Arable growers were warned about the danger of green bridges for vegetable-seed crops at the Foundation for Arable Research (Far) international conference in Christchurch this week.

Scientists at Washington State University in the United States have come up with ingenious methods to reduce green bridges, which refer to the crossover of serious diseases in crops.

A vegetable-seed pathologist from the university, Lindsey du Toit, said 100 years of seed growing in western Washington State had resulted in many disease lessons that would benefit New Zealand growers.

"Many of these seed crops are biennial and it takes two seasons for them to grow, so you have a long season and there is an overlap into the next season. One of the problems is the diseases are spread from one season's crop to the next because they are (both) in the ground for a month or two."

In western Washington, after beet growing almost folded because of the beet-mosaic virus, growers have tried to break the green bridge by requiring seed plants for red-beet seed production to be densely planted off the mainland at Whidbey Island.

The bulbs are transplanted to the mainland in spring and this has stopped the big losses previously experienced.

Du Toit said the process was labour intensive and expensive, but it showed the lengths the seed industry was taking to control disease.

"When the yields were going down tenfold because of the virus, economically it becomes viable to do this."

On the Canterbury Plains, beet production for seed is not as densely planted and does not have the long history of the Washington crop, she said.

"You have not faced the same pressures, but it might be coming down the road so be concerned (about) breaking the green bridge as much as possible."

For spinach-seed crops, researchers have found that burying the woody stems has reduced the carryover of leaf-spot fungi.

However, Washington scientists have yet to solve the bridging problems for a new onion virus, the iris yellow spot, which has appeared in the last seven years.

Onion bulb and seed crops are grown together and "volunteer" plants carrying the virus can survive through to the next year, leaving growers facing big onion losses every year.

Du Toit said disease control was complicated because there were so many types of disease, and new techniques and equipment could affect disease.

In western Washington, the growing of peas, an important rotation crop, was reduced threefold to 2000ha because they became cheaper to process elsewhere.

Burning of stubble is permitted for grass-seed crops in Oregon, but not in Washington.

Du Toit hopes to develop closer ties with New Zealand scientists.

- © Fairfax NZ News