16 March 2001

Control of volunteers is vital tactic

Rotation and volunteer

control are key components to

successful potato production.

Andrew Swallow asked

ADASs David Turley why

GOOD potato agronomy goes way beyond the crop itself; control of volunteers in the rest of the rotation is vital, as a BPC report published this month reveals.

"Volunteer control is important for a whole host of reasons," says report author David Turley of ADAS high Mowthorpe. "Firstly, there are the phytosanitary considerations, and then there are the effects of volunteers on the following crops themselves."

However, growers can easily become complacent about control, especially after a run of easy harvest years, he warns. Despite improvements in harvester design, research work in the report reveals that tuber losses are no lower now than they were 20 years ago. In the worst cases tuber numbers exceed typical ware crop seed rates. Following last autumns awful lifting conditions many growers could be facing such levels of volunteers this spring, warns Mr Turley.

"Waterlogging and frost may have killed off some tubers, but the frost did not go deeper than a few centimetres in many places."

High numbers of volunteers have been shown to reduce the rate of potato cyst nematode decline. But phytosanitary concerns go beyond that, especially for growers with both seed and ware crops.

"Ware crops rarely get sprayed for aphid control so volunteers from such crops are often packed with virus. If you have a lot of volunteers in one field, be aware of the virus risk if the neighbouring crop is intended for seed," stresses Mr Turley.

Potato volunteers also help skin finish diseases such as skin spot, black scurf and silver scurf build up innoculum levels, and volunteer foliage can act as a source of blight innoculum for nearby crops or even potato crops in the field if the volunteers are allowed to persist through the rotation.

Yield loss due to competition from the volunteer potatoes is worst in crops with open canopies, such as sugar beet, onions, peas or even narcissi. Added to that controls are limited, and expensive, in such crops.

"Where these crops are in the same rotation as potatoes, volunteer control is doubly important. There is often only one year of cereals after the potatoes to get on top of the problem," notes Mr Turley.

VOLUNTEER ACTION

&#8226 Reduce harvester web-spacing.

&#8226 Use chat crushers.

&#8226 Use lifting tines, avoid ploughing.

&#8226 Grow competitive following crops.

&#8226 Use two-spray attack in crops and set-aside.

ICM approach

The volunteer research was carried out as part of the LINK Integrated Farming Systems project and the work shows that conventional winter cropping, rather than an integrated rotation, is best to keep volunteers under control.

Volunteer populations were very high in all first and second crops following potatoes, but with spring beans as the second crop and mowing rather than herbicides used on set-aside, an ICM-based rotation allowed high numbers of volunteers to persist into the fourth crop following potatoes.

That has important implications for pest and disease carry over into potato crops, states the BPC report.