Silicon on the control of plant diseases

Fabrício Á. Rodrigues

Universidade Federal de Viçosa, Departamento de Fitopatologia, Laboratório da Interação Planta-Patógeno. Viçosa, MG, CEP 36570-000, E-mail: fabricio@ufv.br.

Silicon (Si) is the second most abundant mineral element in soil and comprises approximately 28% of the earth's crust (Elawad and Green, 1979). Many plants are able to uptake Si. Depending upon the species, the content of Si accumulated in the biomass can range from 1 to greater than 10% (Elawad and Green, 1979; Epstein, 1991). Plant species are considered Si accumulators when the concentration of Si (in dry weight basis) is greater than 1% (Epstein, 1999). Relative to monocots, dicots such as tomato, cucumber, and soybean are poor accumulators of Si with values less that 0.1% of Si in their biomass. Dryland grasses such as wheat, oat, rye, barley, sorghum, corn, and sugarcane contain about 1% of Si in their biomass, while aquatic grasses have Si content up to 5% (Jones and Handreck, 1967; Epstein, 1991, 1999). Silicon is taken up at levels equal to or greater than essential nutrients such as nitrogen and potassium in plant species belonging to the families Poaceae, Equisetaceae, and Cyperaceae (Savant et al., 1997). Although Si has not been considered an essential element for crop plants for lack of supportative data, species such as Equisetum and some diatomaceas cannot survive without an adequate level of Si in their environment (Epstein, 1999).

The beneficial effects of Si, if direct or indirect, to plants under biotic and/or abiotic stresses have been reported to occur in a wide variety of crops such as rice, oat, barley, wheat, cucumber, and sugarcane (Liang et al., 1996; Savant et al., 1997; Hodson and Sangster, 2002). The most significant effect of Si to plants, besides improving their fitness in nature and increasing agricultural productivity, is the restriction of grazing and parasitism (Bélanger et al., 1995; Datnoff et al., 1997; Savant et al., 1997; Rodrigues & Datnoff, 2005). Besides the many agronomic benefits gained by maintaining adequate levels of Si in the soil, this element has reduced the intensity of many diseases in important crops such as rice, cucurbits, grape, wheat, corn, and sorghum which will be described thereafter.

The majority of research using Si to control plant diseases has been conducted with rice. Suzuki (1935) reported that Si application to paddy soils enhanced rice resistance to blast (Magnaporthe grisea (anamorph Pyricularia grisea)). Volk et al. (1958) observed that the number of blast lesions on leaves of Caloro rice cultivar decreased linearly as the Si content in leaf blades increased. Datnoff et al. (1991) noted a significant reduction of neck blast in rice plants growing in a Si-deficient Histosol in southern Florida amended with 5, 10, and 15 Ton ha^-1 of calcium silicate slag. Additional studies conducted with calcium silicate slag revealed that finely ground grades were more effective than more coarsely ground grades at reducing the intensity of neck blast (Datnoff et al., 1992). Datnoff and Snyder (1994) demonstrated that reductions in the severity of neck blast brought about by the application of 0.4 Ton Si ha^-1 did not differ significantly from those achieved by applying a labeled rate of the fungicide benomyl. Studies conducted by Seebold et al. (1997, 1998) revealed that Si alone at 0.1 Ton ha^-1 was as effective as labeled rates of fungicides edifenphos and tricyclazole for the control of leaf and neck blast at low levels of intensity. Furthermore, application of Si plus a 10-25% rate of fungicide provided control equal to the full rate of fungicide under conducive conditions for leaf and neck blast development. Indeed, a single application of Si had a significant effect in controlling leaf and neck blast in the next rice season. At one experimental area in eastern Colombia, Seebold et al. (2000) found that application of Si reduced the severity of rice blast in a partially resistant cultivar to levels of severity observed in a resistant cultivar not amended with Si.

Rodrigues et al. (2001) showed that Si can decrease the severity of sheath blight in both tropical japonicas and an indica type rice cultivar. The authors noted that Si reduced the intensity of sheath blight of two susceptible (Lemont and Labelle) and two moderately susceptible (Drew and Kaybonnet) rice cultivars to levels of intensity observed in two cultivars (Jasmine and LSBR-5) with high partial resistance to sheath blight but not amended with Si. The application of Si to a Si-deficient typic acrustox red yellow latosol significantly reduced the total number of sheath blight lesions on sheaths, the total area under the relative lesion extension progress curve, the severity of sheath blight, and the highest relative lesion height on the main tiller by 37, 40, 52, and 24%, respectively, in six rice cultivars as the rate of Si increased from 0 to 8 Ton ha^-1 (Rodrigues et al., 2003c). Rodrigues et al. (2003b) studied the effect of Si and rice growth stages on tissue susceptibility to sheath blight and observed that as the rates of Si increased in the soil, the intensity of sheath blight was significantly reduced at all rice growth stages.

