Citrus Black Spot Symptoms on Fruit Exposed to Phyllosticta citricarpa Infections at Different Developmental Stages

Citrus black spot (CBS), caused by Phyllosticta citricarpa, is an important fungal disease of citrus. Higher CBS severity has been associated with P. citricarpa infections at the young and green stages of fruit. The length of the fruit susceptibility period may be influenced by the amount of inoculum and the climate of the citrus growing region. This study was conducted in orchards across two South African provinces to assess the intensity of CBS under two conditions: (i) unsprayed fruit of Valencia sweet orange, Nova, and Empress mandarins inoculated with P. citricarpa at 10¹, 10³, and 10⁵ pycnidiospores/mL at various intervals after fruit set, from October (petal fall stage) to March (for mandarins) or June (for sweet orange), and (ii) non-inoculated Valencia fruit from orchards subjected to spray programs ranging from one to nine applications between October and June. Higher CBS intensity was observed in Valencia fruit inoculated between November and January, with an average disease severity index (DSI) of 0.94, compared to a significantly lower (0.07) for fruit inoculated from February to June. Inoculations in November or December for Nova mandarins and in December for Empress with 10⁵ pycnidiospores/mL resulted in higher CBS intensity compared to those with lower pycnidiospore concentrations. The absence of fungicide applications from October to December during the 2017-2018 season led to increased CBS intensity, with a DSI of ~0.47, a value which was not significantly different from that of non-treated control (NTC) trees. Leaving sweet orange trees unprotected from January onwards did not significantly increase the CBS intensity. These results confirm that citrus growers should focus on protecting citrus fruit, against P. citricarpa infection, in their dark green and young stages (c.15-30 mm diameter) to control CBS.