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Crop Advice & Expertise

Phoma Re-Infection Likely As Rain Continues

With twenty days with rain being recorded at ADAS Boxworth by September 11th Phoma leaf spot may be seen earlier than last year in OSR.

To reach 20 days of rain since August 1st this early in September is unusual and ADAS plant pathologist Faye Ritchie says growers need to be extremely vigilant to identify the 10 to 20% plants affected threshold to apply fungicides. “Further wet weather from now on will trigger spore release and it is possible that leaf spots will start to be seen in late September. Regular monitoring of crops to identify when thresholds are met will be key from now onwards.                                                       

“Monitoring crops for re-infection after first sprays will be necessary to identify the appropriate timing for a second spray, which could be between 4 to 8 weeks later depending on whether conditions remain unsettled,” she advises. 

Bayer’s Tim Nicholson expects crops drilled in good time will have sufficient growth to provide more flexibility in application timings, which can be important in a period when spray days can be limited. 

But with so much wet weather from August 1st he agrees that the risk is particularly high this season and has a pressing concern with late drilled crops. “It’s the small crops that are particularly vulnerable as the disease can rapidly spread to the stem, and it’s early infections that cause the most severe stem canker. 

“Phoma losses have rarely matched that of light leaf spot (LLS) in recent years but in severe cases it can sever the stem, killing the plant. We have the possibility that late drilled crops will get come through the ground just in time for peak spore production.” 

For crops still to be planted he urges growers to look for varieties with exceptional vigour. “Here you really need a variety with high autumn vigour and good Phoma resistance. InV1030 or InV1035 would be good options in this situation,” he says. 

But he would always support more resilient varieties with foliar applications. “No variety is immune to Phoma or LLS so foliar support is always essential. Strong Phoma resistance might delay thresholds being reached and even allow for a single application of a product like Proline275 (prothioconazole) to manage both diseases. But I wouldn’t take any chances, especially with rapeseed prices having recovered and so much wet weather already,” he concludes.