J Phytopathol 2010 Blackwell Verlag GmbH
doi: 10.1111/j.1439-0434.2009.01665.x
Vic¸osa Federal University, Department of Plant Pathology, Vic¸osa, Minas Gerais, Brazil
Silicon Suppresses Phytophthora Blight Development on Bell Pepper ´cio Avila Rodrigues2, Gaspar Henrique Korndo ¨ rfer3 and Lawrence Elliot Ronald D. French-M rench-Monar onar1, Fabrı abrıcio orndorfer Datnoff4 AuthorsÕ addresses: 1AgriLife Extension-Texas A&M System, Department of Plant Pathology and Microbiology, 6500 Amarillo Blvd W, Amarillo, TX 79106, USA; 2Vic¸osa Federal University, Department of Plant Pathology, Laboratory of Host–Parasite Interaction, Vic¸osa, Minas Gerais 36570-000, Brazil; 3Uberlaˆndia Federal University, Agronomy Institute, Uberlaˆndia, Minas Gerais 38440-902, Brazil; 4Louisiana State University, Department of Plant Pathology and Crop Physiology, 302 Life Sciences Bld., Baton Rouge, 70803 LA, USA (correspondence to F. A. Rodrigues. E-mail:
[email protected]) Received November 12, 2009; accepted November 13, 2009 Keywords: disease resistance, soilborne plant pathogen, plant nutrition
Abstract The application of silicon (Si) reduces the intensity of diseases in several economically important crops. This study aimed at determining the potential of this element to decrease the symptoms of Phytophthora blight development on bell pepper, caused by Phytophthora capsici. Bell pepper plants (Sakata Hybrid X pp6115) were initially grown in plastic pots with substrate composed of 1 : 1 mixture of sterile fine sand and Fafard No. 2 peat mix amended with calcium silicate (+Si) or calcium carbonate ()Si). Six weeks later, plants were transplanted to new pots that contained the same +Si and )Si substrate but were infested with finely ground wheat grains (1- to 2-mm diameter) colonized by two isolates of P. capsici, Cp30 (compatibility type A1) and Cp32 (compatibility type A2). At the end of the experiment, roots and stems from plants of each treatment were collected to determine Si concentration. The presence of lesions on crowns and stems and wilting of plants were monitored up to 9 days after transplanting (DAT). Data obtained were used to calculate the area under diseased plants progress curve (AUDPPC) and area under wilting plants progress curve (AUWPPC). Relative lesion extension (RLE) was obtained as the ratio of vertical lesion extension to stem length at 9 DAT. There was a 40% increase in the concentration of Si in the roots but not in the stems of bell pepper plants in the +Si treatment compared to the )Si treatment. When comparing +Si to )Si treatments, the AUDPPC was reduced by 15.4 and 37.5%, while AUWPPC was reduced by 29.1 and 33.3% in experiments 1 and 2, respectively. RLE values were reduced by 35% in the +Si treatment. Dry root weights increased by 23.7%, and stem weights were increased by 10.2% in the +Si treatment. Supplying Si to bell
peppers roots can potentially reduce the severity of Phytophthora blight while enhancing plant development.
Introduction Phytophthora blight, caused by the soilborne oomycete Phytophthora capsici Leonian, is the most destructive disease of bell pepper (Capsicum annuum L.) worldwide (Hwang and Kim 1995; McGovern et al. 1998; Tamietti and Valentino 2001). This oomycete causes root rot, crown rot, stem rot and fruit rot on pepper plants, dramatically decreasing yield (Parra and Ristaino 2001; Ploetz et al. 2002). Without the availability of host plants, persistence of P. capsici in field soil is attributed to sporangia, encysted zoospores, or as oospores if both mating types are present (Ansani and Matsuoka 1983; Hausbeck and Lamour 2004; French-Monar et al. 2007). Moreover, weeds may contribute to pathogen survival in the absence of a host crop or when propagules may not readily survive in soil or plant debris (French-Monar et al. 2006). Management of P. capsici requires a multifaceted approach of both cultural and chemical strategies. Establishing P. capsici susceptible bell pepper cultivars in welldrained fields, on raised plant beds, and avoiding the planting of low-lying sections of fields help to manage the disease. The most effective method to combat the pathogen is based on the use of fungicides and soil fumigants even though some cultivars with different levels of resistance to P. capsici have been reported (Cantliffe et al. 1995; Chellemi et al. 1997). Considering that fungicides are perceived to negatively impact the soil–water environment and resistance to these fungicides can occur, alternative and environment-friendly methods of protecting bell pepper against Phytophthora blight need to be investigated.