Brown spot (Cochliobolus miyabeanus), stem rot (Magnaporthe salvinii), leaf scald (Monographella albescens), and grain discoloration (caused by a complex of insects and fungus species) are other diseases controlled by Si (Datnoff et al., 1991, 1992; Deren et al., 1994; Savant et al., 1997; Seebold et al., 2000). Regarding bacterial diseases, Chang et al. (2002), reported reduction in lesion length of bacterial leaf blight (Xanthomonas oryzae pv. oryzae) from 5 to 22% among four rice cultivars upon Si application.

Silicon also shows potential to control diseases in other plants than rice. The application of Si in the form of potassium or sodium silicates to recirculating nutrient solutions reduced the severity of powdery mildew (Sphaerotheca fuliginea) in cucumber (Menzies et al., 1991). Foliar sprays of potassium silicate at and above 17 mM (1000 ppm) were also effective to control powdery mildew on muskmelon and zucchini (Menzies et al., 1992). Chérif and Bélanger (1992) found that 1.7 mM (100 ppm) of potassium silicate significantly reduced plant death, root decay, and yield loss caused by Pythium ultimum on cucumber. The incidency of cucumber plants attacked by P. aphanidermatum also decreased by adding Si to the nutrient solution (Chérif et al., 1992). Fusarium wilt of cucumber is another disease controlled by application of Si to the soil (Miyaki and Takahashi, 1983). Bowen et al. (1992) reported that application of potassium silicate to soil at 1.7 mM did not reduce the number of colonies of powdery mildew (Uncinula necator) on grape leaves while foliar sprays of potassium silicate at the same rate reduced the number of powdery mildew colonies by more than 60% on inoculated leaves. Powdery mildew (Blumeria graminis f.sp. tritici) on wheat has been efficiently controled by application of Si (Rodgers-Gray & Shaw, 2000). Wheat plants amended with Si at both field and greenhouse trials showed reduced severity of leaf blotch (Septoria tritici) and foot rot (Fusarium spp.) (Rodgers-Gray & Shaw, 2000). Silicon also reduced the incidence of stalk rot of corn caused by P. aphanidermatum and Fusarium moniliforme (Sun et al., 1994).

The epidemic rate of some diseases can be dramatically affected by Si. In the rice-M. grisea pathosystem, Seebold et al. (2000) found that application of Si to a Si-deficient soil significantly reduced the epidemic rate of blast on a highly susceptible rice cultivar than in the same cultivar not amended with Si. Based upon these results, it was hypothesized that one or more components of rate-reducing, or partial resistance (Parlevliet, 1979) were affected by Si. In a subsequent study, Seebold et al. (2001) evaluated the effect of Si on several components of resistance in four rice cultivars with different levels of resistance to race IB-49 of M. grisea grown in Si-deficient soil amended with 0, 2, 5, and 10 Ton Si ha^-1. The authors found that the number of sporulating lesions was the component of resistance mostly affected by Si. When Si was applied at 10 Ton ha^-1 to rice cultivars M201 (susceptible), Rosemont (partially resistant), or Lemont (partially resistant), the number of sporulating lesions was reduced by more than 70%. The lower number of sporulating lesions represents a reduction in the production of conidia by M. grisea, and, consequently, a decrease in the epidemic rate which will become important when rice is grown under conducive conditions for blast development.

It is very interesting to know how does Si controls diseases in plants. In the rice-M. grisea pathosystem, increased resistance through Si treatment has been associated with the density of silicified buliform, long, and short cells in the leaf epidermis that act as a physical barrier to impede penetration by M. grisea (Ito and Hayashi, 1931). This physical barrier hypothesis is strengthened by the findings of Yoshida et al. (1962), who reported the existence of a layer of silica of approximately 2.5 µm thick beneath the cuticle of rice leaves and sheaths. Rodrigues et al. (2003a) provided the first cytological evidence that Si-mediated resistance to M. grisea in rice correlated with specific leaf cell reaction that interfered with the development of the fungus. The accumulation of an amorphous material that stained densely with toluidine blue and reacted positively to osmium tetroxide was a typical feature of cell reaction to infection by M. grisea in samples from plants amended with Si. Rodrigues et al. (2004) found also that leaf extracts from plants inoculated with M. grisea and amended with Si showed higher levels of momilactone phytoalexins than leaf extracts obtained from inoculated plants not amended with Si or non-inoculated plants amended or not with Si.

In conclusion, that Si plays an important role in the mineral nutrition of many plant species is not in doubt nor is its ability to efficiently control several diseases. Effective, practical means of application and affordable sources of Si are needed for use in row crop agriculture in particular. As the need for environmentally friendly strategies for management of plant diseases increases, Si could provide a valuable tool for use in crops capable to accumulate it. The use of Si for controlling plant diseases would be well-suited for inclusion in integrated pest management strategies and would permit reductions in fungicide use. As researchers and growers become aware of Si and its potential in agriculture, it is likely that this often overlooked element will be recognized as a viable means of managing important plant diseases in a more sustainable food production way.

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