Now is the time to protect crops from phoma and light leaf spot

18 January 2002

Now is the time to protect crops from phoma and light leaf spot

Oilseed rape disease control

in late winter is the focus

for this weeks baseline

advice article. There is still

plenty to play for

GET out into oilseed rape crops as soon as you can and look for signs of light leaf spot and phoma infection.

Last autumns early phoma epidemic meant many crops were sprayed in October. But those sprays will now be running out of steam, warns Dr Bob Bulmer of Dalgety Arable.

"The fungicides available to growers give six weeks protection. So depending on when spraying took place, fresh lesions may well be appearing by now." Waiting until March for pgr-type fungicide applications could be too late.

Yield losses of up to 2t/ha can be caused where cankers are allowed to develop, so treatment is economic. "Even if you didnt spray in the autumn, you have until mid-February to get a 1t/ha response. So its not too late."

Phoma lesions are relatively easy to identify (see panel), with a spray threshold of 10-20% of plants infected. Dr Bulmer advises growers to use the 10% figure, to allow for spraying delays and poor weather conditions.

Although the varieties in the ground this year have a range of resistance ratings, those should only be used to prioritise spraying. "Even varieties with good ratings need spraying in a bad year," he notes.

Light leaf spot

Phoma is usually the predominant oilseed rape disease in the south and east, but light leaf spot takes over in the north and Scotland.

"Light leaf spot will be coming into crops now. The cold weather over Christmas favoured its development, so it will be apparent from now until the end of February." Some reports suggest it is already evident in the south and east too.

More difficult to identify than phoma, light leaf spot has lesions with white dots. "Pick some leaves and incubate them in polythene bags at 10-15C," recommends Dr Bulmer. "Lesions will become obvious where infection has occurred."

The threshold for light leaf spot spraying is 25% of plants infected. Growers can alsomake use of the light leaf spot forecast on the internet, at to check regional risk.

Dont confuse the disease with white leaf spot, which was a problem in the south-west last year as it likes more humid conditions. "You get a white bleaching of the leaves and very uniform spotting with this one. At the moment, there arent any spray thresholds."

Losses from light leaf spot vary from 0.5-1.5t/ha, making it another disease worth spraying for. "And you can still get a response quite late in the season, so its worth monitoring on a regular basis."

Product choice

Where both light leaf spot and phoma need treating, the best product is Punch C (flusilazole + carbendazim), believes Dr Bulmer.

"If phoma alone needs treating, then Plover (difenoconazole) will do a good job. And for light leaf spot alone, the tebuconazole formulations are effective."

Pricing is very similar, so it is worth using the best product, he adds. "An adjuvant will help in all cases and where you have a very thick canopy, use a medium/fine spray to help the droplets penetrate."

Light leaf spot should need just one spray, but phoma will often need two. "An autumn/spring split application is best where both diseases are present. With Plover use 0.25 litres/ha for each application, with Punch C the rate is 0.4 litres/ha."

Even if crops look clean at this stage, growers should keep checking until mid-February for phoma. "And the smaller crops are the priority, as phoma can move very quickly if it warms up," Dr Bulmer concludes. &#42

Conditions continue to favour phoma (right) and light leaf spot (inset). To prevent yield loss apply a fungicide now if 10% of plants have phoma and/or 25% have light leaf spot, advises Dr Bob Bulmer of Dalgety.

Area NIAB Disease

drilled Resistance Ratings

(%) L leaf spot Phoma

Fortress 17 6 6

Escort 15 7 7

Canberra 9 8 8

Recital 8 7 8

Apex 8 5 5

Pronto 7 6 5

Royal 5 6 4

Pyramid 3 5 5

Source: Dalgety Arable

1Crop inspection – look for signs of new disease or re-infection and keep checking until mid-February.

2Phoma – crops sprayed in autumn will need follow up soon. Crops not treated should be sprayed by end of January if disease showing.

3Light leaf spot – coming into crops now after favourable December conditions.

4Lesions – phoma fawn coloured with black dots; light leaf spot has white spots.

5Thresholds – spray when 10% of plants infected with phoma and/or 25% of plants infected with light leaf spot.

6Varieties – few varieties have good resistance and are still likely to need treating.

7Fungicides – use Punch C for both diseases, Plover for phoma alone and tebuconazole for light leaf spot.

8Spray quality – thick canopies need a medium/fine spray.

9White leaf spot – may occur in south-west, identified by leaf bleaching and uniform spotting.

10Cabbage stem flea beetle – watch for this too. If five or more larvae per plant, spray as soon as possible.

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