DOI: http://doi.org/10.4038/agrieast.v13i1.59

Control of grain discoloration by silicon AGRIEAST 2019 Vol.13(1):1-11

Exploring the efficacy of silicon supplementation on control of rice grain discoloration disease

Nasla, M F F1, Prasannath K1 and Gunapala K R D2

1Department of Agricultural Biology, Faculty of Agriculture, Eastern University, Sri Lanka 2Pathology Division, Rice Research and Development Institute, Batalagoda, Sri Lanka

Abstract

Rice is the most important cereal crop in Sri Lanka. Grain discoloration is an emerging disease complex, reducing grain quality of rice crop. Grain discoloration control is, however, mainly focused on fungicide; their use is limited due to perceived environmental problem and health concern. Silicon (Si) application is known as encouraging eco-friendly alternative to fungicide. Foliar applications of Si- based formulations were evaluated to examine their effect on reducing grain discoloration disease. Field experiments were conducted at Rice Research and Development Institute (RRDI), Batalagoda from May to September 2017. After planting, Si was applied as solution at the rate of 1 mL/L at tillering and early flowering stage. Fungicide and control (sterilized water) were applied as treatments at early flowering stage. Treatments were arranged in a randomized complete block design with three replications. Data on plant height, total number of tillers per plant, chlorophyll content, yield, incidence of plant infection and percentage of grain discoloration were recorded. Laboratory study was carried out to isolate and identify the grain discoloration causing pathogens from the infected seeds by using potato dextrose agar plate method. Results revealed that incidence of plant infection and grain discoloration percentage were reduced by foliar application of silicon supplements as same as fungicide application. Among the Si-based formulations tested, Gainexa UPL gave the best result in controlling grain discoloration in rice. These silicon supplements could be used as alternatives to synthetic fungicide and could reduce the amount of fungicide needed during rice crop cultivation. lunata and Curvularia pallescence were identified as the causal organisms of rice grain discoloration disease in the study area.

Keywords: Grain discoloration, Rice, Silicon supplements, Synthetic fungicide

1. Introduction families are engaged in paddy cultivation in island-wide (Rodrigo,

2014). However, rice crop is threatened Rice is one of the most important cereal by different biotic and abiotic factors. food crops in the world. It is consumed Rice crop is threatened by more than 50 as a staple food and it provides more diseases which may appear at any than one fifth of the calories in the growth stage of the plant (Arshad et al., world for human consumption (Arshad 2009). The quality of rice is directly et al., 2009). Rice is also a single staple affected to marketable yield and food in Sri Lanka. The average per consumption of rice grain. Grain capita consumption of rice in Sri Lanka discoloration disease is an emerging is 114 kg per year per person (Roberia et disease complexes threaten for rice crop al., 2014). About 1.8 million farm seriously. Grain discoloration reduces

