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“Flavescence dorée” (FD)-related phytoplasmas are widespread in alder in Germany and their transmission to grapevine represents a high risk for FD outbreaks when the primary vector, <i>Scaphoideus titanus</i>, becomes present in the future. Therefore, the potential role of the Deltocephalinae leafhopper species in transmitting FD-related phytoplasmas from alder to grapevine was studied in extensive transmission trials conducted between 2017 and 2020. The transmission capacity of autochthonous <i>Allygus</i> spp. and the invasive <i>Orientus ishidae</i> captured on infected alder trees was tested under controlled conditions using various test designs, including grouped insects and single-insect studies. The latter experiments were analyzed in terms of survival probability, transmission success and phytoplasma load in the insects, measured by quantitative PCR. A minimum inoculation titer (MIT) required for successful transmission to alder was defined for both <i>Allygus</i> spp. and <i>O. ishidae</i>. While <i>Allygus</i> spp. exhibited slightly better survival on <i>Vitis vinifera</i> compared to <i>O. ishidae,</i> the latter displayed higher phytoplasma loads and greater transmission success. Although all species were capable of infecting alder seedlings, <i>O. ishidae</i> was able to transmit 16SrV-phytoplasmas directly to single grapevines. Infective adults of <i>O. ishidae</i> were captured from the beginning of July until the end of August, while <i>Allygus</i> spp. were only considered infective towards the end of the season. Thus, <i>O. ishidae</i> likely poses a higher risk for FD transmission from alder to grapevine, albeit at a very low level, as only five out of 90 transmission trials to <i>V. vinifera</i> were successful.