Stay alert for Phoma activity this autumn
Phoma leaf spotting could start to become evident by early October, predicts ADAS plant pathologist, Faye Ritche. But this year growers can be better forewarned and prepared to take action against the most damaging early infections, thanks to Syngenta’s new Phoma Alert.
Managed by Faye and her ADAS team, to monitor reference crops in Herefordshire, Yorkshire and Cambridgeshire, it will give a valuable update of pending disease pressure - to enable more proactive spray timing - along with assessing the relative value of varietal disease resistance ratings.
“We know that many areas have already received the critical 20 days with rainfall from the 1st August, which has primed cankers on last year’s crop trash to release ascospores,” reported Faye.
“With each further rain event, repeated spore releases would be triggered and, with crops infected, the onset of characteristic leaf spotting.”
Faye, (above) warned that, unless leaf spots are controlled, infection will imperceptibly move down the petiole to reach the stem. The earlier that stems are infected, the greater the size of canker that will develop in the spring, and the worse the effect on yield.
“Backward crops are more susceptible, since infection will move to the stem faster on smaller leaves; particularly if weather conditions remain mild when disease development is faster,” she advised.
“Varietal resistance appears to help growers by slowing the development of infection on the leaf, which could gain valuable flexibility in fungicide timing.
"The Phoma Alert monitoring will help give practical guidance as to how the resistance ratings could help tailor fungicide programmes,” advocated Faye.
She pointed out that first treatments should be applied as soon as leaf spotting is identified on a threshold 10-20% of plants. Small plants, warm temperatures, repeated rain showers and susceptible varieties would all be factors to warrant earlier treatment at lower thresholds, she suggested.
Phoma would typically be the prime target for treatments in early to mid-autumn. However, growers may look to introduce some additional Light Leaf Spot activity with later follow-up applications if required, later in November – and then monitor crops through to January or February for LLS development and treat then accordingly.
Syngenta Technical Manager, James Southgate (above), advised growers should look to treat with straight Plover in the first instance, to provide cost effective control of the most damaging early Phoma infections.
“Protecting yields on the significantly reduced area of oilseed rape has been established this season will ensure growers can benefit from any resulting further increase in oilseed values,” he advised.
Phoma trials have repeatedly shown yield loss of 0.5 t/ha without treatment - worth around £155 at current prices.
James pointed out that for later season applications - often as second sprays in the Phoma programme - growers and agronomists should be assessing risk of initial LLS infection and may consider adding tebuconazole to Plover. “That will remain a highly cost effective way of providing Phoma and LLS protection for the winter months.”