9 October 1998

Timing is everything for autumn phoma control

Cuts in oilseed area aid are

likely for rape crops in the

ground as well as those just

harvested, so high yields are

vital to keep budgets on

track. Charles Abel looks at

the scope for making more

of autumn fungicides

TIMING is all when it comes to oilseed rape disease control. Unless fungicides go on at the right time, big yield benefits could be squandered, say crop experts.

Last year an estimated 28% of crops received an autumn spray. According to Peter Gladders, oilseed rape specialist at ADAS Boxworth, it paid off. "For the first time moderate leaf spotting in the autumn was not followed by severe canker in the summer."

Independent trials sponsored by fungicide maker Du Pont showed yield responses of 10-54% over untreated crops, adds Jane Thomas of the National Institute of Agricultural Botany.

More crops could benefit from such an approach this autumn, Dr Gladders maintains. If conditions remain wet crops will come under severe phoma pressure from inoculum built up on old crop stubbles.

The key to successful control is spraying as soon as phoma is found, usually in late October or early November, he stresses. "You need to be out walking crops and checking for phoma leaf spot every week. Timing is everything – sprays must go on as soon as spotting is seen. Last year we had one crop at Boxworth go from 0% to 90% infection in just two weeks."

NIAB trials show crops yielded 0.3t/ha more where the first autumn spray of 0.4l/ha of Punch C (flusilazole + mbc) went on at the start of infection in October, rather in December. In both cases a second spray went on in February.

Fungicides with eradicant activity, including flusilazole (as in Punch C and Sanction), tebuconazole (Folicur) and difenconazole (Plover), all have some ability to remove initial infections, notes Dr Gladders. Prochloraz (Sportak) is better suited to prophylactic use.

It will also be worth considering plant size and soil temperature. Phoma leaf spot infections move to the stem to form a canker. Small leaves, short leaf stalks and mild weather all speed that process. That means crops with small plants should be the priority for sprays, says Dr Gladders.

Varietal resistance also needs considering. Estimates suggest up to a quarter of the crop could be in susceptible varieties with NIAB phoma resistance ratings of five or less, like Pronto and Synergy. Over half the crop is likely to be in moderately resistant varieties rated six, like Apex. "A large proportion of crops will need good disease control," stresses Dr Thomas. &#42


&#8226 Timing critical to success.

&#8226 Late Oct/early Nov key.

&#8226 Check for phoma leaf spots

&#8226 Allow for variety choice, temperature and plant size.

&#8226 Follow-up at stem extension vital.

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