Archive Article: 1997/12/13
The mild autumn is not only encouraging rape and cereal diseases, but is shortening the effective life of fungicides used to control them. Sarah Henly reports.
OILSEED rape diseases are keeping growers and agronomists on their toes, literally. Both phoma and light leaf spot are active and need watching carefully.
The mild conditions may shorten the active life of fungicides, leaving early sprayed rape unprotected by the New Year, warns Dr Peter Gladders at ADAS Boxworth.
"Incorrect spray timing cost growers about £80m in losses to light leaf spot and phoma two seasons ago. With new chemistry and a better understanding of how those diseases work, theres no excuse to get it seriously wrong again," he says.
He expects that treatments may last only four to six weeks under warm conditions rather than the usual eight or so.
"I can identify three scenarios -the first where a fungicide was applied early, the second where the first spray is going on now, and the third where there is no disease at present. Growers are likely to need a mid-winter follow-up for the first situation, a spring stem extension follow-up for the second, and a one-hit spring application for the third," he explains.
It is uncommon to have to apply an autumn fungicide treatment to winter cereals, but this autumn has been exceptionally mild. It has given a headstart to mildew and net blotch in barley, and mildew in wheat. It has also sparked off an unusual problem – barley yellow rust.
But are any diseases severe enough to warrant treatment before the new year? Heres what agronomists from around the country say.
LIGHT LEAF SPOT
There are few sightings of light leaf spot, even in early drilled crops.
According to HGCA-funded forecasts, the likely risk in the north of England is two in five crops with 25% or more of plants infected at early stem extension. In the east it is only one in 14 crops, while in the south and west, 20% to 30% of crops are at risk.
Nevertheless it would seem that no one is taking any chances. Without waiting for symptoms to appear, growers are applying a fungicide, mainly as an insurance against both light leaf spot and Phoma. Many crops around the country have been sprayed with products such as Punch C (flusilazole and carbendazim) at 0.4 litres/ha, which is the rate recommended for a two-way split between the autumn and the spring.
In Yorkshire, Rachel Webster of Yorkshire Arable Advice hopes growers who sprayed in October wont need to go back for light leaf spot control until early flowering. But she will monitor crops regularly for the bleached leaf blotches with a halo of white spores.
Further south, Richard Cartwright of Chichester Crop Consultancy expects early November applied treatments to need following up in February or March at early stem extension.
Dr Gladders warns that early autumn applied fungicides could run out of steam against light leaf spot. There is a limited window for spore movement into rape from crop debris, and the best timings for split treatments are late December and early March, provided symptoms do not appear before that.
PHOMA LEAF SPOT
Last season phoma didnt appear until December. This autumn the characteristic green spots with dark brown pinheads inside could be found from late October in early drilled rape crops in the south.
A surge in development occurred in November. Mr Cartwright recommended all his rape growers use Punch C or Sportak Alpha (prochloraz and carbendazim). When thresholds of one in 20 plants with at least one spot were reached, agronomists in the Midlands commonly followed suit.
In the east, Mrs Hayes was forced to recommend a late October application of Punch C. Where that occurred, she will err on the side of caution and apply a three spray programme. She is concerned early treatments will run out of steam before the traditional follow-up timing in March, so will go in during late December or January as well, using a half rate each time.
Dr Gladders supports that view. With an incubation period of just four weeks, Phoma could be back at worrying levels by later this month. All varieties except Express are at high risk, so crops growing on particularly high risk sites may need a second spray before Christmas.
BEET WESTERN YELLOW VIRUS
This disease hasnt been found in oilseed rape for a couple of years but could stage a comeback following this mild autumn. The vector, the peach-potato aphid, appeared in southern rape crops as early as September, although most came in during October.
Dr Alan Dewar of IACR-Brooms Barn recorded only a moderate catch in suction traps in Suffolk, but is concerned that they are flying later this autumn. That may extend their period of reproduction, provided conditions remain mild.
He was surprised that the October ground frosts didnt kill them off, and surmises that lush plant canopies kept them protected. The generally mild conditions are allowing them to persist and potentially transmit BWYV.
Whether or not there is justification for spraying, and what to use, depends on the strain of aphid in question. If the offenders are one of the two resistant types that exist in this country, then there is cause for concern. There are speculative reports that some of the aphids caught recently are of the MACE type, which are resistant to all currently recommended insecticides.
Aphid populations were so high in West Sussex that Mr Cartwright recommended all rape crops in his area were sprayed in mid-October with the pyrethroid insecticide, lambda-cyhalothrin (Hallmark). By that time rape plants had four or so leaves, thus the aphids were a hard target to reach. Mr Cartwright believes that is probably the reason only 80% kill was achieved.
MILDEW
Symptoms were seen as early as mid-October in early drilled barleys. Levels are apparently higher than last season, particularly in the north and Midlands.
In Rachel Websters area of Yorkshire, 20% of leaves had white, powdery mildew pustules on them by early November. She was starting to consider spraying when night frosts halted disease development.
As for wheat, thick, well-tillered crops are displaying symptoms, albeit at low levels. The early frosts also knocked them out quite well.
Its an age-old question – will you get a yield response from applying a fungicide for control in the autumn? According to Bill Clark, national cereal pathologist at ADAS Boxworth, even this year the answer is no.
"More often than not, the winter will check mildew development. The only situation where it may pay to spray before the new year is on light land where thin crops are at the mercy of frosts, and mildew further weakens them."
YELLOW RUST
Theres little to report about yellow rust in winter wheat. With less Brigadier in the ground, there is possibly less threat this season.
But what about yellow rust of winter barley? Mr Clark hasnt seen it for 15 years, so was a little concerned when symptoms characteristic of yellow rust appeared in a crop of Regina recently.
"We can only assume the inoculum has been ticking over all these years, and the mild conditions coupled with a highly susceptible variety have given rise to the disease again. Its interesting but not worth panicking about," he says.
However fungicide choice may have to be reviewed in future, particularly since there is a flush of highly susceptible varieties on the horizon. Some fungicides commonly used in the spring for broad-spectrum disease control dont kill barley yellow rust, he warns.
NET BLOTCH
Like mildew, net blotch has been severe enough to cause concern. The mild, damp conditions are giving it a boost. Very severe infections inhibited the establishment of some crops, but generally it is no more troublesome than last season.
Once again, frosts in October helped to keep it in check. The worst affected varieties seem to be Glean, Fanfare and Melanie.
Its prudent to keep a close eye on the disease to prevent its spread. Many agronomists are preparing to recommend early spring treatments with a powerful triazole combination.