Efficacy of Trichoderma-based formulates against Mal secco disease of citrus

The tracheomycotic fungus Plenodomus tracheiphilus is a major threat to lemon fruit production in the Mediterranean basin, causing the Mal secco disease. Restrictions on the use of contentious copper-based compounds, due to their negative environmental impact, make the control of this pathogen challenging, especially in organic agriculture. This study evaluated the in vitro and in vivo antagonistic potential of four commercial Trichoderma-based formulations (T. asperellum ICC 012 + T. gamsii ICC 080, T. asperellum T34, T. atroviride I-1237, T. atroviride SC1) against P. tracheiphilus using a stepwise approach and comparing them with a copper compound. Our results showed that all Trichoderma-based biocontrol products significantly inhibited mycelial growth through different mechanisms, exhibiting variable efficacy depending on the specific P. tracheiphilus isolate. Comprehensively, Trichoderma dual culture and volatile organic compounds (VOCs) provided excellent performance in reducing mycelial growth of the pathogen, both higher than those observed for cell-free filtrates. Trichoderma colonies provided inhibition values varying from 30 to 71%, whereas VOCs inhibited from 23 to 75% and CFs up to 44%. Under controlled conditions, all Trichoderma products reduced symptoms of leaf vein chlorosis caused by P. tracheiphilus on Volkamer lemon seedlings. Specifically, T. gamsii ICC 080 + T. asperellum ICC 012 averagely showed the best performance since it significantly reduced disease incidence and severity of 50 and 57% at 28 days after inoculation, respectively, thus confirming once again similar performance recorded for the copper-based treatment. The commercial Trichoderma mixture was further tested to determine whether the application mode can play a role in mitigating infections. To this regard, the data showed that both soil drenches and foliar sprays effectively managed foliar symptoms caused by P. tracheiphilus, reducing disease incidence by 27 and 41% at 28 days, respectively, and symptom severity by 33 and 43%, respectively. Overall, this study confirmed a good efficacy of commercial Trichoderma product against P. tracheiphilus in controlled conditions using different application modes. Highlights. • In vitro and in vivo efficacy of some Trichoderma bioformulates against Plenodomus tracheiphilus (Mal secco) were tested on lemon. • Trichoderma gamsii ICC 080 + T. asperellum ICC 012 mixture was the most effective bioformulate in managing Mal secco infections. • The performance of Trichoderma bioformulate also depends on the application mode. • Although lesser effective than pyraclostrobin, Trichoderma bioformulate showed similar efficacy to Cu compound.