Siderophore containing extract from Serratia plymuthica AED38 as an efficient strategy against avocado root rot caused by Phytophthora cinnamomi

Phytophthora cinnamomi Rands. is a widely distributed pathogen in avocado crops. Root rot caused by Phytophthora cinnamomi decreases the yield and growers are to establish new productive areas in order to meet demand which in turn is causing the loss of native forests. Therefore, sustainable production of avocado is only possible by addressing yield issues and the efficient use of lands. In this work, the effectiveness of two promising antagonistic bacterial isolates associated to healthy avocado trees to protect avocado seedlings against P. cinnamomi was assessed. First, the virulence of five isolates of P. cinnamomi from different avocado growing regions in Colombia was evaluated. The most virulent isolate was used to infect five-month-old avocado seedlings. The native bacterial isolates, both identified as Serratia plymuthica, were used to produce extracts in solid medium and their inhibitory activity against P. cinnamomi was tested. One of the extracts was selected and submitted to a bioguided metabolite isolation. A siderophore molecule (serratiochelin C) was purified and fully characterised by Nuclear Magnetic Resonance. Finally, the protective capacity of one bacterial extract was assessed in vivo against the infection of Phytophthora cinnamomi on avocado along with a commercial product based on fosetyl-aluminum. The crude metabolic extract of S. plymuthica AED38 was found to be effective and nontoxic on seedlings at 60 µg.mL−1. The results were comparable with the effect of fosetyl-aluminum at 0.5 µg.mL−1. S. plymuthica extracts possess a promising potential as bioproduct for the control of P. cinnamomi.