Alternative control approaches against gray mold of Pelargonium zonale.

In collaboration with a nursery located near Aprilia (central Italy), we started to investigate the use of antagonistic fungi of the genus Trichoderma (T. atroviridae P1, T. harzianum T22 and T67, T. reseii T34, Trichoderma spp. 8009, T. hamatum T382 and a commercial product containing the isolates ICC012 of T. harzianum and ICC080 of T. viride) and resistance inducers in the control of Pelargonium gray mold. In dual-culture tests, each Trichoderma isolate caused a significant reduction in the growth of an isolate of Botrytis cinerea collected in the nursery, which was iprodione-resistant as confirmed by in vitro assays. The effect was significantly higher when Trichoderma isolates were inoculated two days before the pathogen rather than simultaneously. Moreover, the isolates P1, T22, T34 and ICC012+ICC080 caused an abnormal hyphal branching of the pathogen, while T67 formed coiling around them. Preliminary in vivo assays seemed to confirm the effectiveness of all the antagonists with the exception of T22, as they reduced significantly the severity of gray mold. Among the resistance inducers, Biochikol, which contains chitosan, reduced the disease severity, while treatments with pure chitosan dissolved in 1% acetic acid (0.04%, w/v) or 0.3 mM acibenzolar-S-methyl were ineffective and phytotoxic. As expected, pelargonium plants treated with iprodione (34 ppm) were not completely protected from gray mold, confirming the necessity for alternative control approaches.