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The beneficial effects of silicon (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). Che´rif and Be´langer (1992) found that cucumber plants growing in hydroponic culture containing 100 ppm of Si had less root decay, yield loss and plant death caused by the oomycete Pythium ultimum than plants grown in hydroponic culture without Si. The incidence of cucumber plants infected by P. aphanidermatum also dramatically decreased when plants were grown in hydroponic culture containing Si (Che´rif et al. 1992). Fusarium wilt of cucumber is another disease managed by application of Si to the soil (Miyaki and Takahashi 1983). Resistance of corn to stalk rot, caused by P. aphanidermatum, has been attributed to Si (Sun et al. 1994). However, Walker and Morey (1999) reported that potassium silicate applied to the soil did not reduce the isolation frequency of P. nicotianae and P. ultimum from citrus roots or the severity of root rot caused by these pathogens, but it did reduce the population of the nematode Tylenchulus semipenetrans in the soil. Many important crops are not capable of taking a large amount of Si from the soil; therefore, they may not be able to benefit from the positive effects that this element can provide by alleviating several biotic and abiotic stresses (Datnoff et al. 2007). If these plants are able to accumulate Si in roots, then this element may affect pathogen development. Heine et al. (2007) studied the effect of Si supply on the infection and spread of P. aphanidermatum in the roots of tomato and bitter gourd and found that this element did not affect severe root-growth inhibition by P. aphanidermatum in either crop. However, continuous Si supply significantly inhibited the basipetal spread of the pathogen from the infected root apex in bitter gourd. The application of Si alone or combined with other control methods may offer a viable alternative for reducing yield losses caused by Phytophthora blight on bell pepper and other vegetable production systems. As a paucity of information in the literature exists about the direct influence of Si on bell pepper diseases, this study determined the potential use of this element to decrease Phytophthora blight development on bell pepper plants.
Materials and Methods Plant growth and Si amendment
Bell pepper seeds (Sakata Hybrid X pp6115, Sakata Seed America, Inc., Fort Myers, FL, USA) were surface sterilized in 10% (v ⁄ v) NaOCl for 2 min, rinsed in sterilized water for 3 min and sown in plastic pots (20-cm diameter) (Fisher Scientific Co., Pittsburgh, PA, USA) filled with 2.5 kg of substrate made from a 1 : 1 mixture of sterile fine sand and Fafard No. 2 peat mix soilless medium (Conrad Fafard Inc., Agawan, MA, USA). Calcium silicate (+Si treatment) (Calcium Silicate Corp., Lake Harbor, FL, USA)
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(CaSiO3; 22% Si, 42% Ca) was incorporated in the amount of 31 g ⁄ kg of substrate (6.8 g Si ⁄ kg of substrate). This amount of calcium silicate was selected to provide enough Si for plant uptake based on previous studies with several Gramineae species (Rodrigues et al. 2001b) and also considering the low amount of Si (2.4 mg Si ⁄ dm) naturally present in the substrate. Calcium carbonate ()Si treatment) (40% Ca; Sigma– Aldrich, St. Louis, MO, USA) was added to separate pots in the amount of 32.6 g ⁄ kg of substrate to equilibrate the amount of Ca present in this treatment with the treatment corresponding to substrate amendment with calcium silicate. The amount of Ca between the calcium silicate and calcium carbonate treatments was equilibrated to 13 g ⁄ kg of substrate. Magnesium carbonate (12.3% Mg; Sigma–Aldrich, USA) was also added to the pots that received calcium silicate or calcium carbonate treatments in the amount of 14.6 g ⁄ kg of substrate so as to provide magnesium to the plants. The calculated Ca : Mg ratio in the substrate was 2 : 1 (Rezende et al. 2009). Substrate in each pot that received either calcium silicate or calcium carbonate treatments was incubated for 30 days under 60% moisture content at 25C. A total of three seeds were sown per pot, and 5 days after seedling emergence, each pot was thinned to one plant. Soil in each pot was fertilized with 100 ml of a nutrient solution containing N 100 mg, P 300 mg, K 150 mg, S 40 mg, B 0.81 mg, Cu 1.33 mg, Fe 1.55 mg, Mn 3.66 mg, Mo 0.15 mg and Zn 4.00 mg ⁄ kg substrate (Rodrigues et al. 2001b). The nutrient solution was prepared using deionized water and applied before sowing as well as every 2 days after seedling emergence in the amount of 30 ml per pot. Plants were otherwise watered as needed with deionized water. Inoculum production and inoculation procedure
