development while protecting plants against diseases and abiotic stress (Datnoff et al., 2001; Ma et al., 2001; Marschner, 1995). Savvas et al. (2002) reported that gerbera plants amended with Si in the nutrient solution had significantly thicker flower stems and a higher proportion of flowers graded Class I. Moreover, Richter (2001) revealed that vase life of different gerbera cultivars can be extended and the number of flowers with bent neck can be reduced by supplying plants with Si. Foliar and root applications of Si reduced the number of colonies of powdery mildew developing in cucurbits such as cucumber, muskmelon and zucchini squash (Menzies et al., 1992). Powdery mildew colony number in grape leaves was reduced to 11% of the control leaves when foliar Si sprays were used (Bowen et al., 1992). Powdery mildew development in Arabidopsis thaliana was observed rarely when plants were watered with a nutrient solution containing soluble Si (Ghanmi et al., 2004). Application of Si to soil or hydroponic cultivation resulted in suppression of powdery mildew in the highly susceptible Toyonoka strawberry cultivar (Kanto et al., 2004; 2006). Belanger et al. (2003) found that Si amendments to the soil mix or added to the nutrient solution protected wheat (Triticum aestivum) from powdery mildew. The protective role of Si has been attributed to the accumulation of Si in the leaves, which creates a physical barrier to pathogens (Adatia and Besford, 1986; Samuels et al., 1991a, 1991b). Alternatively, Si may have a more active role by inducing the plant s own defense mechanisms (Fauteux et al., 2006; Remus-Borel et al., 2005; Rodrigues et al., 2004; 2005). For instance, Fawe et al. (1998) demonstrated that the addition of Si to cucumber (Cucumis sativus) plants enhanced resistance to powdery mildew by increasing antifungal activity in the plant. Similarly, Liang et al. (2005) and Rodrigues et al. (2005) found that root-applied Si enhanced the activity of pathogenesis-related (PR) proteins and thus increased resistance to pathogen attack on cucumber and rice (Oryza sativa) plants, respectively. The effect of Si for the control of powdery mildew in gerbera daisy has not been investigated. Since Si was effective in controlling powdery mildew in other crops, we hypothesized that adding Si to the growing media of gerbera plants might decrease disease severity caused by P. fusca. The objective of this study was to evaluate the efficacy of Si for the management of powdery mildew in gerbera daisy grown under greenhouse conditions in Florida.

Silicon for Powdery Mildew of Gerbera Daisy 2133 MATERIALS AND METHODS Experiments were conducted under greenhouse conditions from May 2006 to January 2007 at the University of Florida, Gulf Coast Research and Education Center, Wimauma, Florida. Effect of Calcium Silicate on Powdery Mildew Development in Gerbera Plants Calcium silicate (CaSiO 3 ) slag 20% Si (Calcium Silicates Corporation, Lake Harbor, FL) at rates of 0, 0.9, 1.8, 3.6, 5.53, and 7.3 g/pot was added to the growing medium CMA mix made with peat moss, perlite coarse, A3 coarse and rock 65:20:15:3 (v:v:v:v) (Verlite Company, Tampa, FL). Subsequently, Osmocote Plus (15 9-12) controlled release fertilizer (The Scott s Company, Marysville, OH) at 11g/pot was added and all mixed with a concrete mixer (Gilson mixer 59015C, CF Gilco, Inc. Grafton, WI.). Gerbera cultivar Snow White highly susceptible to powdery mildew was used for this study. Tissue culture liners ( 3-month old; Twyford International, Inc., Apopka, FL) of this cultivar were transplanted into 15-cm diameter plastic pots on 8 May 2006. Pots were placed on greenhouse benches and watered by drip irrigation. Symptoms of powdery mildew caused by P. fusca developing on plants from natural inoculum were first seen two weeks after transplant. Disease evaluations were made at 7-day intervals beginning on 24 May and ending on 19 July 2006. The disease severity wasratedusinga0to5scale,where0= no powdery mildew present, 1 = 1to 20%, 2 = 21 to 40%, 3 = 41 to 60%, 4 = 61 to 80%, and 5 = 81 to 100% of upper leaf surface covered with