Citrus black spot caused by Guignardia Citricarpa Kiely, is the most serious fungal disease affecting citrus in the Republic of South Africa. The disease is also reported from Swaziland, Rhodesia and Mozambique and in different countries overseas, notably in Australia and also in China, Japan, Formosa, India, Philippine Islands, Java, Brazil, Argentine and the U.S.S.R. The disease is most serious on Valencia oranges but may also cause serious losses on grapefruit, lemon and on early maturing oranges as well. It is also reported on citron, Jaffa orange and kumquat and in China on mandarins. At present, this disease costs the South African citrus industry over 1 ½ million rand to control annually. Notwithstanding the application of preventative measures, losses frequently result due to post-harvest development of the symptoms on export citrus in transit from the packhouses to the ports, prior to shipment overseas.