Plant health top tips - May: Control volunteer and ratoon cotton plants; and break the green bridge!

Farm hygiene is at the heart of pest and disease management, but it's one of the most under-rated tactics. Maintaining a farm that is free of weeds including volunteer and ratoon cotton plants breaks the green bridge needed for pests and pathogens to overwinter until the following season. These unwanted plants provide a starting population for pests to move quickly into the next crop the following season and increase the chances of pest outbreaks.

Volunteer and ratoon cotton plants can also act as a reservoir for plant viruses such as cotton bunchy top (CBT). Cotton bunchy top is spread by aphids that can also be found overwintering on volunteer and ratoon cotton plants. Past CRDC-supported research, conducted by QLD DAF, found around 40 per cent of volunteer and ratoon plants alongside roadsides in cotton growing regions were infected with cotton bunchy top. Cotton bunchy top can have sporadic outbreaks if conditions are favourable, such as the presence of the virus and a high aphid population. In 1998-99, cotton bunchy top was estimated to have caused around 25 per cent yield loss on 21 per cent of the cotton hectares planted that season. This equates to a 5 per cent yield loss across the whole industry. More recently, there were significant outbreaks in 2011 where several cotton crops in Central Queensland had yield losses of up to around 40 per cent. Controlling volunteer and ratoon cotton, as part of an integrated pest and disease management approach, is one of the best ways to minimise the availability of hosts for pests and diseases overwintering and the chance of outbreaks occurring in your next crop.

In-field volunteer surveys were conducted by DAF across Queensland and Northern NSW assessed the distribution and abundance of early season volunteer cotton plants in the 2014-15 season. This CRDC-funded research found that crop history plays a major factor in the presence of volunteer cotton plants in a field. A vast majority of the volunteer cotton populations (93 per cent) were detected in back-to-back cotton rotations, with the remainder following a summer fallow or grain rotation.

Past surveys highlighted that you will often see volunteer cotton plants in areas where viable seed have been left such as in fields and on farm along roadsides and fence lines. In addition to being more common in back-to-back cotton fields, they are also likely to be seen in fields where a single control tactic is used or no specific management plan is in place. In situations where high levels of volunteer cotton control were achieved, growers had been timely, persistent and used multiple management tactics.

• Reduce the amount of viable seed left in fields – through clean pick and stubble management, and on farm – through clean up after module removal and spillages.
• Cultivation will act as a cultural control for volunteer cotton of differing sizes.
• Manual removal (chipping) can be effective in where plants are in low densities or in non-field areas.
• Herbicide controls are available that target seedling volunteer cotton to ratoon cotton.

The simplest and most cost effective way of controlling ratoon cotton is preventing these plants from occurring in the first place. Recent research (conducted by QLD DAF with support from Nufarm) assisted with identifying three herbicide options for the control of large volunteer or ratoon cotton plants in fallow using an optical spot spray technology providing high plant mortality in research trials.

Three herbicide options have since been registered for both optical and broad acre application, which means all growers should be able to make use of these control measures. Products must be used in accordance with label instructions.