Isolates Cp30 (compatibility type A1) and Cp32 (compatibility type A2) of P. capsici obtained from bell pepper (C. annuum) were used for inoculum production. These isolates were obtained from the Phytophthora Collection, Plant Pathology Department, University of Florida, Gainesville, FL, and maintained on BBL (Becton, Dickinson and Company, Sparks, MD, USA) corn meal agar (CMA). Inoculum of P. capsici was produced on autoclaved grains of wheat (Triticum aestivum L.) according to the method described by Mitchell and Kannwischer-Mitchell (1992). Approximately, 20 g of grain plus 25 ml of deionized water was added to a 250-ml flask and allowed to soak overnight. Flasks were then autoclaved for 30 min on each of two consecutive days. After the two isolates were individually grown on 20% clarified V8 juice agar for 5 days, four 5-mm-diameter disc plugs of each isolate were added to each autoclaved flask, and each flask was incubated for 3 weeks at 25C under darkness and shaken every 2–3 days to prevent grains from clumping. Infested grain was finely ground in a Black and Decker HC3000 Mincer ⁄ Chopper (The Black and Decker Corporation, Towson, MD, USA).
Silicon Suppresses Phytophthora Blight on Bell Pepper
Oospores were observed on the finely ground wheat grains, but they were still immature. Mycelium was the major propagule for infesting the substrate. Any germinating oospores would produce a germ tube and behave like mycelium. Six-week-old bell pepper plants grown in pots containing substrate amended with calcium silicate or calcium carbonate were then transplanted to other plastic pots (15-cm diameter) (Fisher Scientific Co., USA) containing 1 kg of substrate that also received calcium silicate (31 g ⁄ kg of substrate) or calcium carbonate (32.6 g ⁄ kg of substrate). Only plants of equal size were used. One day before transplanting, 1 g of finely ground infested grain was mixed with 1 kg of substrate in each plastic pot. As a check treatment, 1 g of finely ground grain not infested with P. capsici was mixed with 1 kg of substrate per pot. Infested and non-infested grain was homogeneously mixed with the substrate by shaking thoroughly the mixture in a plastic bag. Disease assessment
The presence of lesions on crown and stem of diseased plants as well as plants showing symptoms of wilting was monitored up to 9 days after transplanting (DAT) them to pots previously infested with P. capsici. At 9 DAT, most of the pepper plants were dead, mainly in the substrate containing calcium carbonate ()Si treatment). Area under diseased plants progress curve (AUDPPC) and area under wilting plants progress curve (AUWPPC) for each treatment were calculated using the trapezoidal integration of diseased and wilting plants progress curves over time using the formula proposed by Shaner and Finney (1977). Relative lesion extension (RLE) was obtained as the ratio of vertical lesion extension to stem length at 9 DAT. Samples of diseased plants were plated on PARPH medium (Mitchell and Kannwischer-Mitchell 1992) immediately after mortality onset, while any surviving plants were plated at 10 DAT to confirm that P. capsici was the causal agent of plant disease by its presence in the roots. Determination of dry root and stem weight and plant tissue analysis for concentration of Si and Ca
At the end of the experiment, plant roots and stems for each replication of both treatments were removed from the pots, shaken to remove the bulk of the soil, washed in sterile deionized water and dried for 72 h at 65C. Dry root and stem weight was obtained prior to having root and stem tissue ground to pass through a 40-mesh screen using a Thomas-Wiley mill. Concentration of Si in roots and stems was determined by a colorimetric analysis on 0.1 g of dried and alkali-digested tissue (Rodrigues et al. 2001a). Concentration of Ca was determined by atomic absorption spectrophotometry. Experimental design and data analysis
A 2 · 2 factorial experiment was arranged in a completely randomized design with four replications. Each replication consisted of one plastic pot containing 1 kg
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of substrate and one plant. The factors studied were pathogen inoculated and non-inoculated plants and plants amended with calcium silicate (+Si) or calcium carbonate ()Si). The experiment was repeated once. A total of 16 plants were used in each experiment. Data from the two experiments were pooled for analysis as indicated by the CochranÕs test for homogeneity of variance (Gomez and Gomez 1994). The experiment-treatment interactions were not significant (P ‡ 0.05) when compared with the main effects of the treatments. Data were analysed by analysis of variance (anova) and treatment means compared by t-test (P £ 0.05) with SAS (Release 8.02 Level 02M0 for Windows, SAS Institute, Inc., 1989, Cary, NC, USA). Pearson correlations of Si in the root tissue with RLE, AUDPPC and AUWPPC were calculated for all treatment combinations.
