Apple tissue culture contamination by Rhodotorula spp. Identification and prevention

Shoot cultures of apple cv. Pinova were contaminated with faint pink pigmented yeast. Yeast isolates were identified as Rhodotorula slooffiae with standard physiological methods and molecular analysis. Growth of isolated yeasts was tested against different fungicides. The following fungicides inhibited the growth of yeast isolates, and were not phytotoxic to apple shoots at concentrations lower than the minimal phytotoxic concentrations (MPC): ProClin® 300, mancozeb, triforine, myclobutanil, thiabendazole, mancozeb + zoxamid, and silver nitrate. Some fungicides inhibited growth of yeasts, but were phytotoxic. These included miconazole, PPM™, copper sulfate, potassium sorbate, and cycloheximide. Benomyl was not phytotoxic, but was effective only at high doses. Decontamination of shoots was achieved using a combination of two treatments. Shoots were first soaked in half-strength Murashige and Skoog (MS) liquid medium containing silver nitrate (588 μM) and Silvet 77 (0.01%) for 1-2h, and then transferred to a solidified MS medium containing both mancozeb (15 mg l^-1) and thiabendazole (40 mg l ^-1).