Cotton bunchy top (CBT) is a viral disease that is spread by cotton aphids.
It can only survive in living plants.
Aphids prefer ratoon cotton but can also live on various other plants, including weeds.
They can move between plants, leading to patches of infected areas.
Location
CBT has been detected in the Alice Springs region.
Symptoms
Symptoms of CBT include pale green angular patterns on the leaf edges with darker green centres. The leaves may also become leathery and brittle.
After infection, new growth has small leaves, short internodes, and small bolls, while older growth remains normal.
If infected early, such as in seedlings, the entire plant becomes compact and severely stunted.
Infected roots appear dark brown and hairy, with small knots, unlike the light yellow-brown colour of healthy roots.
Symptoms may take 3 to 8 weeks to appear and are harder to spot in older plants with limited new growth.
Host plants
Known natural hosts include:
cotton
Sturt’s desert rose
Rosella
Velvetleaf
spurred anoda
asthma plant
bladder ketmia
deadnettle
marshmallow weed
prickly malvastrum
burr medic
Paddy’s Lucerne
black pigweed.
The virus may have a wider host range, include other non-Malvaceae species.
Critical alternative host plants that survive between seasons are ratoon and volunteer cotton.
Control measures
Eliminating host plants, particularly between crop rotations, is the most effect way to minimise the risk of CBT.
CBT can only survive in living plants. Creating a break in the cycle of living host plants between cotton seasons will reduce the risk of CBT surviving. Effective crop destruction if cotton volunteers, regrowth and ratoons is critical for control.
Good on-farm management of broadleaf weeds. They can also host aphids or may be a host for CBT.
Early control of cotton volunteers and ratoons before new cotton emerges greatly reduces the risk of aphids moving into nearby crops.
