Rapid Spread of Ratoon Stunting Disease during Manual Harvesting of Sugarcane and the Effect of Knife Cleaning on the Rate of Spread

In a field experiment, spreader plants of variety N13 infected with ratoon stunting disease were interplanted with healthy plants of indicator variety NC0376. When cane knives were not disinfected during manual harvesting, 62% ofstalks ofNC0376 down-row from the spreader plants were found to be infected after one harvest, and almost all the stalks were infected after three harvests. This compared with 16%stalks infected after three harvests when the knives were disinfected with diluted carbolic acid after every metre of row cut. When the spreader plants were first removed and the remainder of the row then cut with knives disinfected every metre, only 3% of stalks of the indicator variety were infected after three harvests. Rapid spread of Clavibacter xyli subsp xyli can therefore be expected during routine commercial harvesting, once ratoon stunting disease is introduced into a field by planting infected seedcane, or if it persists in newly planted fields from a previously infected crop. There was some evidence of limited natural spread of C. xyli subsp xyli but this can only have occurred at a low rate compared with mechanical spread during harvesting with uncleaned cane knives.