Count the number of days in which rain falls from 1 August to determine when phoma infections are likely to start in oilseed rape, Peter Gladders, ADAS plant pathologist, told growers.
“A key driver for phoma is the rainfall pattern in August and September – rain is needed to produce airborne spores.”
Just 20 days of rain – even as little as 0.1mm – were needed to trigger phoma infections, he said. “It’s worth ticking off the days with rain from 1 August.”
Phoma was a difficult disease to control, he said. “Spores are every-where in the environment – it is not just what you do on your farm that matters. And you need to be tuned in to early epidemics.”
Infection in the current crop came in early, he said. “We’re seeing the consequences of that now with early ripening. Two sprays look like they will have paid this season.”
Appropriate fungicide dose trials suggested there was not much difference between most products in controlling phoma leaf spotting. “Half-rate sprays applied twice will be very robust.”
Caramba (metconazole) was a bit weaker at preventing subsequent stem cankers than competitor products, such as Proline (prothioconazole) and Punch C (carbendazin + flusilazole), however. “It makes timing a bit more critical, but it does get the stem canker index below 30 where yields are unaffected.”
Against light leaf spot Proline and Prosaro (tebuconazole + prothioconazole) were ahead of competitor products in terms of a yield response in Scottish trials, he said. “But in English situations [where pressure is generally lower] other products will do a good job for you.”