powdery mildew. All leaves for each plant were rated and an average of disease calculated. This experiment was arranged in a randomized complete block design with six treatments and four replications. Disease ratings were used to calculate the area under disease progress curve (AUDPC) for each treatment by the midpoint rule method (Campbell and Madden, 1990) as follows: AUDPC = n 1 i=1 [(y i + y i+1 )/2] x (t i+1 t i ) where n is number of disease assessment times, y is the disease severity and t is the time duration of the epidemic. The AUDPC values obtained were analyzed by linear regression using SAS for Windows version 9.0 (SAS institute, Cary, NC). Effects of Calcium Silicate on Horticultural Traits of Gerbera Flowers Gerbera plants started blooming four weeks after transplant. Flowers were harvested once they were fully developed, i.e., when the second circle of disks in the flower showed pollen development (Rogers and Tjia, 1990). Flowers were counted and flower diameter, stem diameter, and flower height were recorded

2134 C. Moyer et al. for all flowers harvested. Data were analyzed by linear regression using SAS for Windows version 9.0. Evaluation on Uptake of Calcium Silicate and Potassium Silicate over Time This experiment was designed to determine a timeline for Si uptake into gerbera leaves, using two silicon sources: calcium silicate and potassium silicate. On 16 August 2006, gerbera seedlings Snow White Sunburst series (Twyford International, Apopka, Florida) were transplanted into 15-cm diameter plastic pots filled with the growing medium as previously described. Silicon was applied as calcium silicate (CaSiO 3 ) slag 20% Si (Calcium Silicates Corporation Lake Harbor, FL) at 5.4 g/pot or as potassium silicate (K 2 SiO 3 ) 12.4% Si supplied as Kasil R 6 (PQ Corporation, Valley Forge, PA) at 0.27 ml/l (1.22 mm SiO 2 ). Calcium silicate was incorporated into the growing medium before transplant and potassium silicate was applied three times a week as a drench (100 to 300 ml depending on plant age). Control plants received no additional Si. All plants were watered by drip irrigation. This experiment was conducted as a randomized complete block design with 15 treatments, three plants per treatment and three replications. Gerbera plants were evaluated for Si content on 2, 5, 9, 16, and 23 days after transplanting (DAT). On each sample date, four to five new leaves were collected from nine plants of each treatment to determine Si uptake. This experiment was repeated in January 2007. Data were subjected to analysis of covariance using SAS for Windows version 9.0. Disease evaluations were recorded at 9, 16, and 23 DAT. The disease severity was rated using a 0 to 5 scale, as described above. Disease ratings were used to calculate AUDPC for each treatment as described above. The AUDPC values were subjected to analysis of variance (ANOVA) and means separated by the Waller Duncan k ratio t test (P 0.05). Silicon Extraction from Gerbera Leaves Four to five new leaves from gerbera plants were oven dried at 70 Cforthree days. Dried tissue was ground finely with a sample mill (Cyclotec 1093, Foss Analytical, Denmark) to pass through a 0.425 mm mesh. Silicon content was determined by a modification of the autoclave-induced digestion procedure of Elliot and Snyder (1991). Briefly, 100 mg of dried, ground leaf tissue were placed in 100-mL polyethylene tubes with 2 ml of 50% hydrogen peroxide (H 2 O 2 ) and 3 ml of 50% sodium hydroxide (NaOH). Each tube was shaken gently and covered with loose fitting plastic caps. The tubes were then placed into an autoclave at 103 kpa (15 psi) for 30 min. If tissue was not completely digested, 2 ml of hydrogen peroxide were added to each tube, vortexed and

