Developing methods to screen sugarcane varieties for resistance to Fiji leaf gall.

RESISTANCE to Fiji leaf gall (previously Fiji disease) is a prerequisite for all cultivars of sugarcane released in central and southern Queensland and New South Wales. Fiji leaf gall can cause total crop loss. The disease is caused by the Fiji disease virus and is spread by the planthopper, Perkinsiella saccharicida. Traditionally, clones have been screened for resistance by planting small plots of the test clones between rows of infected cane. Natural planthopper populations transmit the disease. For more than five years, the populations of the insects in southern Queensland have been low and screening trials have failed. This paper reviews the past 28 years of field Fiji leaf gall resistance trials. Trials failed in 11 years because of low infection levels (9 years) or excessive infection (2 years). The optimum discrimination between clones is obtained when NCo310, a susceptible standard variety, has disease levels between 60-80% but 40-60% and 80-100% ranges gave acceptable results. Only 35 of the 68 trials that were rated during this period had disease levels in this range. Two greenhouse-based techniques have been developed to provide reliable ratings for resistance to Fiji leaf gall. In the first method, plants were grown in the field under a poly-tunnel and the planthoppers were released into the tunnel. The second method is similar but the plants of the test clones were exposed to planthoppers in a greenhouse for two weeks and were then transplanted into the field. Both methods have given high levels of disease and the reaction of a set of standard clones was highly correlated with their known field reaction. The number of planthoppers on each clone was recorded and the percentage of the planthoppers carrying the virus was monitored with a reverse transcription-polymerase chain reaction (RT-PCR) assay. There was no correlation between planthopper numbers on the test clones and their reaction to the disease. Planthoppers have a significant preference for taller plants. These methods are being further tested and should provide reliable ratings for resistance to this important disease