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ROLE OF SILICA IN Plant DISEASE MANAGEMENT 1

INTRODUCTION Silicon is the second most abundant element in the earth crust. It is readily taken up by plants and often present in relatively high concentration of plant tissues.Silicon concentration in plant tissue some time even exceed the concentration of nitrogen and potassium (Epstein, 1994) there for silicon is often major constituent of plant tissue. Some plant such as many dicot are known as silicon non accumulator and have tissue concentration of 0.5% or less (Marschner 1995).

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Other plants like wetland grasses are known as Si accumulator because they tend to have relative high concentration of (5% or higher) of tissue Si (Epstein, 1994). Si has been shown to be beneficial elements for many terrestrial plants. The beneficial effects of adequate Si includes decreased susceptibility to fungal pathogen and insects.It helps in decreasing abiotic stresses and increase growth in some plants.3

Silicon(Si) isthesecondmostabundantmineralelement in soilcomprisingapproximately28%of theearth'scrust.

Silicon is deposited in the form of silica gel or biogenetic opal as amorphous SiO2 n H2O in cell walls and intercellular spaces of root and leaf cells as well as in bracts.

Silicon also can be found in the form of monosilicic acid, colloidal silicic acid, or organosilicone compounds in plant tissues.4

HistoryIsenosuke Onodera, Japanese plant nutrient chemist the first researcher who suggested that Si was involved in rice resistance to blast.

5Onodera, published a milestone paper entitled Chemical studies on rice blast disease. This is the first report on Si research published in a scientific journal of agronomy.

Hayashi and Miyake and Ikeda also showed that the application of Si increased resistance to blast as well as increased silica content of rice.

Kawashima first demonstrated under controlled conditions that application of Si to rice plants increased resistance to blast as well as increased Si content in rice. 6

7This study focused on the uptake and deposition of Si in sugercane as well as its severity of brown rust of sugercane caused by Puccinia melanocephala.Both trials consisted of 9 treatments i.e. 100 ,200 ,300, 400,800, 1200, 1600, 2000 mg/ lit. potassium silicate, applied on a week for 8 weeks and Calmasil. A commercially available form of calcium silicate , applied at the recommended dosage of 52 g / lit incorporated potting soil at planting.

CONTENTSIntroductionRice Diseases.History. Role of silica in disease managementCase studiesConclusionFuture silicon research needs.8

Rice diseasesBlast of rice co: Magnaporthe griseaBacterial leaf blight co: Xanthomonas oryzae pv. oryzaeBrown spot of rice co: Drechslera oryzaeGrain discoloration of rice co: Bipolaris oryzaeStem rot of rice co: Sclerotium oryzaeNeck blast of rice

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Grain discolorationNeck blastBrown spotLeaf blast

Bacterial leaf blight10

11Dry weight and silicon concentrations in shoots and roots of rice plants amended with silicon (+Si) or not (Si) andinoculated with M. grisea (+M. grisea) or not (M. grisea)

12Leaf blast symptoms

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Influence of calcium and mercuric fungicides alone and in combine blast incidence and grain yield/ha % of Neck blast Grain wt.( g/2.9m2)Treatment 50kg/ha 75kg/ha 50kg/ha 75kg/haSilicon ( Si)* 12 11.2 1398.7 1415.7Fungicide ( Fu)** 10.1 7.4 1302.0 1357.3Si*Fu 1.7 2.5 1425.0 1504.7Control 26.5 42.5 1018.0 1012.7

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Calcium silicate applied at 2.25 ton/ha.** Mercuric fungicides= phynel murcuric acetate:calcium carbonate mixture (1:5) applied at 40kg/ha.18

Effect of propiconazole and silicon on brown spot development19Treatment Lesion no. per cm2 Brown spot severitybControl 2.5ac 87aPropiconazole (P) 2.0b 61bSilicon 1.6c 37cP+Si 0.6d 14d

a-area under disease progress curved, b-Brown spot severity based on 0-9 disease scale,where,0= no disease 9=76% or more leaf each numerical rating was used for establishing differences between treatments, c- means followed by different letter are significantly different based on FLSD P=0.05.

Si and greenhouse cucumber diseases Greenhouse cucumber are often grown in nutrient solution( hydroponically) without added Si. This provides an excellent setting for investigating the effects of Si on cucumber growth and disease.The effects of Si fertilization on greenhouse cucumber infection by powdery mildew and Pythium spp. have been studied very throughly.Cucumber leaves were inoculated with known conidial concentration ( Menzies et.al., 1991 ).The investigation found that Si fertilization reduce the leaf area covered by powder by as much as 98%.

