Apple tissue culture contamination by <Emphasis Type="Italic">Rhodotorula</Emphasis> spp.: Identification and prevention

Summary Shool 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 (588μM) and Silvet 77 (0.01%) for 1–2h, and then transferred to a solidified MS medium containing both mancozeb (15mgl⁻¹) and thiabendazole (40mgl⁻¹).     

Control of <Emphasis Type="Italic">In vitro</Emphasis> contamination of explants from greenhouse- and field-grown trees

Summary Controlling fungal and bacterial contamination of woody plant material can be extremely difficult. Isothiazolone biocides and sodium dichloroisocyanurate have been used singly and in combination to reduce microbial contamination in bud explants derived from greenhouse- and field-grown citrus trees. Explants from greenhouse-grown trees were effectively disinfested (<5% vs. 85% contamination) using a ‘standard’ disinfestation (SD) procedure followed by culture on medium containing 5 ml l⁻¹ Plant Preservative Mixture, or by the SD procedure but substituting 300 ppm sodium dichloroisocyanurate for 48 h for the treatment with 1.05% NaOCl in the SD procedure. Disinfestation of explants from field-grown trees was less effective than explants from the greenhouse, but was improved (10% vs. 47% contamination) by using a combination of the SD procedure with 1.05% NaOCl, or 100 or 300 ppm sodium dichloroisocyanurate followed by culture onto medium containing 5 ml l⁻¹ Plant Preservative Mixture. Mention of a trademark, warranty, proprietary product, or vendor does not constitute a guarantee by the US Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable.