15 October 1999

Early phoma in OSR spotlights timing of spray

With oilseed rape prices on

the floor it is tempting to

cut inputs. But when it

comes to phoma control that

would be false economy,

agronomists warn.

Andrew Blake reports

WITH phoma leaf spotting showing on winter oilseed rape at least 10 days earlier than normal this season, well-timed autumn fungicides will be even more important than usual.

That is the warning from crop specialists in the south and east, where the disease tends to be most damaging. But with the crops margin eroded extra attention, especially to spray timing, will be needed to get maximum reward, they stress.

"The phoma threat is very high this year," says Cleanacres agronomist Tim Horton, who was already finding lesions in Hants and Wilts crops last week. "I reckon it has come in two to three weeks early, mainly because of the wet and warm September.

"We still hope to get away with a single autumn spray in most cases, but on some occasions we may have to go twice.

"We tend to use Lyric (flusilazole). At about £22/ha for two 0.3 litres/ha treatments it starts to look expensive. But when you consider that with an early infection we could get 0.75t/ha response it should still be economic. We have never yet gone twice in the autumn. We shall spray once and then reassess at the end of November."

Product choice is a toss-up between Lyric and Plover (difenoconazole), though metconazole is waiting in the wings, says Mr Horton.

"They are the two real alternatives for phoma. We have had some very good results from Plover in high risk situations. Tebuconazole and prochloraz are certainly not as good on phoma, especially in the autumn."

A 15-20% drop in fungicide prices makes the operation more attractive than last year, he notes.

Suffolk-based Crop Care agronomist Neal Boughton was particularly concerned to hear phoma reported on small plants as early as last week. "People were asking me if it was too soon to spray. I replied that where crops were still healthy and growing away they should try to hold off for a couple of weeks. But those still backward needed treating fairly quickly."

His reasoning is simple. Spraying too soon on strong crops risks having to repeat the treatment in three to four weeks to protect new growth. Small plants, especially those restricted by water-logging, are more vulnerable to canker damage and so merit application sooner. That is because the fungus has less far to travel from leaf to stem where it is hard to control, he explains.

"Everyone has their own opinion about thresholds for phoma," he adds. "We find most people go on as soon as they see it."

His fungicide choices match Mr Hortons, but he stresses the importance of good crop coverage. "A cheap and cheerful non-ionic wetter can help break the surface tension caused by waxy leaves."

In Lincs, where establishment has generally been good, John Bayes of Agrochem South is wary of splitting the autumn fungicide dose. That is even though increased seed rates in many cases have produced extra dense disease-prone stands.

Only under extreme disease pressure would he consider two autumn treatments. "You cant afford to throw all your budget at phoma control. A full 0.5 litres/ha of Plover costs about £20.50/ha." Crop production as much as protection is the aim, he explains. "You can get good disease control, but that is not always transferred to yield.

"Timing is more important than dose. Id prefer to go with a targeted 0.25 litres/ha of Plover or 0.4 of Punch C (flusilazole + carbendazim) and monitor the crop properly. You often cant get on with a second autumn treatment anyway."

Combined with a follow up treatment at early stem extension this approach is potentially much more rewarding, he suggests. &#42

PHOMA LEAF SPOT

&#8226 Hitting crops earlier.

&#8226 Higher canker risk.

&#8226 Dual autumn treatment?

&#8226 Economic even at £110/t.

Severe stem canker (above left) can be the penalty for not stopping phoma. Partial control can come from a spring spray (above right). But the best approach usually involves both autumn and spring treatments (left). This season a second autumn spray may be needed, say some agronomists.

Treatment cost well worth the price

Phoma is much easier to control at the leaf spot stage than later when it has spread to the stems to cause canker, notes NIAB pathologist Jane Thomas. "Even at todays prices treatment is well worthwhile. If you have 10% of plants infected by mid October the potential yield loss is very high."

Yield increases from stopping phoma at this stage have been 0.5-0.75t/ha (4-6cwt/acre) in NIAB trials. "At individual sites you can get up to 1t/ha response from reasonable control."

The key difficulty for growers is that the 10% spray trigger threshold varies widely from year to year and site to site, explains Dr Thomas who is in her fourth season of monitoring untreated HGCA trials. At Wye, Kent in 1997 it came before the end of October. But a year later in Norfolk it wasnt hit until the third week in November. On average it occurs in the first week of November.

Some varieties, notably Contact and Escort, are more resistant to phoma than others. "They have a rating of 7 which is better, but not perfect," she says. "Its hard for breeders to incorporate resistance and maintain all the other desirable characteristics."

On-site monitor

From next week farmers weekly, in conjunction with NIAB and spray manufacturer Du Pont, will publish details of phoma monitoring at five sites to help growers decide when to treat crops for best effect.

"It is all about targeting inputs correctly," says Du Ponts Andy Selley. "Many more rape crops get sprayed in the autumn nowadays, but too many are sprayed at the wrong time and there is still too much fungicide going on in the spring."