Yellows diseases, caused by phytopathogenic bacteria of the genus Phytoplasma, are a major threat to grapevines worldwide. Because conventional applications against this pathogen are inefficient and disease management is highly challenging, the use of beneficial bacteria has been suggested as a biocontrol solution. A Dyella-like bacterium (DLB), isolated from the Israeli insect vector of grapevine yellows (Hyalesthes obsoletus), was suggested to be an endophyte. To test this hypothesis, the bacterium was introduced by spraying the plant leaves, and it had no apparent phytotoxicity to grapevine. Fluorescent in situ hybridization analysis showed that DLB is colonizing grapevine phloem. Because phytoplasmas inhabit the same niche, DLB interactions with this phytopathogen were examined. When the isolate was introduced to phytoplasma-infected Chardonnay plantlets, morphological disease symptoms were markedly reduced. The mode of DLB action was then tested using bioinformatics and system biology tools. DLB genome analysis suggested that the ability to reduce phytoplasma symptoms is related to inhibition of the pathogenic bacterium. These results provide the first step in examining the potential of DLB as a biological control agent against phytoplasmas in grapevine and, possibly, other agricultural crops.

Candidatus Phytoplasma, the causative agent of yellows disease, inflicts substantial damage on several hundred plant species including perennials and annual plants. The endophytic bacterium Frateuria defendens reduces the symptoms of yellows disease in a number of agricultural crops. One possible mode of action is that the bacterium secretes antimicrobial metabolites. To test this hypothesis, the substances secreted by the endophyte during 10 days of growth in an artificial medium were identified by GC-MS (gas chromatography–mass spectrometry). Synthetic analogues to these substances were then used on periwinkle, a nurse culture plant infected by phytoplasma. Phytoplasma quantities were evaluated by quantitative PCR, and disease symptoms were monitored and recorded. It was found that specific compounds identified by the biochemical analysis caused a significant reduction in both the titer of phytoplasma and the disease symptoms in periwinkle when compared to untreated infected plants. Further research is required to examine the potential of these compounds as an effective treatment against yellows disease.

Bois Noir is a major yellows disease in grapevine with no current means of control. The endophyte Frateuria defendens (Frd), isolated from the insect vector of Bois Noir, is a potential biocontrol agent for this disease. The aim of this study was to determine an efficient way to introduce Frd into mature vines under field conditions. Shoots of young and field-grown vines were sprayed with Frd suspension supplemented with either Tween 20, BB5, DX, or Triton X-100 as surfactants. The presence of Frd was confirmed by PCR in sprayed leaves, and in leaves below and above the sprayed site. The results showed that cells of Frd penetrate and move inside the vine shoots under field conditions. Highest penetration rate was achieved when leaves were sprayed with 108 or 109 colony forming units per mL (CFU mL-1) of Frd with addition of 0.1% Tween 20. The addition of a surfactant is essential to increase the proportion of shoots with Frd.