Cotton growers in most valleys remain very concerned about the high prevalence of Cotton Bunchy Top (CBT) this season.

As part of the cotton industry pathology survey, a recent tour by Cotton Australia’s Greg Kauter and QDEEDI’s Emerald based cotton extension officer Susan Mass has helped identify that current levels of the disease are now a cause of concern for next season.

Crop Consultants Association also remain concerned by the disease potential stating, “Cotton Bunchy Top is currently the greatest threat to high yielding cotton production in Australia.”

According to Crop Consultants Australia, “Cotton Bunchy Top cannot be prevented by controlling aphids” and growers should not rely on this. Not only are there limited insecticide options for aphid control, but their management has become particularly challenging in recent years due to growing resistance issues.

CBT symptoms were visible in almost half of the fields inspected as part of the industry’s disease survey this year, with some problematic patches. On some fields on the Downs for example, up to 70% of plants were affected and significant yield impacts are likely. This is an industry-wide problem requiring an industry-wide response, and Cotton Australia has taken a coordinating role.

Cotton Bunchy Top is a viral disease spread by cotton aphids, with the last big outbreak back in the 1998-99 season. It causes stunting in cotton including reduced leaf and fruit size and reduced internode and petiole length, all having significant impacts on yield. Symptoms include leaves having pale green angular patterns around the margins, darker green centres and a leathery texture.

Cotton Australia is working with the CRC Extension Team to deliver the latest research findings and best practice information is shared with growers as well as advising CRDC on the research needs of growers in this area.

The TIMS Committee, led by Cotton Australia is also in the process of consulting with industry on the development of the Insect Resistance Management Strategy for 2011-12, with CBT set to be addressed by this plan.

CSIRO Program Leader Dr Greg Constable addressed the Cotton Australia General Meeting on the Cotton Bunchy Top issue, updating members on the research program’s efforts to address it. The meeting expressed a clear desire for new varieties that are resistant to the disease, with Dr Constable informing the group that commercialisation of such varieties is about two years away.

Because this technology is still in the pipeline, growers need to be on top of aphid control, and have strategies in place well ahead of next season.

Host management during winter is crucial in minimising the number of severely impacted fields next year. A number of broadleaf weeds such as marshmallow weeds can be hosts, but the biggest problem is volunteer and ratoon cotton as they provide the “green bridge” that allows aphids to survive season to season. If these are not controlled, CBT can fast become an area-wide problem.

Adding to the problem is the fact that volunteer and ratoon cotton can appear symptomless, because there is a latent period from initial infection to the presence of visual symptoms. The best rule of thumb is to remove any weeds, volunteer and ratoon cotton from the farm over winter and to not leave it until after cotton planting. If left too late, growers run the risk of CBT carrying aphids leaving these host plants, resulting in sudden and widespread crop infestation and the risk of high yield loss.