Citrus black spot intensity is influenced by timing of infection by Phyllosticta citricarpa and fruit developmental stage in South Africa

Abstract Text: Citrus Black Spot (CBS), caused by Phyllosticta citricarpa, is an important disease affecting citrus in South Africa (SA), due to the phytosanitary import regulations imposed by the European Union, a key export market for SA. To identify the critical period for fruit protection in South African orchards, we assessed CBS symptom intensity on fruit inoculated with P. citricarpa or protected with fungicides at different growth stages. ‘Valencia’ orange, ‘Nova’ and ‘Empress’ mandarin fruit were inoculated with 10¹, 10³ and 10⁵ pycnidiospores/mL from October to March (mandarin) or June (orange). ‘Valencia’ orange trees were also sprayed with 1 to 9 fungicide applications between October and June. Inoculations of orange fruit at young and dark green stages (15–30 mm diam.), from November to January, resulted in the highest disease severity index (DSI) of 0.94. Inoculations performed January onwards had a lower DSI below 0.07. Inoculations in November or December for ‘Nova’ and December for ‘Empress’ with 10⁵ pycnidiospores/mL resulted in higher CBS levels compared to the lower concentrations. The absence of fungicide protection between October and December resulted in greater CBS intensity. Spray programs with applications from October to January significantly reduced CBS compared to non-treated trees. These findings indicate that the early fruit developmental period, from November to January, is the most critical for P. citricarpa infection and fruit protection. Citrus growers need to prioritize fungicide applications during this period to achieve effective CBS control.