Results Only the factors pathogen inoculated and non-inoculated plants as well as plants amended with calcium silicate (+Si) or calcium carbonate ()Si) were significant (P £ 0.05) for both Si and Ca concentration in the plant roots and stems. When plants were grown in substrate amended with calcium silicate (+Si), the concentration of Si in the roots, but not in the stems, was greater than when plants were supplied with calcium carbonate ()Si) regardless of the inoculation status of the plants (Table 1). In general, there was a 40% increase in concentration of Si in the roots when plants were amended with calcium silicate when compared to calcium carbonate treatment. There also was a 92.8% increase in the concentration of Si in the roots of inoculated vs. non-inoculated plants. The concentration of calcium was significantly increased by 59.6% in the roots and 21% in the stems of plants grown in substrate amended with calcium carbonate in comparison with calcium silicate (Table 1). Calcium content in the roots was 134.6% greater in the inoculated plants than in the non-inoculated plants. The pattern of Phytophthora blight symptom development on roots and stems of bell pepper plants grown in a substrate with )Si and +Si treatments is illustrated in Fig. 1. Roots of plants grown in the
Table 1 Concentration of silicon (Si) and calcium (Ca) in roots and stems of bell pepper plants grown in pots amended with calcium carbonate ()Si) or calcium silicate (+Si) and inoculated or non-inoculated with Phytophthora capsici Si (%) Treatments Inoculated plants Non-inoculated plants t-test (P £ 0.05)* )Si (calcium carbonate) +Si (calcium silicate) t-test (P £ 0.05)*
Ca (%)
Roots
Stems
Roots
Stems
0.54 0.28 0.10 0.35 0.49 0.09
0.07 0.04 0.06 0.09 0.04 0.07
0.89 0.38 0.15 0.91 0.57 0.14
0.44 0.42 0.05 0.46 0.38 0.04
*Values for t-test are the least significant difference.
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120
Exp. 1
100 80
Si–
60
Si+
Inoculated
Non-inoculated – Si
+Si
– Si
+Si
Fig. 1 Root rot and stem rot caused by Phytophthora capsici blight on bell pepper plants grown in pots containing substrate amended with calcium carbonate ()Si) or calcium silicate (+Si) and compared to non-inoculated plants that received the same treatments
Diseased plants ( )
40 20 0 1
2
3
4
5
6
7
8
9
3
4
5
6
7
8
9
120
Exp. 2 100 80 60
absence of Si were severely damaged by the pathogen as shown by the intense necrosis in the root system. This was complemented by necrosis in the crown and main stem as disease progressed. In contrast, on plants from the +Si treatment, the roots were less damaged and necrosis on crown and stems was greatly reduced. RLE on the lower stem of inoculated bell pepper plants was reduced by 35% for the +Si vs. )Si treatment after data from the two experiments were pooled (Table 2). The AUDPPC and AUWPPC values were decreased significantly by the addition of Si to the substrate (Table 3, Figs 2 and 3). When comparing +Si to )Si treatments, the AUDPPC was reduced by Table 2 Relative lesion extension (RLE) on lower stems of bell pepper plants amended with calcium carbonate ()Si) or calcium silicate (+Si) and inoculated with Phytophthora capsici Treatments
RLE (%)
)Si (calcium carbonate) +Si (calcium silicate) t-test (P £ 0.05)*
37.49 24.34 6.86
*Values for t-test are the least significant difference.
Table 3 Area under diseased plants progress curve (AUDPPC) and area under wilting plants progress curve (AUWPPC) of bell pepper plants amended with calcium carbonate ()Si) or calcium silicate (+Si) and inoculated with Phytophthora capsici AUDPPC Treatments )Si (calcium carbonate) +Si (calcium silicate) t-test (P £ 0.05)*
AUWPPC
Exp. 1
Exp. 2
Exp. 1
Exp. 2
233 197 14
208 130 21
127 90 11
150 100 17
*Values for t-test are the least significant difference.
