In vitro Control of Microbial Contamination of Sweet Potatoes Cultured with Nodal Explants

The use of nodal explants for in vitro propagation promotes direct regeneration of cultures, but may cause high levels of microbial contamination due to large size of the explants. We evaluated different surface sterilization protocols, and inclusion of antibiotics and fungicides in growth medium to control microbial contamination of sweet potatoes (Ipomoea batatas) propagated in vitro with nodal explants. Three surface sterilization methods (low- 70% ethanol for 1 min, followed by 10% NaOCl for 15 mins; moderate- 70% ethanol for 3 mins, followed by 20% NaOCl for 10 mins; and high- 90% ethanol for 3 mins, followed by 30% NaOCl for 10 mins), plus a control where only distilled water was used to rinse the explants, were assessed. In addition, hormone-free Murashige and Skoog (MS) basal medium was amended with different rates of Chloramphenicol and Benomyl. The results showed high levels of direct regeneration of plantlets, but microbial contamination was also high, which emanated from both endogenous and exogenous sources. Surface sterilization without medium amendments controlled mainly the exogenous contaminants, but endogenous contaminants were still problematic, leading to the contamination of cultures even 4 wks after inoculating. Amending the culture medium with Chloramphenicol and Benomyl, reduced microbial contamination but it inhibited the growth of plantlets. In general, the moderate surface sterilization plus amending the culture medium with 0.5 g l-1 Chloramphenicol and 0.1 g l-1 Benomyl was the most effective in reducing contamination and causing minimal inhibition to plantlet growth.