Histological responses to downy mildew in resistant and susceptible grapevines

Downy mildew in grapevines, caused by Plasmopara viticola, is a very serious disease throughout the grape-producing nations, especially in more humid climates. Downy mildew mainly affects the cultivated varieties of Vitis vinifera. A promising way to minimize or eliminate P. viticola infections is by the adoption of resistant cultivars. Chinese wild grapevines are reported to possess resistance to many fungal diseases. In this study, three Chinese wild grapevines (Vitis pseudoreticulata Baihe-35-1, Vitis davidii var. cyanocarpa Langao-5, and Vitis piasezkii Liuba-8) and a European cultivated variety (V. vinifera cv. Pinot noir) were inoculated with P. viticola, and a histological survey was undertaken. Macroscopic observations revealed no sporulation in V. piasezkii Liuba-8, little sporulation in V. pseudoreticulata Baihe-35-1 and V. davidii var. cyanocarpa Langao-5, but serious sporulation in V. vinifera cv. Pinot noir. Aniline blue staining indicated callose deposition in V. pseudoreticulata Baihe-35-1, V. davidii var. cyanocarpa Langao-5, and V. piasezkii Liuba-8. Cells with distinctive fluorescence were also observed in V. pseudoreticulata Baihe-35-1. After staining with 3,3-diaminobenzidine, production of H₂O₂ was observed early on, after infection in V. davidii var. cyanocarpa Langao-5 and V. piasezkii Liuba-8. No H₂O₂ accumulation was observed in V. vinifera cv. Pinot noir. It is concluded that V. piasezkii Liuba-8 should be classified as "highly resistant" to downy mildew, V. pseudoreticulata Baihe-35-1 and V. davidii var. cyanocarpa Langao-5 as "resistant," and V. vinifera Pinot noir as "susceptible." The possible roles of stomatal callose deposition in the defense r6eactions of the mildew-resistant grapevines are discussed.