Reports of Phoma leaf spotting on a crop of oilseed rape at the four-leaf stage near Thame in Oxfordshire this week (1st October) has highlighted the risk to backward crops that have been late to emerge this season, warns Syngenta Field Technical Manager, Chris Rowsell. The outbreak has occurred nearly two weeks ahead of the Rothamsted Research predicted 10% spray threshold for the area.
“With the high incidence of Phoma stem cankers in last year’s crop and the recent wet weather, infective spore release has already occurred,” he advised.
“Although conditions are far cooler than last autumn, it is still mild enough for the Phoma infection to develop and move down the leaf, albeit slower than in warmer temperatures.”
Rowsell added that this season’s crops with small leaves and that are growing slowly, especially late-sown or slow-emerging crops, are a priority for Plover fungicide treatment as soon as thresholds are reached.
“With slow-growing plants that are not quickly putting on new leaves, the older leaves could be slower to senesce and drop off this autumn,” he said. “That means any infection on these old leaves could continue to develop down the petiole and infect stems, unless leaves are protected in time.” He believes it could also increase the need for a repeat application after four to six weeks if new disease lesions appear.
With backward crops Rowsell advises it will be especially important to avoid using fungicides with strong growth regulatory effects, such as metconazole or tebuconazole, which could cause further checks in crop development.
Whilst Plover can be tank-mixed with a grass weed or volunteer cereal herbicide, the importance of the Phoma control on small plants this season means it will be crucial not to compromise the fungicide timing to fit in with herbicide application, he added.