Silicon for Powdery Mildew of Gerbera Daisy 2135 autoclaved again. Otherwise, tubes were removed and the volume was increased to 50 ml with distilled water. Si was determined colorimetrically as follows: a 1-mL aliquot was taken from the digested plant tissue and mixed in 10 ml of distilled water. Then, 0.25 ml hydrochloric acid, 0.5 ml of ammonium molybdate solution (100 g/l, ph 7.0), 0.5 ml tartaric acid (200 g/l), and 0.7 ml of a reducing solution were added. The reducing solution was prepared by dissolving 4 g sodium sulfite (Na 2 SO 3 ), 0.8 g 1-amino-2-naphthol-4-sulphonic acid, and 50 g sodium bisulfite (NaHSO 3 ) in 500 ml water. Five minutes elapsed between the addition of the ammonium molybdate and the tartaric acid. A series of standard Si (Si reference solution, Fisher Scientific) were developed (Si range from 0 to 2 ppm) and used to generate a regression equation for determining final Si content in leaves (mg/kg). After 10 min, the absorbance was measured at 650 nm with a spectrophotometer (PC 910, Brinkmann Instruments, Inc.). RESULTS Effect of Silicon on Powdery Mildew Development in Gerbera Plants Powdery mildew developed from natural inoculum. The first symptoms were observed 12 DAT and at 26 DAT, the average disease severity was about 50% in all treatments. Disease severity increased until the end of the experiment when powdery mildew covered most of the leaves. Disease severity was not related to the amount of Si applied at any of the evaluation dates (Table 1). At the end of this experiment, leaves were collected to determine the Si concentration which ranged from 0.037 to 0.079%. Linear regression analysis showed no relationship between the Si content of the leaves and the different levels of Si applied to the potting mix (P > 0.38). Effects of Silicon on Horticultural Traits of Gerbera Flowers Gerbera flowers were evaluated for the effect of Si on flower number, height, flower diameter and stem diameter. There was an average of 3 flowers per plant with an average height of 21 cm, flower diameter of 7 cm and stem diameter of 0.5 cm. No significant relationship was found between the amount of Si applied and the average number of flowers, flower height, flower diameter and stem diameter (Table 2). Evaluation of Silicon Accumulation over Time This experiment was conducted twice and since the interaction between treatment and experiment was significant (P 0.05), results of each experiment are

2136 C. Moyer et al. Table 1 Powdery mildew severity and silicon concentration (%) in gerbera daisy supplied with six calcium silicate (CaSiO3) rates by addition to the soil medium Si Disease severity z Days after transplant Si in g/pot 12 19 26 33 40 47 54 61 68 AUDPC y leaves (%) 0 0.78 1.73 2.22 2.92 3.25 3.92 3.94 4.16 3.94 131 0.049 0.9 0.98 2.22 2.44 2.81 3.26 3.83 3.81 3.94 3.94 119 0.046 1.8 0.73 1.82 2.09 2.52 3.05 3.68 3.78 3.91 3.94 110 0.079 3.6 1.11 2.33 2.60 2.98 3.11 3.78 3.88 3.69 3.82 122 0.041 5.5 0.80 1.92 2.28 2.75 3.05 3.88 4.00 3.94 4.00 116 0.048 7.3 0.92 1.88 2.28 2.82 3.34 3.87 3.88 3.94 3.91 117 0.037 Pr > F x 0.62 0.88 0.79 0.94 0.90 0.92 0.63 0.47 0.89 0.43 0.38 z Disease severity rated on a 0 to 5 scale, where 0 = no powdery mildew symptoms and 5 = 81 to 100% of upper leaf surface covered with powdery mildew symptoms. y AUDPC = area under the disease progress curve. x Pr > F: significance level of F value by linear regression analysis. presented separately. For the first experiment, no significant difference among the treatments was observed, and there was no significant interaction between treatment and DAT. However, the effect of DAT on Si content was significant and Si content decreased over time according to the regression equation: Si (%) = 0.055 0.00104 DAT. For the second experiment, Si treatment (potassium silicate) had a significant effect and the interaction of potassium silicate and DAT was significant (Table 3). The accumulation of potassium silicate in gerbera leaves showed a slight increase over time which was described by Table 2 Horticultural traits of gerbera flowers supplied with silicon using calcium silicate (CaSiO3) by addition to the soil medium Flower number Flower height Flower diameter Stem diameter Si (g/pot) (no/plant) (cm) (cm) (cm) 0 3.27 19.81 7.13 0.54 0.9 3.21 19.89 7.08 0.53 1.8 3.63 21.46 7.27 0.54 3.6 3.33 22.53 7.81 0.56 5.5 3.23 21.54 7.45 0.53 7.3 3.67 22.16 7.14 0.55 Pr > F x 0.68 0.06 0.52 0.48 x Pr > F: significance level of F value by linear regression analysis.

