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Recent outbreaks and continued spread of diseases caused by phytoplasmas in potato, tomato and other vegetable crops in the USA accentuates the need for practical strategies to mitigate the impact of the phytoplasmal diseases. The purpose of this study was to investigate whether exogenous application of salicylic acid (SA) on healthy tomato seedlings would enhance the plants' defence against a subsequent potato purple top (PPT) phytoplasma infection. Our results showed that a twice-applied SA pretreatment, at 4 and 2 days before phytoplasma inoculation, significantly decreased the rate of PPT phytoplasma infection and reduced disease symptoms. At 40 days postinoculation (dpi), while 94% of the PPT phytoplasma-inoculated control plants exhibited characteristic PPT symptoms, 53% of the SA-pretreated, PPT phytoplasma-inoculated plants remained symptom-free with no molecular evidence of phytoplasma infection. The remaining 47% of SA-treated plants became infected, but symptoms were much milder and the average phytoplasma titre was more than 300 times lower compared with that in control plants. Real-time qRT-PCR assays revealed that SA-pretreatment caused transcriptional reprogramming of three defence-related genes. Up-regulations of a WRKY-type transcriptional factor gene (LeWRKY1) and a mitogen-activated protein kinase gene (LeMPK3) in an early stage following graft inoculation, and a sustained higher level expression of a downstream pathogenesis-related protein gene (LePRP1), were observed in SA-pretreated, PPT phytoplasma-inoculated plants. With the dosage and treatment regime implemented in this study, no noticeable phytotoxic effects or other negative impact on vegetative growth or reproductive development was observed in SA-treated plants throughout our experiment. Our findings encourage field trials of SA pretreatment as a possible approach to protect crops from phytoplasmal diseases.