Resistance to powdery mildew in grapevine induced by Plasmopara viticola

Prior inoculation of grapevine plants with the downy mildew fungus, Plasmop ara viticola, induced a resistance to the powdery mildew fungus, Uncinula n ecator. Fourteen days after exposure to airborne conidia of U. necator, 1.5 and 23.9 colonies of powdery mildew per leaf developed on Plasmopara-inocu lated and on control (noninoculated) plants, respectively. This resistance was confined to leaf tissues colonized by P. viticola and was not transferr ed to the new developing leaves. No protection was achieved when P. viticol a failed to infect the plants. Microscopic examination indicated that conid ia of U. necator could germinate on leaves infected with downy mildew, but could not produce secondary hyphae and colonies. This protection was revers ed by the exogenous foliar application of 0.1% sucrose solution to leaves i nfected by P. viticola, which resulted in the restoration of susceptibility to powdery mildew. Application of a 0.1% solution of sucrose enhanced the development of U. necator on both water-treated control plants and plants i noculated with P. viticola. Sucrose had a 2x and 3x stimulatory effect on p owdery mildew colony numbers on water-treated control plants and plants ino culated with P. viticola, respectively. Reciprocal inoculations indicated t hat infection by U. necator did not inhibit the development of P. viticola.