Removal of Persistent Bacterial Contaminants from In Vitro Shoot Cultures of Raspberry (Rubus idaeus L.) Using Vacuum Infiltration and Its Effect on Multiplication Efficiency

The aim of this study was to find a way to remove persistent bacteria inhabiting in vitro shoot cultures of raspberry. Often, decontamination treatments fail to reach bacteria residing in internal tissues, leading to contaminated cultures later. Three raspberry cultivars, each harboring a unique bacterial contaminant, were used in this study. Experiments were conducted to assess the potential for eliminating these bacteria using biocide infiltration at 30 mbar. The following biocides were used: mercuric chloride (HgCl2 at 0.05 and 0.1%), Plant Preservative Mixture (PPMTM 0.2–4%), rifampicin (50–200 mg L−1), and sodium hypochlorite (NaOCl 0.1–60%). Only 0.05 or 0.1% HgCl2 applied via infiltration successfully eliminated all of the bacteria from the shoots, which remained bacteria-free for several years, as confirmed by indexing explants on bacterial media at each subculture. While most treated shoots became necrotic and died due to infiltration, the surviving shoots remained vital and provided bacteria-free material for long-term propagation. Results from experiments comparing micropropagation potential in bacteria-contaminated and bacteria-free cultures showed that bacteria-free shoots produced longer shoots, and the total number of shoots did not differ, except for “Norna”/Curtobacbacteria-free cultures, which were more productive. Bacteria-contaminated shoots rooted at higher percentages, but roots were much shorter, and plantlets initiated growth during acclimatization later. Cultures that were contaminated did not survive storage at 4 °C in the dark for 4–6 months.