In vitro and in vivo activity of QoI fungicides against Colletotrichum gloeosporioides causing fruit anthracnose in Citrus sinensis

Anthracnose of sweet orange caused by Colletotrichum gloeosporioides was recently reported as a severe epidemic disease in pre-harvest conditions in several orchards of Sicily, one of the most important orange production areas for Europe. Nowadays, this emerging pathogen strongly reduce the amount of marketable fruits worldwide and fungicide applications are needed. In this study, sensitivity profile to QoI fungicides was preliminarily established for 37 C. gloeosporioides isolates collected from Sicilian orchards and fungal capacity to activate alternative respiration in presence of QoIs, a mechanism known for many phytopathogenic fungi when normal respiration is inhibited, was also determined. Tested isolates showed different fungicide sensitivity levels depending on salicylhydroxamic acid (SHAM) amendment of media. Medians of EC50 values ranged from 0.36 to 102 μg mL–1 for azoxystrobin and from 0.047 to 0.49 μg mL–1 for trifloxystrobin with or without SHAM amendment, respectively. Otherwise, EC50 medians of pyraclostrobin calculated within the same subset of fungal isolates were very similar in presence or in absence of SHAM (0.11 and 0.14 μg mL–1, respectively), thus indicating that only pyraclostrobin among the tested QoIs do not induce alternative respiration for this fungus. Subsequently, four field experiments were carried out in 2013–2014 in the same orange orchards, from which tested isolates were before collected, to determine QoIs performances against C. gloeosporioides. Although under low-medium disease pressure conditions, our data clearly showed that field applications of QoIs significantly managed both the incidence (reductions from 83.7 to 92%) of anthracnose infections on sweet orange in pre-harvest conditions and, approximately reduced up to 81 and 91.6% disease incidence and severity in postharvest stage. Pyraclostrobin, always applied at low rate (about 1/3 of label rate recommended for other targeted fungi of Citrus sinensis) showed good performances in reducing pre- and postharvest anthracnose infections on two cultivars, “Scirè” and “Alcalà”; of sweet orange. Based on our data, this latter molecule could be considered as a reliable chemical option to counteract this threatening disease on sweet orange, reducing environmental impact and the likelihood of fungicide resistance arising within Italian C. gloeosporioides population.