Table 3 Analysis of covariance for silicon content in gerbera leaves at 2, 5, 9, 16, and 23 days after transplant (DAT) Exp 1 x Exp 2 y Source DF Mean Square F value Pr < F DF Mean Square F value Pr < F Silicon treatment 2 0.00017337 1.05 0.352 2 0.00037337 4.49 0.013 z DAT 1 0.00992459 60.25 < 0.0001 z 1 0.00003427 0.41 0.5221 Silicon DAT 2 0.00004737 0.29 0.7505 2 0.00034887 4.19 0.0172 z Error 0.00016472 0.00008317 x Experiment 1 conducted from 16 August to 26 September 2006. y Experiment 2 conducted from 18 January to 16 February 2007. z Significantly different. 2137

2138 C. Moyer et al. Table 4 Silicon content (%) in gerbera leaves at 2, 5, 9, 16, and 23 days after transplant Silicon treatment Control (no Si supplied) Calcium silicate Potassium silicate DAT exp 1 y exp 2 z exp 1 exp 2 exp 1 exp 2 2 0.035 0.030 0.040 0.021 0.042 0.031 5 0.052 0.025 0.046 0.028 0.051 0.048 9 0.048 0.039 0.048 0.037 0.060 0.036 16 0.027 0.022 0.027 0.027 0.037 0.044 23 0.018 0.024 0.024 0.029 0.027 0.043 y Experiment 1 conducted from 16 August to 26 September 2006. z Experiment 2 conducted from 18 January to 16 February 2007. the regression equation: Si (%) = 0.037 + 0.00036(DAT). According to this equation, the expected Si concentration of gerbera leaves at 2 DAT would be 0.038% of dry weight and it would increase to 0.045% by 20 DAT. Consequently, although the accumulation of Si in gerbera leaves is significant, the amount of Si did not vary greatly over time. Powdery mildew severity was evaluated for the last three weeks of this experiment to determine whether Si content was related to disease severity. Although by the end of the experiment, plants treated with potassium silicate showed a slightly higher Si content of 0.042% compared to 0.032% in the untreated control (Table 4), the potassium silicate treatment did not reduce disease significantly in comparison to the control plants (Table 5). DISCUSSION Silicon mixed with the soil or applied as a drench to gerbera plants did not reduce the severity of powdery mildew nor did it improve flower quality. The lack of control of powdery mildew with Si may have been due to the high inoculum concentration at the beginning of the experiment and to insufficient time for Si to act before the fungus infected the plants. It appears that Si should be present in the plant tissue before the infection takes place which means that plantlets would need to be disinfected at transplant in order to provide the plant with a disease-free period in which Si could be taken up. Silicon has the potential to trigger the plant s defense mechanisms (Cherif et al., 1994; Ghanmi et al., 2004; Rodrigues et al., 2004, 2005). Nevertheless, the exact time between uptake of Si, movement throughout the plant and the

Silicon for Powdery Mildew of Gerbera Daisy 2139 Table 5 Effect of silicon supplied as calcium silicate (CaSiO3) (5.4 g/pot) or as potassium silicate (K2SiO3) (0.27 ml/l) on powdery mildew severity in gerbera leaves Days after transplanting Si treatment 18 25 32 AUDPC Experiment 1 y 0 0.60 b z 1.00 a 1.79 a 15.37 a Calcium silicate 0.70 a 1.03 a 2.02 a 16.73 a Potassium silicate 0.64 ab 0.89 a 1.59 a 14.03 a Experiment 2 x 0 0.22 a 0.27 ab 0.27 b 2.95 a Calcium silicate 0.11 ab 0.30 a 0.47 a 3.30 a Potassium silicate 0.08 b 0.20 b 0.50 a 3.00 a z Means in columns and within experiment followed by different letters are significantly different according to the Waller-Duncan k ratio t test (P 0.05). y Experiment 1 conducted from 16 August to 26 September 2006. x Experiment 2 conducted from 18 January to 16 February 2007. production of defense response is not clear. In some cases, plants have been supplied with Si a week or two before inoculation. Ma et al. (2004) characterized the Si uptake in rice in a time-course experiment where roots and xylem sap were analyzed for Si content. The concentration