Corresponding author: [email protected] http://orcid.org/0000-0003-4588-1983 Received: 19.10.2018 1 Accepted: 31.10.2018 grain quality very seriously and also and concern of food safety, reduces consumer preference on rice environmental friendly alternatives (Phat et al., 2005). Du et al. (2001) should be investigated to control rice reported that grain discoloration grain discoloration disease. affected milling recovery, cooking and eating quality, seed germination and Silicon (Si) application may offer an seedling height. encouraging alternative to fungicide use while enhancing host plant There are several reasons to cause the resistance (Datnoff et al., 1997; grain discoloration on rice crops such as Korndorfer et al., 1999). Si is beneficial panicle lodging, and climate for plants in terms of yield (Ghasemi et changes, and microbial infection. al., 2013), growth and resistance to Among these reasons, microbial biotic and abiotic stresses (Datnoff et al., infection is the major reason for the rice 1992). Addition of Si can decrease the grain discoloration disease (Agarwal et incidence of several diseases in rice al., 1989). It occurs due to the infection such as blast, brown spot, sheath blight, of microorganisms on the glumes, grain discoloration and disease kernel or both (Ou, 1985; Arshad et al., bacterial leaf blight (Jayawardana and 2009). A large number of fungi and Weerahewa, 2016). bacteria is associated with rice grain discoloration disease (Ou, 1985). The Various mechanisms have been fungi that are reported to be associated proposed to explain the increased level with discoloration of grains are Bipolaris of disease resistance mediated by Si. It oryzae, Alternaria padwickii, Pyricularia acts as a physical barrier, deposited oryzae, Fusarium moniliforme, Fusrium beneath the cuticle, to impede grminearum, Nigrospora oryzae, , penetration by fungal appressoria, Curvularia spp, Phoma sorghina, thereby disrupting the process of Dichotomophthoropsis nymphacearum and infection (Jayawardana et al., 2015). Si Heterosporium echinunulatum etc. (Ou, may signal a biochemical mechanism to 1985; Arshad et al., 2009). Suryadi and suppress fungal infection and disease Kadir (2009) reported that several (Weerahewa and Somapala, 2016). Si bacteria such as Pseudomonas avenae, could induce defense responses such as Pseudomonas fuscovaginae, Pseudomonas systemic acquired resistance syringae var. oryzicola, and Pseudomonas (Jayawardana and Weerahewa, 2016). glumae could cause rice grain Hence, inorganic form of Si would be discoloration disease. used as solution form to spray on the plant. The bioactive form of silicon Most of the studies showed that seed (orthosilicic acid) would be provided to treatment with synthetic fungicides plant as foliar spray to enhance the and foliar application of synthetic plant resistance against the rice grain fungicide are most effective to control discoloration. Therefore, the present rice grain discoloration disease study was conducted to determine the (Agarwal et al., 1989; Arshad et al., effect of silicon supplements on rice 2009). However, continuous use of grain discoloration disease and identify fungicides can lead to health and the pathogens causing rice grain environmental problems (Korndorfer et discoloration disease in the research al., 1999) and development of fungicide area. resistance of pathogen populations (Jayawardana and Weerahewa, 2016). Therefore, to address the current demand in sustainable crop production

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2. Materials and Methods allowed for the natural infestation of grain discoloration pathogens. 2.1 Evaluation of the effect of silicon supplements on control of grain 2.3 Preparation and application of discoloration treatments

A field experiment was carried out at Test solutions of the Si supplements the Rice Research and Development were prepared according to the label Institute (RRDI), Batalagoda, Sri Lanka, instructions. They were prepared at 1 during Yala season from May to mL/L concentration (1 ppm). Si September 2017. The following five supplements were applied two times treatments with three replications were during the crop life cycle. First arranged in Randomized Complete application of Si supplements was done Block Design. Treatments include three 40 days after transplanting (tillering different silicon supplements, a stage or just before the booting stage). synthetic fungicide and sterilized Second application was done 70 days water. Bg 370 variety was selected as after transplanting (at early flowering object for the field trial because more stage of the plant). The fungicidal seed discoloration has been observed in solution was prepared at the Bg 370 variety in Batalagoda. concentration of 1 mL/L (1 ppm). It T1 - Sterilized water (Control) was applied 70 days after transplanting T2 - Silicon 1 (Orthosilicic Acid in liquid (at early flowering stage of rice plant). form – Silicon solution) T3- Silicon 2 (Orthosilicic Acid in liquid form – Imported sample (Gainexa 2.4 Isolation and identification of UPL)) pathogens causing grain discoloration T4- Silicon 3 (Sodium metasilicate in Solid form (Silicon 17)) The laboratory experiment was carried T5- Fungicide (Azoxystrobin 200 g/L + out at the Pathology Laboratory of Cyproconazole 80 g/L SC fungicide) RRDI. Potato dextrose agar plate method was used to isolate the 2.2 Establishment of rice seedlings in pathogens from infected grains. The the field discolored seeds of Bg 370 were collected by using random sampling Healthy seeds of Bg 370 variety were method from each plot in the field. seeded at parachute nursery bed. The They were mixed thoroughly. From the seedlings were kept for 12 days in seed lot, 50 discolored seeds were nursery bed. After 12 days, selected randomly collected. The seeds were healthy seedlings were established in kept under the running tap water for 30 the field. Before the establishment of minutes. After prewashing of seeds, 25 rice seedlings, the field was divided seeds were directly planted on the into 15 plots. Each plot area was 18 m2. potato dextrose agar plates. Five seeds One meter spacing was maintained were planted on each agar plate. within the plots. Fertilizers were Another 25 seeds were surface applied according to the sterilized by using 70% of ethanol for 1 recommendation of the Department of minute. The surface sterilized seeds Agriculture, Sri Lanka. The field was were thoroughly washed with distilled irrigated regularly. Weed control was water three times. The sterilized 5 seeds done manually and the field sanitation were planted on each plate. Three days was also maintained. All plots were after planting of seeds in agar plates, clearly observed colonies of fungi

