29 October 1999

Phoma remains at high levels

RAIN and temperatures over 10C (50F) mean phoma leaf spot remains at much higher levels, much earlier in the season, than in recent years, according to the PhomaWatch service run by farmers weekly, NIAB and Du Pont.

Monitoring at NIAB trials sites shows levels are uniformly high and above threshold for spraying. In current conditions phoma will be growing down petioles towards crop stems, where it forms cankers, at more than 3.5mm a day, warns Andy Selly for PhomaWatch sponsor Du Pont.

Last weeks lowest score was 12% plants infected at NIABs Hants site. This week the same site is 31% infected. The predominantly small plants are showing more and larger spots, suggesting the fungus is developing rapidly, notes NIABs southern region manager, Duncan Connabeer.

Time may now be short for getting sprays on to prevent infection moving from petioles to stems, warns NIABs David Kenyon. On small plants even late infections could produce damaging cankers next year, he adds.

Higher fungicide rates to boost eradicant activity may be appropriate where spots have been present for several weeks already, or where there are delays getting on to crops with high infection.

Observations in Apex and Madrigal show spotting is about the same as in Pronto. There is still less spotting on more resistant Escort.

Results from previous years show that where 5% of plants were affected in Oct/Nov growers treating in the autumn and at stem extension can boost yield by 0.64t/ha (5cwt/acre), says Mr Selley. &#42