Resistance to powdery mildew in grapevine induced by Plasmopara viticola

Prior inoculation of grapevine plants with the downy mildew fungus, Plasmopara viticola, induced a resistance to the powdery mildew fungus, Uncinula necator. Fourteen days after exposure to airborne conidia of U. necator, 1.5 and 23.9 colonies of powdery mildew per leaf developed on Plasmopara-inoculated and on control (noninoculated) plants, respectively. This resistance was confined to leaf tissues colonized by P. viticola and was not transferred to the new developing leaves. No protection was achieved when P. viticola failed to infect the plants. Microscopic examination indicated that conidia of U. necator could germinate on leaves infected with downy mildew, but could not produce secondary hyphae and colonies. This protection was reversed by the exogenous foliar application of 0.1% sucrose solution to leaves infected 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 inoculated with P. viticola. Sucrose had a 2x and 3x stimulatory effect on powdery mildew colony numbers on water-treated control plants and plants inoculated with P. viticola, respectively. Reciprocal inoculations indicated that infection by U. necator did not inhibit the development of P. viticola.