40 20 0 1
2
Days after inoculation Fig. 2 Percentage of bell pepper plants growing in pots containing substrate amended with calcium carbonate ()Si) or calcium silicate (+Si), showing symptoms of Phytophthora blight on crown and stem. Data points are means of four replications for each treatment. Bars represent standard deviation of the mean for each experiment
15.4 and 37.5%, while AUWPPC was reduced by 29.1 and 33.3% in experiments 1 and 2, respectively. PearsonÕs correlation values between Si content in bell pepper roots and the variables RLE, AUDPPC and AUWPPC were negative and significant (P £ 0.05) (r = )0.52, )0.60 and )0.45, respectively). Only the factors pathogen inoculated and non-inoculated plants as well as plants amended with calcium silicate or calcium carbonate were significant (P £ 0.05) for both dry root and stem weights of bell pepper plants. Dry root and stem weights were significantly increased for plants grown in the substrate amended with calcium silicate (Table 4). Root weights increased by 23.7%, while stemÕs increased by 10.2% for the +Si in comparison with the )Si treatment. Plants inoculated with P. capsici showed a reduction in root and stem dry weights of 69.6 and 26.2%, respectively.
Discussion The mechanism responsible for the variation in accumulation of Si in the above-ground parts of different plant species is still poorly understood. For
Silicon Suppresses Phytophthora Blight on Bell Pepper
5
120 100
Exp. 1
80 60
Si–
40
Si+
Wilting ( )
20 0 1
2
3
4
5
6
7
8
9
3
4
5
6
7
8
9
120 100
Exp. 2
80 60 40 20 0 1
2
Days after inoculation Fig. 3 Percentage of bell pepper plants growing in pots containing substrate amended with calcium carbonate ()Si) or calcium silicate (+Si), showing symptoms of wilting. Data points are the means of four replications for each treatment. Bars represent standard deviation of the mean for each experiment
Table 4 Dry weight accumulation by bell pepper plants inoculated or noninoculated with Phytophthora capsici and grown in pots amended with calcium carbonate ()Si) or calcium silicate (+Si) Dry weight (g ⁄ plant) Treatments Inoculated plants Non-inoculated plants t-test (P £ 0.05) )Si (calcium carbonate) +Si (calcium silicate) t-test (P £ 0.05)*
Roots
Stems
0.62 2.04 0.80 0.97 1.20 0.15
2.05 2.78 0.14 2.15 2.37 0.17
*Values for t-test are the least significant difference.
instance, monocotyledonous plants such as barley, oat, rice, rye, sugarcane and wheat are considered efficient Si-accumulator plants with a shoot dry matter Si content of more than 5% (Rodrigues et al. 2001b). On the other hand, some dicotyledonous plants cannot accumulate more than 1% Si content in their shoots (Liang et al. 2006). Silicon transport occurs both passively and actively in cucumber, maize, rice, sunflower and wax gourd (Liang et al. 2006). In rice, the Si trans-
porter genes Lsi1 and Lsi2 are responsible for the high capacity of the lateral roots to take up Si from the soil solution in the form of monosilicic acid (H4SiO4), an uncharged molecule (Ma et al. 2001, 2007; Ma and Yamaji 2006). Lsi1 is responsible for an influx transporter protein of silicic acid, while Lsi2 is an active efflux transporter protein, both localized in the root exodermis and endodermis (Ma and Yamaji 2006; Ma et al. 2007). The gene Lsi6, involved in Si distribution in rice shoots, has been cloned (Yamaji et al. 2008). Some dicotyledonous plant species, despite being inefficient in the uptaking of high amounts of Si from the soil solution and translocating this element to the shoots, are still able to obtain some benefits that this element brings to plants under a number of biotic and ⁄ or abiotic stresses (Datnoff et al. 2007). In the current study, the availability of Si to bell pepper plants did not result in increasing Si concentration in stems, but the plants were able to uptake and accumulate it in the roots from where P. capsici gains access to the plant tissue. The concentration of Si in roots, especially in stems, of bell pepper plants can be considered very low when compared with Si concentrations ranging from 0.5 to 6% as reported for leaf tissues of corn, cucumber, oat, rice, rye, sorghum, strawberry or wheat plants (Kanto et al. 2004; Liang et al. 2006; Rodrigues et al. 2001a,b; Seebold et al. 2001). However, its concentration in the roots of peppers was quite enough to reduce disease symptoms on roots, crown and main stem of pepper plants. According to Voogt and Sonneveld (2001), for plants of sweet pepper (Capsicum frutescens) grown in soil or in a soilless medium and supplied only with ambient Si in the root environment, the content of this element in young leaves ranged from 12 to 22 and from 15 to 17 mmol ⁄ kg dry matter, respectively. However, there is a lack of information in the literature about the Si content in roots, stems and even on leaves of pepper plants. As Ca is a normal constituent of the cell wall