Sunday, November 19, 2017

Act now to control light leaf spot and phoma

November 1, 2017 by  
Filed under Crops

Growers are advised to continue monitoring forecasts for light leaf spot and phoma amid an ongoing risk of infection this autumn.

Regional phoma risk forecasts based on phoma canker incidence last season and rainfall in September and October are available from the Rothamsted Research website. They can help decide fungicide timings this autumn, says Hutchinsons technical development director David Ellerton.

“Phoma spores need 20 days or more rain from 1 August in order to mature on stubble. More rainfall than this causes release of mature spores, which are then able to infect the crop – if there is a minimum of four hours leaf wetness.”

“From infection, it takes an accumulated mean temperature of 120 day degrees – six days at 20˚C, or 10 days at 12˚C – for a mature spore to produce the characteristic leaf spot, from which the mycelium will migrate down the petiole of the leaf and into the central stem.”

Stem cankers

At low temperatures, this migration may be only 1mm per day, says Dr Ellerton. But it is essential to control the disease before it reaches the stem, as control then becomes almost impossible and stem cankers will result later in the season, he adds.

All rape crops should be monitored and fungicides applied once crops have reached a threshold of 10-20% of plants infected with phoma leaf spot – and the risk forecast will indicate when this is likely to occur.

Dr Ellerton advises that priority should then be given to spraying small plants of varieties with high susceptibility to phoma, where there is a shorter distance for the mycelium to travel before reaching the stem.

Sprays applied for phoma control will also inhibit light leaf spot. But if no spray has been applied for phoma, then a routine protectant fungicide should be applied for leaf spot in late October or early November – although symptoms may not be found in crops until late November.

Second spray

“Even if a phoma spray has been applied, a second spray may also be required, particularly if development of phoma leaf spots have prompted very early application of the first fungicide,” says Dr Ellerton.

Risk forecasts for light leaf spot in the autumn – again available on the Rothamsted website – are based on region, amount of pod disease the previous summer and deviation from the 30 year mean summer temperature. All indications are that the risk is likely to be high this autumn.

Symptoms shown are large, mealy blotches on the leaves, with a pinkish white centre and white spore droplets around the edge of the lesion. It may be necessary to incubate them for a couple days in a plastic bag for these droplets to develop, says Dr Ellerton.

The Rothamsted Research website is at www.rothamsted.ac.uk

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