3 which were grown surrounding the of fungal colonies were observed to planted seeds on agar plates were sub- identify the pathogens. Later, cultured. The sub-culturing of fungal microscopic examination was colonies was continued until getting performed to confirm the pathogens on pure cultures of fungal colonies for the basis of mycelial growth and clear identification. After getting the shape morphology (Fig. 1). pure cultures of pathogens, characters

b a

c d

Fig.1. a) Discolored seeds b) Planting the seeds on PDA plate c) Fungal growth surrounding the seeds d) Pure culture of fungal pathogen

2.5 Data collection and analysis Percentage of Number of infected infected plants = plants × 100% Data were collected from sampled Total number of plants selected by quadrant method sampled plants per from each replicate of each treatment. plot The quadrant method was done 5 times per plot. Plant height, chlorophyll Percentage of discolored grains was content of leaves and total numbers of estimated using the following equation. tillers per plant were measured at Grain discoloration was estimated by maximum tillering stage (70 days after counting grains with more than 25% of transplanting). The yield was glume surface affected (IRRI, 2014). determined from the total plants in Percentage of Number of each plot of each treatment at discolored discolored seeds × harvesting stage (106 days after grains = 100% transplanting). Percentage of infected Total number of plants was determined at ripening seeds in 25 g of seed stage (95 days after transplanting) lot using the following formula. A plant with at least 50% of grains showing Arc-sin transformation technique was discoloration was considered as an used to transform the non-parametric infected plant (IRRI, 2014). data such as percentages of infected plants and discolored grains, which

4 were then confirmed normally Chlorophyll content rice plant was not distributed by performing normality significantly (p>0.05) affected by silicon test. Data were subjected to Analysis of supplements and fungicide application Variance and the significance of (Table 1). In the investigation, no treatments was tested using Duncan’s significant differences were observed Multiple Range Test at α=0.05 level. among silicon treated, fungicide treated and deprived rice plant in the term of 3. Results and Discussion chlorophyll content. These results are in 3.1 Evaluation of foliar application of accordance with the finding of Bhalali silicon on rice grain discoloration et al. (2013) reporting that there was no disease significant correlation between chlorophyll a and logarithm Height of rice plants was not chlorophyll a with silica in wetland significantly (p>0.05) affected by silicon plants. supplements and fungicidal application (Table 1). In the Yield of rice plants was significantly investigation, no significant differences (p<0.05) affected by silicon were observed between silicon treated supplements and fungicidal and deprived rice plants in term of application compared to control (Table plant height confirming the reports by 1). Si supplements and fungicidal Liang et al. (2005), Guevel et al. (2007) application resulted in a similar effect and Guntzer et al. (2012). Liang et al. on yield of rice grains as there were no (2005) stated that silicon has not been significant (p>0.05) differences between considered important for vegetative the Si supplements and fungicidal growth but it aids the plant in healthy solution. This result was obtained development under stresses in different because fungicide which has the ability grasses especially in rice. Guevel et al. to reduce the disease to the same (2007) found that Si feeding did not general degree as silicon could reduce yield a direct measurable effect on plant the severity of disease and increase growth. In their experiments, Si yield in rice plants. Rodrigues et al. amendment, either through the roots or (2010) reported that the severity of leaf the leaves did not increase growth of angular disease in beans was reduced plants. Si, an element forming up by fungicide application, through that to 60% of the wheat ash had no increased the yield of beans. In the metabolic role in wheat plant investigation, significantly higher (p<0.05) yield was observed in Si There were no significant (p>0.05) treated plants compared to the control differences observed among the plants, confirming the reports by treatments in the total number of tillers Soratto et al. (2011) and Ahmed et al. per rice plant at the maximum tillering (2013). Soratto et al. (2011) showed that stage (Table 1). In the investigation, no grain yield was significantly increased significant differences were observed by Si leaf application compared to the between silicon tested and deprived control (without Si application), as a rice plant in term of number of tillers result of the higher photosynthetic area, per plant. These results are in line with due to the higher dry matter production Ahmed et al. (2013), who reported that and higher number of spikes per m² in increase in applied silicon amount did white oat and wheat. Ahmed et al. not enhance the number of tillers per (2013) demonstrated that foliar plant in rice. application of silicon solution significantly increased paddy yield

5 compared to control (without silicon application).

