© Australasian Plant Pathology Society 2004 10.1071/AP03094 0815-3191/04/020197 www.publish.csiro.au/journals/app Australasian Plant Pathology , 2004, 33, 197–202 CSIRO PUBLISHING Cotton bunchy top: an aphid and graft transmitted cotton disease A. Reddall A , A. Ali B , J. A. Able B , J. Stonor B , L. Tesoriero C , P. R. Wright D , M. A. Rezaian B and L. J. Wilson A,E A CSIRO Plant Industry and Australian Cotton Cooperative Research Centre, Locked Bag 59, Narrabri, NSW 2390, Australia. B CSIRO Plant Industry, Horticulture Research Unit, PO Box 350, Glen Osmond, SA 5064, Australia. C NSW Agriculture, Elizabeth McArthur Agricultural Institute, Woodbridge Road, Menangle, NSW 2568, Australia. D NSW Agriculture, Locked Bag 21, Orange, NSW 2800, Australia. E Corresponding author; email: lewis.wilson@csiro.au Abstract. 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. AP03094 Coton bunchytop disease A.Redal etal. Additional keywords: integrated pest management, Gossypium hirsutum. Introduction Cotton (Gossypium hirsutum) is an economically important crop to Australia worth approximately A$1.6 billion in exports annually. Several viral diseases of cotton are of economic significance worldwide, including cotton leaf crumple and cotton leaf curl but neither has been observed in Australia (Briddon and Markham 2000; Rahman et al. 2001). A new syndrome, termed cotton bunchy top (CBT) after the bunched appearance of affected plants, has recently become a problem in major cotton-growing areas in Australia. Growers first reported the disease in the 1998–99 cotton-growing season, from several regions including the Darling Downs and McIntyre Valleys in Queensland and the Namoi and Gwydir Valley in New South Wales. It has since been reported from the Macquarie Valley in the south to the Emerald region in the north. In many instances, growers or their pest management consultants noted that CBT usually occurred in portions of fields that had been infested with cotton aphids (Aphis gossypii). This paper reports the first studies of CBT including characterisation of the symptoms and evidence that it is transmissible by cotton aphid and by grafting. Methods Plant culture Cotton seeds were sown in individual 10 L pots containing a grey vertisol soil typical of cotton growing regions (Constable et al. 1992). Seedlings were grown either in a glasshouse at 20–35°C under natural light or in a growth room at 25–30°C and a 16 h photoperiod. Thrive fertiliser (16 g dissolved in 1.5 L water, 27% N; 5.5% P; 9% K plus trace elements) was applied to the plants once every 3 weeks. Magnesium sulphate was also added twice during plant growth at a rate of 20 g per pot as the soil is known to be magnesium deficient. Characterisation of plant responses to cotton bunchy top in the field Twenty plants (ten with CBT symptoms and ten healthy) were selected from each of two commercial cotton fields, one of cv. Sicot 40 at ‘Hazeldene’ near Boomi, NSW, the other of cv. NuPearl at ‘Pindara’ near Wee Waa, NSW, in mid-February 2000 when the plants were at the boll maturation phase. Symptomatic and healthy plants were collected in pairs within 1 m of each other to minimise any differences in soil or other environmental conditions. Plant characteristics including plant height, number of nodes, flowers, squares, bolls and dry weight of bolls, flowers, roots, squares and stems were recorded. Leaf area was measured at every third node upwards from node six, to the top of the plant using a leaf area meter (LI-3100, Lincoln, NE, USA). Specific leaf weight was also determined for each of these leaves by dividing the leaf dry weight by the leaf area. Photosynthetic rate was measured on