The October update of the AHDB-funded phoma leaf spot forecast shows the disease has got off to a head start this season.

The forecast uses temperature and rainfall data (from the summer to early autumn period) to provide information on potential disease pressure.

According to the forecast issued today (6 October 2017), treatment thresholds have already been reached at some sites. These sites are located in the South West, the South and East Anglia. Visual reports from fields in these areas confirm the forecast findings.

Application timing is the key to success for phoma control and the forecasts can be used to focus crop walking and guide spray decisions against phoma.

The UK forecast map uses a colour-coded system to show how long it will take for each site monitored to reach the spray threshold of 10 per cent of plants affected.

Dr Neal Evans, Weather INnovations Consulting LLP (WIN), who puts the forecast together, said: “The mild autumn and wet August and September have really pushed the development of the fungus on this year. It’s led to the early release of airborne spores and the appearance of leaf infections at the earliest time for several seasons.”

Recent additions to the AHDB Recommended List (RL) have added strength to the overall resistance to phoma stem canker, with several varieties having a resistance rating of 8.

There are several varieties, however, with relatively low resistance ratings (3 to 4). These should be the priority for treatment, particular for backward crops.

Making a spray application as close as possible to the threshold timing can help maximise the effect of fungicides.

The latest fungicide performance information shows good control can be achieved with half rates applied as a two-spray programme

A second spray should be made when re-infection is evident – typically four to ten weeks after the first spray.

Fungicide performance information and the phoma forecast can be accessed via cereals.ahdb.org.uk/osrdisease

About phoma

Phoma stem canker is caused by two species of fungi: Leptosphaeria maculans and L. biglobosa. In autumn, fungal spores are released from infected crop stubble. The spores infect young susceptible leaves which then develop the characteristic leaf spot. Fungi grow from the leaf into the leaf petiole and then infect the plant stem – the smaller the plant, the quicker this step can occur. Once in the stem, the fungi kill tissue causing visible phoma stem cankers in the late spring and through the summer. Cankers caused by L. maculansare generally associated with the stem base and are considered to be more severe than the upper stem lesions often associated with L. biglobosa. The forecast is based on L. maculansdevelopment.

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