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The inoculated cucumber receiving Si fertilization were productive as they uninoculated control in this study. However, among the uninoculated (disease free ) plants, those receiving Si fertilization did not differ significantly in productivity from those not receiving Si fertilization. This seems to indicate that Si improve cucumber health and productivity in the presence of pathogen.

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Nutrient solution concentration was 100 to 200 ppm SiO2 of potassium silicate.It reduces the root mortality,root decay and yield losses on the plant inoculated with pythium ultimum.Furhtermore,in Si potassium silicate fertilized by Pythium inoculate plants, root dry weights and number of fruits ( especially high grade fruits)were significantly higher than for Pythium inoculated plants without Si fertilization ( Cherif and Belanger,1992).22

The effect of potassium silicate on Phakopsora pachyrhizi on soybeanPhakopsora pachyrhizi, the causal organism of soybean rust, is a fungal pathogen causing significantly soybean losses throught the world.To determine the effect of variouse concentration of potassium silicate, two trials were conducted where Si was applied as a foliar spray and a root drench.For the foliar treatment trials were follows: 0,1000,2000,4000mg/lit Si, root drench (100mg/lit Si ) and Punch C (800ml/ha)+10000mg/lit Si Punch C (400ml/ha ), Punch C ( 40oml/ha) +10000 mg/lit Si. 23

Si, Fly ( 95, 189, 378 kg/ha ) K Si slow release fertilizer ( 59, 118, 236 kg/ha ) and Na Si slow release fertilizer ( 62, 124, 248 kg/ha ), applied twice during the season.For the foliar trial, Punch had a significantly lower area under the disease progress curve and higher yield than all other treatment.All foliar Si application had a significantly lower AUDPC compare to the untreated control. 24

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AUDPC decreases of 20, 12 and 14% for 1000, 2000 and 4000mg/lit Si and yield increases 12, 6 and 10% for 1000, 2000 and 4000mg/lit Si were observed.For the drench trial, all Punch C treated plants had significantly lower AUDPC and higher yield than all other treatments.AUDPC of Si treatments were not significantly different from that of the .untreated control.25

26This study focused on the uptake and deposition of Si in sugarcane as well as its effects on the severity of brown rust of sugercane , caused b y Puccinia melanocephala.Both trials consisted of 9 treatments i.e. 100, 200, 400, 800, 1200, 1600, 2000 mg/lit potassium silicate, applied once a week and casmil, a commercially available form of the calcium silicate, applied at doses 52g/lit incorporated in the potting soil at planting.For the disease severity trials ,plants were naturally infected with P,melanocepha by placing them in a tunnel with brown rust infected spreader plant. Effect of silicon on rust of sugarcane

From 3 weeks after planting,plants were rated weekly for 5 weeks for percentage disease severity using a rating scale.Percentage disease severity was reduced from 85% in the control to 64% in plant treated with silicon at 200mg/lit.

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The effect of potassium silicate on the control of Phakopsora pachyrhizi on soybean Phakospora pachyrhizi the causal organism of soybean rust a fungal pahtogen causing significant yield losses throught out the world.To determine the effect of variouse concentration of silicon (potassium silicate ) two trials were conducted where Si was applied as a foliar spray and as root drench.For the foliar trials treatment were as follows 0,5000,10000mg/lit. Si Punch C (800ml/ha), Punch C (800mg/lit)+10000mg/lit Si,Punch C (400ml/ha) and 10000mg/lit Si.

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CONCLUSIONAlthought silicon is not consider to be an essential nutrient for plant. But beneficial to many plants.Si has the potetial to significantely decrease the susceptibility of certain plants to both abiotic and biotic disease.In plants such as rice, Si fertilization may even increase growth and yield in addition to reducing disease severity.As we learn more about the important of silicon in plant physiology, we may find more ways to use this important element in plant health and disease resistance.29

FUTURE SILICON RESEARCH NEEDSSource of silicon and their management practices should be develop and practices in integrated disease management programme for those crop where Si has demonstrated to have positive effect.Some Si sources have residual activity that persist over time, raising the possibility that applicaton need not be applied annually.There is need to find or develop cheaper and more efficient Si sources.Effective, practical means of application and affordabe sources of Si are needed for used row crop agriculture.30

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