Cotton bunchy top: an aphid and graft transmitted cotton disease

A new disease, termed cotton bunchy top (CBT), has been observed in Australian cotton fields since the 1998 - 99 cotton-growing season. Symptoms included short petioles and internodes, pale, light-green, angular patterns on the leaf margins, and a leathery texture of mature leaves. Affected plants had a reduced photosynthetic rate, leaf area, plant height, number of bolls, dry weight of bolls, roots and stem and ultimately yield. CBT was demonstrated to be graft-transmissible in glasshouse experiments. In the field, CBT hotspots appeared to correlate with cotton aphid ( Aphis gossypii) density and this species was identified as a CBT vector in controlled transmission tests. CBT symptoms and plant responses recorded in graft and aphid-inoculated plants were similar to those seen in the field. Seed transmission of CBT appears unlikely as none of 3930 plants grown from seed of CBT-affected plants developed symptoms.