| Summary: | <i>Candidatus</i> Liberibacter solanacearum (<i>Ca</i>Lso) is associated with diseases in tomato crops and transmitted by the tomato psyllid <i>Bactericera cockerelli.</i> A polymeric water-dispersible nanobactericide (PNB) was evaluated against <i>Ca</i>Lso as a different alternative. PNB is a well-defined polycationic diblock copolymer designed to permeate into the vascular system of plants. Its assessment under greenhouse conditions was carried out with tomato plants previously infected with <i>Ca</i>Lso. Using a concentration as low as 1.0 mg L<sup>−1</sup>, a small but significant reduction in the bacterial load was observed by real-time qPCR. Thus, to achieve an ecologically friendly dosage and set an optimum treatment protocol, we performed experiments to determine the effective concentration of PNB to reduce ~65% of the initial bacterial load. In a first bioassay, a 40- or 70-fold increase was used to reach that objective. At this concentration level, other bioassays were explored to determine the effect as a function of time. Surprisingly, a real reduction in the symptoms was observed after three weeks, and there was a significant decrease in the bacterial load level (~98%) compared to the untreated control plants. During this period, flowering and formation of tomato fruits were observed in plants treated with PNB.
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