and middle lamella of plants, the relationship between Ca ions and the cell wall partially explains the increased resistance to invasion by certain pathogens induced by this element (Rahman and Punja 2007). In the literature, there are no reports on the effect of Ca in reducing Phytophthora blight on bell pepper, but this element can decrease the symptoms of P. cinnamomi on avocado (Messenger et al. 2000) and P. nicotianae on citrus (Campanella et al. 2002). Even though plants grown in a substrate amended with calcium carbonate mostly showed an increase in Ca concentration in roots, they were not protected against P. capsici when compared to what was observed for plants supplied with Si from calcium silicate. The results of this study show that resistance to soilborne pathogens can be increased even in plant species with a limited capacity to accumulate Si in roots and incapable of translocating this element to the shoots. Significant reduction in the values of the components of resistance RLE, AUDPPC and AUWPPC on plants
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supplied with Si clearly demonstrates the crucial role played by this element in reducing Phytophthora blight development. This finding is in corroboration with what has been reported from other soilborne pathogens (Datnoff et al. 2007). Cucumber plants growing in hydroponic culture containing Si significantly reduced root decay caused by P. ultimum (Che´rif and Be´langer 1992). The incidence of cucumber plants infected by P. aphanidermatum also dramatically decreased when plants were grown in hydroponic culture containing Si (Che´rif et al. 1992). The supply of Si to tomato and bitter gourd plants did not affect severe root-growth inhibition by P. aphanidermatum in either species (Heine et al. 2007). However, continuous Si supply to these two plant species inhibited the basipetal spread of the pathogen from the infected root apex in bitter gourd but not in tomato. Silicon application to the roots only during pretreatment or only during or after root infection failed to inhibit the spread of P. aphanidermatum. Determination and compartmentalization of Si in the roots of bitter gourd revealed that symplastic Si, rather than apoplastic Si, was associated with the ability of the plant to reduce the spread of the pathogen in roots. These authors found that Si accumulation in the root cell walls does not represent a physical barrier to the spread of P. aphanidermatum in bitter gourd and tomato roots, but the maintenance of elevated symplastic Si content is a prerequisite for Si-enhanced resistance against this pathogen. According to Jeun and Hwang (1991), no significant differences were found between a susceptible and a resistant pepper cultivar or plant growth stages to Phytophthora blight development and its relationship to the concentration of Si and micronutrients such as sodium, iron, zinc and manganese in plant tissue. The authors concluded that the expression of age-related resistance of pepper plants to Phytophthora blight may be due to host morphological changes and a decrease in the amounts of mineral nutrients such as nitrogen, phosphorus, potassium, calcium and magnesium. Increases in bell pepper dry matter accumulation could be attributed to a decrease in the intensity of Phytophthora blight symptoms in the presence of Si. Infection by P. capsici in pepper causes intense root necrosis that changes the dynamic balance of water and nutrient uptake and, consequently, the gain in shoot dry weight (Erwin and Ribeiro 1996). Rodrigues et al. (2001a) reported that the dry matter accumulation in rice plants from six cultivars was significantly greater in the presence of Si. In the absence of disease, Si enhanced dry matter accumulation by 15% when compared to inoculated plants, whereas Si more than doubled the mean dry matter accumulation in infected plants. The results of this study, in association with previous reports from other pathosystems, clearly suggest that supplying Si to bell pepper roots can suppress Phytophthora blight development while enhancing plant development. In this study, pepper plants were
subjected to P. capsici, which is very aggressive on pepper and other vegetable crops. In case of mature plants infected by P. capsici at the flowering growth stage, reduction in disease progress by Si could allow one or more harvests during the growing season. Decreasing disease progress on pepper roots, crown and stem by Si could be combined with systemic fungicide treatments to achieve better disease management and increase pepper production in compromised fields. Acknowledgements Profs. F.A. Rodrigues and G.H. Korndo¨rfer thank the CNPq for their fellowships. The authors also thank University of Florida, Department of Plant Pathology, for providing operational and financial support to carry out this study.
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