Cotton Bunchy Top is a disease of cotton that was first reported in the 1998-99 cotton season. Symptoms of the disease include shortening of internodes and petioles, . CBT affected plants also have a reduced photosynthetic rate, and reduced cotton lint and seed yields. Research in this project aimed to (i) identify the causal agent of CBT and develop a probe to detect it (ii) identify the vector(s) of the disease and investigate its transmission characteristics and alternative hosts and (iii) Assist in selection of resistance to CBT in elite cotton germplasm. These are discussed below;

• Identification of the causal agent. Despite exhaustive efforts the agent causing CBT remains unidentified. Tests looked for viruses (common RNA virus, circular DNA Geminivirus and nanovirus, double stranded RNA viruses and a range of virus specific ELISA test including Barley Yellow Dwarf Virus), viroids, phytoplasmas and endoparasitic fungus hyphae. Testing also employed subtractive hybridisation, which did not reveal any non-cotton genes, and cDNA microarrays, which did reveal three clones (genes) that did not match the cotton genome. Unfortunately the project ended before these genes could be characterised.

• The disease is vectored by Aphis gossypii (cotton aphid) and attempts to transmit it using Myzus persicae (green peach aphid) or Aphis craccivora (cowpea aphid) failed. Cotton aphid transmits CBT semi-persistently, meaning they can transmit the disease to multiple plants over several days. Aphids can acquire CBT after feeding on CBT affected plants for five minutes and can inoculate a healthy plant after feeding for one hour. As few as one aphid can transmit the disease though more aphids do it more effectively. Cotton plants infected with CBT are capable of infecting aphids with the disease after a period of about 16-17 days, about 2 weeks before symptoms were obvious in the plants.

• Gossypium barbademse cv Pima S7 is a symptomless host of CBT, while the resistant variety DeltaOpal did not host the disease.

• Two alternative hosts of the disease have been identified, both from the family Malvacae. These are Malva parviflora and Malvastrum americanum..

• Over the past four seasons we have screened cotton varieties in the field and glasshouse for resistance to CBT. This has lead to selection of CBT resistance in several elite lines for commercial release of the first variety in 2005-06.

This project has provided the basic information to begin to understand the field epidemiology of this disease. It is likely that it is acquired by aphids feeding on an alternative host. Aphids transmit the disease semi-persistently, so they are infective for days after leaving alternative hosts and entering cotton crops. Cotton plants can become infected by the feeding of a single aphid, though more aphids results in higher transmission rates. Cotton plants colonised by aphids become new CBT sources after about 17 days and show clear symptoms after 3 – 5 weeks. Initial spread of the disease is slow as aphids are in the apterous form but as populations build alates begin to appear and secondary transmission to other parts of the field can occur.

CBT continues to threaten the industry both due to yield reductions and due changes in growers and consultants perceptions of aphids – resulting in lower thresholds, increase spraying and selection for insecticide resistance in aphids. Potential problems due to CBT have been hidden in recent years due to drought reducing over winter survival of aphids. There is a need for continued research to select for CBT resistance in elite germ plasm, to identify the agent so that a probe can be developed to facilitate understanding field epidemiology and breeding and there is a need to further study the field epidemiology so that outbreaks of CBT can be predicted and managed.