Project to tackle aggressive strain of lettuce disease

AHDB has commissioned work on lettuce fusarium wilt following the first confirmed outbreak of the disease in the UK, on a soil-grown protected lettuce crop in Lancashire. It was also recently found on a crop in Ireland.

The UK outbreak, reported in October, has been confirmed as race 4 of Fusarium oxysporum f.sp. lactucae, the particularly aggressive strain that has been causing problems for growers in Belgium and the Netherlands for the past three years. The origin of the UK outbreak remains unknown but information published by AHDB suggests it can be carried on symptomless young plants. Research in Italy indicates that it can also be seed borne though the extent to which this may be a factor in commercial production is not known.

The disease can be transmitted in water, too, so hydroponic crops are potentially at risk, if young plants are already infected.

The pathogen has been listed on Defra’s UK plant health risk register since 2011 but no statutory action is required and there is no official surveillance or outbreak monitoring. No fungicides currently available are effective against it and no varieties are resistant, though at least two seed companies are believed to be breeding for it.

AHDB crop protection scientist Kim Parker said that although the soil-borne disease could affect both outdoor and protected lettuce, its requirement for relatively high temperatures means that in the UK the main impact would be on crops under glass, should it become established here. “It has become a problem under glass in the Netherlands,” she said. “The spores can survive for several years in soil. Growers in the Netherlands have either had to move to new glass or restrict themselves to winter production only, when the pathogen is less active.”

AHDB has commissioned Warwick Crop Centre pathologist Andrew Taylor to undertake a review of all known information about the disease. “As well as reviewing published research he’ll be liaising with seedhouses and propagators and with Dutch growers to assess the current situation and put together some detailed best practice advice, and with scientists in Italy who have done a lot of research on the pathogen’s biology,” she said.

His report is due in February but any information he finds that could be of immediate use to growers will be published as it arises, she added. As part of a wider existing AHDB project on fusarium diseases, Warwick Crop Centre will be accepting samples from lettuce crops with suspect wilt symptoms for free testing.

Initial symptoms include leaf yellowing and wilting with reddish-brown necrosis to internal tissues of upper roots and crowns but these can sometimes be confused with other diseases such as Pythium tracheiphilum.

“We are advising growers that avoidance is the best strategy,” said Dr Parker. “Growers should review their hygiene measures on the nursery and with their plant propagator to avoid the disease getting onto their sites.”

Propagators should check with suppliers whether seed has been cleaned and to request seed-testing results. Trays should be treated, as the disease can be transmitted in any soil and debris they carry, and steps taken to cut the risk of spread on tools, equipment and footwear.

AHDB is organising a meeting in Lancashire on December 14, where ADAS associate plant pathologist Tim O’Neill will discuss practical measures to reduce the risk and impact of the disease.