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Phylloxera galls as Plasmopara viticola infection and sporulation sites on leaves of grapevines partially resistant to downy mildew

ChengFang Hong: University of Georgia

<div>Downy mildew (<em>Plasmopara viticola</em>) and phylloxera (<em>Daktulosphaira vitifoliae</em>) are two important pests in many grape production areas worldwide. Both species can produce severe symptoms on leaves, but no interactions between them have been reported previously. In 2017, <em>P. viticola</em> was found sporulating on galls of phylloxera-affected leaves of interspecific hybrids ‘Blanc du Bois’ and ‘Lenoir’ in vineyards in southern Georgia and northern Florida; no signs of infection were observed on leaves not affected by phylloxera. ‘Blanc du Bois’ is generally considered resistant to downy mildew once leaves have matured. Leaves harboring <em>P. viticola</em> were collected and the pathogen was subcultured on <em>Vitis vinifera</em> ‘Chardonnay’. Artificial inoculations with two isolates of <em>P. viticola</em> (Pvv731S1 and Pva1068) were conducted on ‘Blanc du Bois’ leaf disks with or without phylloxera galls to determine the effects of the galls on infection efficiency and sporulation of the pathogen. Each leaf disk was inoculated with 5 µl of a sporangium suspension (10<sup>4</sup> sporangia/ml), and disks were incubated at 20<sup>o</sup>C for 16-24 h in the dark followed by 6 days under diurnal light. Preliminary results revealed that phylloxera galls increased infection efficiency of isolate Pva1068 significantly (<em>P</em> = 0.0069 based on Chi-Square analysis). This is the first report showing that phylloxera galls can compromise resistance to downy mildew and serve as a reservoir for <em>P. viticola</em> on resistant grape cultivars.</div>