Bacillus velezensis MT9 and Pseudomonas chlororaphis MT5 as biocontrol agents against citrus sooty mold and associated insect pests

Sooty mold is a plant disease caused by Aleurocanthus spiniferus, an alien insect spread widely in the South of Puglia region (Italy) that causes extensive damage to citrus crops. Sooty mold is a complex disease: the insect sucks the sap of the plant and expels a sugary liquid called honeydew which supports the growth of a black biofilm on the plant. This biofilm is made up of various types of fungi and, while not causing disease, it con-tributes to plant malaise by inhibiting photosynthesis. In this study, microbiological and chemical methods have been used to characterize two bacterial strains, Bacillus velezensis MT9 and Pseudomonas chlororaphis MT5, which were isolated from soil samples taken from vine-cultivated land and can be used as biocontrol agents (BCA). These bacteria were shown to produce antibacterial and antifungal activities in the form of water-soluble mol-ecules and were effective in vitro in inhibiting the growth of two fungal strains, Penicillium sp. YM1 and YM2, isolated from citrus soot. In addition to water-soluble molecules, P. chlororaphis MT5 was shown to produce antifungal volatile organic compounds (VOCs). The VOCs produced by B. velezensis MT9 and P. chlororaphis MT5 were then identified by gas chromatography - mass spectrometry (GC-MS). Interestingly, some of the identified VOCs have previously been reported to have semiochemical activity in insects and, therefore, can interfere with chemical signaling in plant-insect-microbe interaction. Altogether these in vitro findings and pilot experiments with a limited set of plants indicate that Bacillus velezensis MT9 and Pseudomonas chlororaphis MT5 can effectively combat sooty mold possibly by acting on both the insects and the fungi that inhabit the black biofilm, paving the way for an innovative and sustainable tool to fight this disease.