Biological control of black rot (Guignardia bidwellii) in mildew-resistant grapevine cultivars

Abstract: Guignardia bidwellii (anamorph: Phyllosticta ampelicida) causes sporadic outbreaks of black rot in grapevines but has so far had relatively little economic impact on European viticulture. The pathogen is commonly suppressed as an incidental effect of fungicides targeting the more economically significant pathogens of the grapevine, Plasmopara viticola and Erysiphe necator. In recent years, the increasing cultivation of resistant grapevine cultivars with increased tolerance to downy and/or powdery mildew (DM/PM) has led to a reduction in fungicide applications, which has facilitated the emergence of other fungal pathogens, such as G. bidwellii. Resistance to black rot has been observed in selected grapevine cultivars, and initial QTLs associated with this trait have been identified.
In this study, DM/PM-resistant grapevine cultivars with differing susceptibility to black
rot were phenotyped under greenhouse conditions in artificially inoculated potted grapevine
plants. Disease severity was classified and associated with the presence of Rgb1 and Rgb3
resistance alleles using a newly established multiplex PCR assay. A clear association between
leaf resistance and Rgb1 resistance alleles was observed. Resistance alleles for Rgb1 and Rgb3
were identified for the first time in the cultivar Souvignier Gris, confirming its potential value
for resistance breeding.
However, especially for integrated plant protection concepts that rely on preventive
measures such as the cultivation of DM/PM resistant varieties, effective yet environmentally
friendly protection solutions are needed to prevent black rot infections in susceptible cultivars.
In vitro bioassays demonstrated a significant inhibitory effect of several Trichoderma-based products and Bacillus amyloliquefaciens on mycelial growth of G. bidwellii. In addition,
Primula veris root extract showed strong antifungal activity in vitro and significantly reduced
disease severity on grapevine leaves under greenhouse conditions. The results provide new data for resistance breeding and support the integration of biological control strategies into black rot management in sustainable viticulture.