The current disease situation on the commercial estate of Ramu sugar, Papua New Guinea.
Papua New Guinea (PNG) is the centre of origin for several Saccharum species, including S. officinarum, the original species used in commercial production. There are a number of endemic diseases that affect commercial sugar production in PNG on the commercial estate of Ramu Sugar Limited at Gusap, Madang Province. These include downy mildew (caused by Peronosclerospora sacchari), Ramu stunt and the leaf diseases orange rust (Puccinia kuehnii), brown rust (Puccinia melanocephala) and yellow spot (Mycovellosiella koepkei). Of these, downy mildew is of greatest significance. The disease has reduced the number of foreign varieties able to be grown commercially by up to 50% and also affects the selection of new clones in the Ramu breeding programme. Ramu stunt has also had a very serious effect on crop production, particularly in the 1986 season when it caused very low yields in the cultivar Ragnar. Ramu stunt is only known from PNG and is transmitted by the plant hopper Eumetopina. Resistance screening trials are undertaken routinely for both downy mildew and Ramu stunt. In recent years, two previously exotic major diseases, leaf scald (caused by Xanthomonas albilineans) and ratoon stunting disease (RSD; caused by Leifsonia xyli subsp. xyli), have been identified on the estate and RSD is now a very serious problem for Ramu Sugar. Control programmes are being established. Other diseases present on the estate include Fiji leaf gall (caused by Fiji disease virus transmitted by Perkinsiella saccharicida), Ramu orange leaf and Ramu streak. Diseases will be of on-going importance for commercial sugar production in PNG.