First confirmed report of citrus black spot caused by Guignardia citricarpa on sweet oranges (Citrus sinensis) in Uganda

Oranges are locally abundant in the Teso district of Uganda, and are important cash crops for subsistence farmers. In November 2006, orange fruits showing black spot symptoms were brought to the rural plant clinic at Katine market near Soroti, run by the local Christian organisation SOCADIDO. Small 4-5mm lesions with light centres, surrounded by a darker red to purple rim were observed on the surface of the fruit (Fig. 1). Within the lesions, numerous dark black fruiting bodies were seen, typical of Guignardia citricarpa infection (Fig. 2). G. citricarpa is a damaging pathogen on Citrus species and causes black spots on leaves and fruits. Fruit quality is affected by the disease and blemishes reduce the aesthetic value and saleability of the fruit.

Fruits were brought back to the Global Plant Clinic, UK, and pycnida picked directly from the lesions and placed onto oat agar. Cultures were incubated for 2-3 days at 20-23°C before being subjected to a 12h dark/ 12 h near UV light cycle. The Phyllosticta state of G. citricarpa was consistently isolated from symptomatic tissues. In culture, colonies were dark brown to black with thick, prostrate mycelium. Pycnidia were immersed in the agar, globose in shape and dark-brown to black. A yellow pigment was produced on oat agar which is characteristic of G. citricarpa (Baldassari et al., 2008). Conidia were hyaline, obovoid to broadly ellipsoid, aseptate, (6-)8-10.5(-13) x (5-)5.5-7(-9) µm, lacking a visible mucoid sheath and with a subulate apical appendage. The culture was positively identified by taxonomists at CABI , UK as G. citricarpa and a culture placed in the CABI, Genetic Culture Collection (ref. IMI 394918). Further molecular identification was performed by the Central Sciences Laboratory in York, who confirmed G. citricarpa using TaqMan real-time PCR. (van Gent-Pelzer, et al., 2007).

This is the first confirmed record of Guignardia from Uganda , although it has been reported from other African countries including Kenya , Mozambique , South Africa , Zambia , and Zimbabwe (EPPO, 2006). Guignardia citricarpa is an IAPSC (Interafrican Phytosanitary Council) A2 listed pest, therefore it was important to confirm the presence of the pathogen. In areas where black spot is well established, fruit losses may periodically be severe and devastating. G. citricarpa is considered to be the most important pathogen of citrus in China, Australia and South Africa, where the citrus industry is of major importance (McOnie, 1967).