Cucumber target spot control and Corynespora cassiicola inhibition by uni-and multi-site fungicides

Target spot, caused by Corynespora cassiicola, is among the main disease that may reduce cucumber yield in worldwide. However, few target spot control strategies have been evaluated. The study aimed to evaluate the effect of fungicides in controlling C. cassiicola in vitro and cucumber target spot in a greenhouse. The effect of seven and thirteen fungicides in conidial germination and mycelial growth, respectively, of three pathogen isolates was evaluated in culture medium. The most efficient fungicides in vitro were applied on cucumber plants, at the recommended commercial dose. After three or seven days of the preventive treatment, plants were sprayed with suspensions of the pathogen (10⁴ conidia/mL). For the curative treatment, plants were inoculated 24 h before receiving the fungicide. Evaluation included target spot severity, number and diameter of lesions and sporulation on the leaf. Fungicides of the groups Methyl Benzimidazole Carbamates (MBC) and Quinone outside Inhibitors (QoI) were inefficient (EC₅₀ > 100 μg/mL) in inhibiting the pathogen in vitro. Chlorothalonil, metiram, boscalid, difenoconazole, epoxiconazole, tebuconazole, and flutriafol showed EC₅₀ < 1 or 1–10 μg/mL. Chlorothalonil, metiram + pyraclostrobin, boscalid + kresoxim-methyl, and azoxystrobin + benzovindiflupyr controlled the disease, providing greater severity reduction as preventive treatment, without any difference between the two application times. The number and diameter of lesions decreased when fungicides were applied as preventive treatment. Chlorothalonil reduced the number of lesions in curative treatment, while fluxapyroxad + pyraclostrobin, azoxystrobin + benzovindiflupyr, boscalid + kresoxim-methyl, and difenoconazole decreased the diameter of lesions. Sporulation on the leaves also reduced, especially when fungicides were preventively employed. A preventive management is recommended in areas with frequent reports of target spot outbreaks, adopting the alternation of the four most efficient fungicides in controlling the disease in order to reduce possible emergence of resistant isolates of the pathogen to the site-specific fungicides boscalid and benzovindiflupyr.