SOLUBLE SILICON SPRAYS INHIBIT POWDERY MILDEW DEVELOPMENT ON GRAPE LEAVES

The effect of root or leaf applications of soluble Si on severity of grape (Vitis vinifera L.) powdery mildew [Uncinula necator (Schwein) Burrill] was determined. On potted plants, root-feeding at 1.7 mM Si had no effect on disease severity, but foliar sprays at 17 mM Si substantially reduced the number of mildew colonies that developed on inoculated leaves. Scanning electron micrographs showed that, on Si-sprayed leaves, hyphae did not develop in areas where thick Si deposits were present on the leaf surface; and where surface deposits were not present, Si was translocated laterally through the leaf and surrounded the appressoria. Leaves on plants that were fed Si via roots showed a similar deposition of Si surrounding the appressoria. On water-sprayed leaves and leaves from untreated plants, internal deposition of Si was more variable and generally less than on Si-sprayed or root-fed plants. Conidia germination and germtube development on agar media were weakly promoted by the presence of Si. Reduced severity of grape mildew by Si sprays may be partly due to a physical barrier to hyphal penetration and to a resistance response involving the lateral movement of Si and its deposition within the leaf at fungal penetration sites.