of Si in the roots and in the xylem sap was measured after 8 h of Si exposure and the Si content increased over time with an increase of Si concentration in the external solution. This is in agreement with a similar study in cucumber where Si content increased with time depending upon the Si supplied in the external solution (Liang et al., 2005). Hence, in rice and in cucumber, the uptake of Si can be a rapid and active process. In the current study, increasing the amount of Si applied to gerbera plants did not result in increasing Si concentration on the leaves. Silicon has also been proposed to act as a physical barrier on leaf surfaces (Ma et al., 2001; Datnoff and Rodrigues, 2005). Because Si is deposited in plant cell walls, it is reasonable to expect that Si may strengthen plant tissues and thus creates a physical barrier that impedes direct penetration by fungal pathogens. Savvas et al. (2002) stated that Si improved gerbera flower quality by providing mechanical strength to the stems since their diameter increased with increasing Si concentration in the nutrient solution. However, in their study they have not analyzed Si content and the enhanced uptake of calcium by the addition of Si is more likely to be the reason for the greater stem diameter. In our study, stem diameter was not affected by the application of Si alone. Another possible reason for the lack of powdery mildew control could have been that environmental conditions such as temperature might have inhibited

2140 C. Moyer et al. Si uptake. When experiments were conducted during the summer, plants were placed in a shaded area of the greenhouse in an effort to lower temperatures and provide better conditions for the development of powdery mildew even though greenhouse temperatures were above 20 C. Schuerger and Hammer (2003) conducted a study to understand the discrepancy of powdery mildew control by Si between studies conducted in Florida and Canada. Three horticultural parameters (cultivar, nutrient solution and rooting medium) and two environmental factors (light intensity and temperature) were tested to determine their influence on Si effectiveness to reduce powdery mildew severity in cucumber. Of all the factors tested, temperature had the most significant effect in reducing powdery mildew severity. The number of colonies was fewer at 20 C and with Si at 100 mg/l than at 25 or 30 C. The lack of powdery mildew reduction may have been due simply to the failure of gerbera plants to accumulate Si. Savvas et al. (2002) reported that the addition of Si in the nutrient solution improved flower quality by increasing the proportion of class I flowers and the thickness of flower stems. The authors stated that Si also enhanced the uptake of calcium by the plants, but never measured Si contents of the plants. The amount of Si in leaf tissues found in gerbera daisy, Snow-White Sunburst series, supplied with different rates of calcium silicate averaged 0.05% and did not increase with higher Si rates. With rose and sunflowers, an increase in the amount of Si applied to the soil or the nutrient solution resulted in increased Si uptake from 0.02 to 0.09% and 0.5 to 3%, respectively (Gillman et al., 2003; Liang et al., 2006). Similarly in rice, Seebold et al. (2000) demonstrated that an increase in Si supply produced an increase in Si content of the leaves. Soil-applied calcium silicate at 500 kg/ha resulted in a Si content of 3.1% and at double the rate, the concentration in the leaves increased to 3.5%. In our study, the same applied rates resulted in a Si concentration in the leaves of 0.05% of dry weight and was not significantly different from the untreated control. Experiments on Si uptake were conducted to determine a time course for Si uptake in gerbera leaves. Silicon content in leaf or shoot tissues for 735 plant species was reported recently (Hodson et al., 2005), but the time required for uptake was not stated. Tamai and Ma (2003), in their characterization of Si uptake by rice roots, determined that the uptake of Si increased linearly with time, taking only hours for Si to be absorbed by roots. This is in agreement with Liang et al. (2006) who showed that for maize, rice, sunflower and wax gourd, it only took 2 to 10 hours for Si to accumulate and the amount absorbed increased with an increase in Si supply. Since Si alone apparently does not have fungicidal activity (Bowen et al., 1992), it must be absorbed before infection to provide protection against diseases and reduce disease severity. Usually, Si treatments are initiated when the seedlings are transplanted and then plants are inoculated hours (Menzies et al., 1992) or weeks (Gillman et al., 2003; Rodrigues et al., 2001) later. Disease symptoms are then observed within hours (Datnoff and Rutherford, 2004) or

Silicon for Powdery Mildew of Gerbera Daisy 2141 days (Gillman et al., 2003; Rodrigues et al., 2001; Menzies et al., 1992) suggesting that, in most cases, Si uptake and Si response occurs in a matter of hours or days. However, this was not the case in our study with gerbera daisy. Plants treated with potassium silicate showed some evidence of Si uptake 14 DAT and, at that time, symptoms of powdery mildew became apparent. However, no disease reduction was observed thereafter suggesting that even though the plants had some Si accumulation, it was insufficient to reduce disease. Alternatively, the timing of observation may have been too short to make such conclusions and perhaps a longer time course experiment would have shown greater Si uptake, higher levels of disease, and therefore differences in disease control. In our study on Si content in gerbera leaves receiving calcium silicate and potassium silicate, the Si content ranged from 0.01% to 0.06% on a dry matter basis. This concentration is very low compared with Si concentrations of 0.5% to 6% found in cucumber, strawberry, sunflower or rice tissues (Kanto et al., 2006; Liang et al., 2006; Menzies et al., 1991; Seebold et al., 2000). In those experiments, Si was successful in reducing disease severity. In the study with gerberas, Savvas et al. (2002) did not quantify Si accumulation nor did they clearly state that the improvement in flower quality could have been due to an indirect effect of Si and not to Si itself.. Therefore, the low Si content in gerbera leaves may explain the lack of powdery mildew control. In conclusion, gerbera plants apparently do not accumulate Si as rapidly nor in the amounts accumulated by rice, cucumber or sunflower and, therefore, Si does not have an application for powdery mildew management under greenhouse conditions in Florida. REFERENCES Adatia, M. H., and R. T. Besford. 1986. The effects of silicon on cucumber plants grown in recirculating nutrient solution. Annals of Botany 58: 343 351. Belanger, R. R., N. Benhamou, and J. G. Menzies. 2003. Cytological evidence of an active role of silicon in wheat resistance to powdery mildew (Blumeria graminis f. sp. tritici). Phytopathology 93: 402 412. Bowen, P., J, Menzies, D. Ehret, L. Samuels, and A. D. M. Glass. 1992. Soluble silicon sprays inhibit powdery mildew development on grape leaves. Journal of the American Society for Horticultural Science 117: 906 912. Campbell, C. L., and L. V. Madden. 1990. Introduction to Plant Disease Epidemiology. New York: Wiley & Sons. Cherif, M., A. Asselin, and R. R. Belanger. 1994. Defense responses induced by soluble silicon in cucumber roots infected by Pythium spp. Phytopathology 84: 236 242. Datnoff, L. E., and F. A. Rodrigues. 2005. The role of silicon in suppressing rice diseases. APSnet February: 1 28. Available at: http://www.apsnet.org/ online/feature/silicon/.

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