14 April 1995

Watch out – ascochytas about

HIGH levels of ascochyta in winter beans are casting doubt on the value of seed certification, says ADAS. But others stress outbreaks are no reason to drop testing.

ADAS says symptoms of Ascochyta fabae, dark brown to black spots which can be confused with chocolate spot and frost damage, are showing up widely – not just in fields sown with farm-saved seed. "This year its everywhere – on home-saved crops and those grown from certified seed," says Cambs-based pathologist Bill Clark.

"Its an interesting anomaly. Historically ascochyta is a textbook seed-borne disease with very stringent certification standards." For home-savers the advice is not to use seed with more than 1% infection because even the best treatments are only 50% effective, he adds.

However, many crops from certified seed are badly infected. A field of C2 Punch at ADAS Boxworth has extensive symptoms. "Originally we advised people to go back to their suppliers to see if there was anything wrong with particular seed lots."

But it was soon noticed that many diseased crops were close to fields with last years bean stubbles, he says.

Since 1989 it has been known that a second spore-producing stage of the fungus, Didymella fabae, occurs on dead stem tissue.   

Mr Clark speculates that the mild autumn may have triggered far more than usual of the airborne ascospores leading to "long distance" disease spread. "We still havent got to the bottom of it. But if it has happened were in a completely new ball game, with seed certification out of the window."

Fortunately spring beans are unlikely to be affected, he says. This is because most stubbles and trash will have decayed or been cleared by now.

Provided the weather stays dry, this years inoculum may have little effect on output. If it turns wet yield losses of 25-30% can be expected, predicts Mr Clark.

Dr Anthony Biddle of the Processors and Growers Organisa-tion agrees there is much more ascochyta about than normal. But most seed stocks – both home-saved and certified – were "basically very healthy". So there are grounds for thinking that windblown Didymella is more to blame for this springs even disease distribution.

Few growers ploughed their bean stubbles, so there was plenty of crop debris to provide inoculum, he adds. The mild winter may also have favoured ascospore survival.

Fungicides are "not particularly effective" against the established disease. The best one can hope for is to protect new growth with chlorothalonil, says Dr Biddle. Despite this years events he believes seed-testing remains vital. Airborne spread may have occurred. "But it doesnt happen every year, and its just as important as ever to start with healthy seed." &#42

&#8226 Normally seed-borne.

&#8226 Can cut yield by 30%.

&#8226 Fungicides not very effective.

&#8226 1995 outbreaks suggest new spore type involved.

&#8226 Seed certification queried.

Beans at Boxworth look fine at first glance. But closer examination (inset) reveals extensive ascochyta, possibly blown in from infected autumn stubbles nearby, says ADAS pathologist Bill Clark.