Table 1. Effect of silicon supplements and fungicide application on incidence of plant infection and discoloration grains

Treatments Plant height Number of Chlorophyll Yield per (cm) tillers per plant content (SPAD) plot (kg) Control 102.71 a 10.92 a 37.55 a 6.10 b Silicon 1 98.67 a 10.29 a 39.71 a 10.17 a Silicon 2 98.35 a 11.72 a 38.56 a 9.38 a Silicon 3 100.14 a 11.21 a 40.16 a 10.03 a Fungicide 100.80 a 10.24 a 39.43 a 8.80 a Values are means of three replicates. Mean values with similar letter are not significant at α=0.05.

There were significant (p<0.05) There were significant (p<0.05) differences observed among the differences found among the treatments in the percentage of infected treatments in the discoloration plants (Table 2). The result revealed percentage of grains (Table 2). The that percentage of infected rice plants result revealed that the discoloration was significantly affected by silicon percentage of grains was significantly supplements and fungicidal affected by silicon supplements and application compared to control. The fungicidal application compared to the application of Si foliar sprays caused a control. The application of Si foliar significant reduction (p<0.05) in sprays caused a significant reduction of percentage of infected plants compared grain discoloration compared to plants to plants receiving control treatment. receiving control treatment. There were Si-based products such as silicon 2 and no significant (p>0.05) differences 3 resulted in a similar disease control as between Silicon 1, Silicon 3 and the fungicidal solution but Silicon 1 fungicide application, but Silicon 2 treatment did not show the better significantly reduced (p<0.05) grain performance on the disease incidence discoloration compared to the compared to the other Si based fungicide. Si-based products resulted in products and fungicidal solution. In a similar disease control as the present experiment, all three Si-based fungicidal solution but Silicon 2 products applied as a foliar solution did treatment showed the better not perform equally. Silicon 2 and 3 performance in control of disease than were as equally-capable as fungicide the other Si-based products and application for controlling the disease. fungicidal solution.

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Table 2. Effect of silicon supplements and fungicide application on incidence of plant infection and discoloration grains

Treatments Percentage incidence Percentage of plant infection (%) of discolored grains (%) Control 78.21 a 8.12 a Silicon 1 54.96 b 4.42 b Silicon 2 43.04 c 3.75 c Silicon 3 42.49 c 4.07 b Fungicide 30.95 c 4.15 b Values are means of three replicates. Mean values with similar letter are not significant at α=0.05.

