The mode of host resistance to Plasmopara viticola infection of grapevines.

ABSTRACT The resistance and susceptibility of grapevines to downy mildew (DM) disease caused by Plasmopara viticola were compared among different cultivars/accessions belonging to Vitis vinifera, V. rotundifolia, and 10 oriental Vitis species. After inoculation with P. viticola pathogen, no symptom was found in V. rotundifolia grapevines at all, while oriental species V. davidii and V. piasezkii, like V. vinifera, were susceptible to DM disease. The other eight oriental Vitis species showed various resistance levels to DM disease. Intraspecific resistant variations were also observed in V. amurensis. Microscopy studies were conducted on various time courses after pathogen infection on grape leaves. P. viticola hyphae were not observed in V. rotundifolia cultivars, while symptoms with varying degrees of severity were observed among the Euvitis species. In general, the DM resistant oriental species showed a slower development of hypha and less formation of haustoria than DM susceptible V. vinifera grapevines. Cells with distinctive fluorescence were observed in V. rotundifolia and the oriental species V. pseudoreticulata, and callose deposits were observed in V. rotundifolia, V. pseudoreticulata, and V. amurensis grapevines. Based on the results of morphological observations and microscopy studies, we concluded that there were five levels of grapevine resistance to P. viticola pathogen: (i) immune, (ii) extremely resistant, (iii) resistant, (iv) partly resistant, and (v) susceptible.