Role of silicon in plant health has been rice genotype decreased by SiO2, investigated in silicon accumulating amending to the soil increased. crops and it seemed significantly effecting (Jinab et al., 2008). Research Foliar application of Si-based products evidences proved that adequate uptake has been proved to reduce the disease of silicon can increase the tolerance of severity and incidence in plants by agronomic crops especially rice to both some studies. Ashtiani et al. (2012) abiotic and biotic stress (Ma and demonstrated that foliar application of Takahashi, 2002). It has been reported Si reduced disease severity and disease in previous studies that application of incidence of blast disease in rice. Foliar silicon materials and silicon-based application of Si has been reported to be fertilizers such as potassium silicate; effective in inhibiting powdery mildew sodium silicate and silica gel improved development on cucumber, growth and yield of rice plants as well muskmelon and grape leaves (Bowen et as enhanced plant resistance against al., 1992; Ma, 2004). Studies showed that biotic and abiotic stresses (Belanger et Si supplied via leaf application, at lower al., 1995; Datnoff et al., 1997; Seebold et rates, can be a viable alternative to al., 2001). In the present study, the provide this to plants, with results showed that application of particular beneficial effects, meeting the silicon supplements effectively reduced Si requirements and/ or stimulating Si the incidence of rice grain discoloration. uptake (Figueiredo et al., 2010; Sousa et These results further confirm previous al., 2010). Ma and Yamaji (2006) reports by Korndorfer et al. (1999), reported that Si applied to leaves Seebold et al. (2000) and Prabhu et al. deposit on the surface of the leaves and (2001). Korndorfer et al. (1999) showed play a similar role to that of Si taken up that the influence of silicon on rice grain from the roots. discoloration in Brazil. They reported that application of wollastine (source of 3.2 Identification of causal organisms silicon) increase the plant available Si in of grain discoloration disease in the soil dramatically reduced rice grain Batalagoda discoloration disease in Brazil. Seebold et al. (2000) reported that increased level Two species of genus Curvularia were of Si in the rice plant is associated with identified as causative agents of rice decreased level of grain discoloration at grain discoloration disease in harvest. Prabhu et al. (2001) showed Batalagoda area. Curvularia lunata and that the severity of grain discoloration Curvularia pallescence were isolated disease in several irrigated and upland from the discolored seeds. Curvularia

7 pallescens and Curvularia lunata have original description of Curvularia lunata been identified as causal agents of grain by Ou (1985) and Mew and Gonazales discoloration in rice by Ou (1985) (2002). Colonies on PDA at 28–30 °C whereas Curvularia lunata has been grew fast and attained an 8.40 cm reported as causative agent of rice grain diameter in 5 days. Colonies were discoloration by Kato et al. (1988) zonated and felted; the conidial area was greenish gray with a 3 mm sterile Curvularia lunata advancing margin and abundant grayish mycelial tufts (Fig.2 c). The Macroscopic features showed that colony appearance on the reverse side of colony characters resemble with the the agar plate was zonated and gray

b c a Fig.2. a) Discolored rice seeds b) Fully discolored single rice seed c) Curvularia lunata on PDA media

Microscopic features showed the Curvularia pallescence original spore and mycelial characters Macroscopic features resemble the of Curvularia lunata described by Ou original colony characters of Curvularia (1985) and Mew and Gonazales (2002). pallescence described by Freire et al. Mycelia were septated, branched, (1998). Cultures of C. pallescens subhyaline to light brown. developed on PDA showed that Conidiophores were dark brown, colonies were effuse gray in color, unbranched, septated, sometimes bent becoming black when older, black on and knotted at the tip. Conidia was the backside, presented slightly dark brown, boat-shaped, rounded at irregular borders and cottony growth the tip, mostly a little constricted at the with concentric zones (Fig. 4 c). base; with hilum scarcely or not at all protuberant, smooth walled, light to dark brown, with three septa; the 2nd cell was larger than the 1st, 3rd, and 4th cells; bent on the 2nd cell; borne at the tip, arranged in a whorl one over another or more or less spirally arranged or in thick panicles (Fig. 3). Fig.3. Curvularia lunata mycelia, conidiophores and conidia (40×10)

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a b c

Fig. 4 a) Discolored rice seeds b) Fully discolored single rice seed c) Curvularia pallescence on PDA media

Microscopic features showed the study revealed that Si-based original spore and mycelial characters formulations had the ability to reduce of Curvularia pallescens described by Ou the grain discoloration disease in rice. (1985) and Freire et al. (1998). Conidia Among the tested silicon supplements, were slightly curved, three septate, Gainexa UPL (Orthosilicic acid) gave with the medium septum somewhat the best result in term of suppression of off-center. In general, central cells were rice grain discoloration than the other broader and darker than the end cells. silicon supplements. Silicon application Some mature conidia were slightly did not yield a direct measurable effect collapsed after being liberated from the on plant height and chlorophyll conidiophores. Mycelium was mostly content, but showed considerable effect immersed, forming straight or on yield. Silicon foliar application flexuous. The conidiogenous cell was controlled the disease to the same broader at the apex and seemed to be general degree as a fungicide, thereby, inflated at first, with sympodial can reduce the amount of fungicide formation of conidia (Fig. 5). needed. Therefore, it could be an alternative viable method to synthetic fungicide application.

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