These were the main conclusions reached by East Malling Research's (EMR) Dr Angela Berrie after carrying out a number of studies on zero-residue apple production at EMR and on seven farms in Kent and Gloucestershire beginning in 2001.
Her work was funded by Defra, World Wide Fruit and the Horticultural Development Company. She was assisted by EMR workers and growers, Robert Mitchell and his manager Bob Little, Peter Checkley, Sandys Dawes, Clive Maile, Oliver Doubleday and his UAP adviser Neil Franklin and staff at North Court Farm and Gaskains.
"Good pest and disease monitoring is essential to the success of zero-residue systems," she told an association members' day on tree fruit at EMR on 24 November. "And once you start reducing (conventional) pesticides, other problems may crop up such as mussel scale, fruit tree red spider and woolly aphid."
She explained that the main period for pest and disease activity is early in the season when applied pesticides are unlikely to leave residues because harvest is a long way off. After petal fall the zero-residue programme includes pest control products such as Bacillus thuringiensis and Granulosis virus together with sulphur against mildew that won't leave residues.
Post-harvest urea sprays help reduce overwintering scab inoculum, while the removal of primary mildew and canker lesions mean a smaller reservoir of infection of these diseases at the start of the spraying season.
During the first six years of Berrie's studies the weather was not conducive to scab infection, she reported, so scab control in the zero-residue plots was as good as - or better than - that in the conventionally sprayed plots, even on a susceptible variety such as Gala. Not until 2008, when it was very wet early and late in the season, did scab become a problem on zero-residue Gala.
In contrast, the mildew threshold of 30 per cent infected shoots occurred in most conventional and zero-residue plots at all times. The exception was on the EMR site, where infection was below the threshold under both systems in all years thanks largely to good orchard hygiene.
Berrie's general conclusion on the incidence of storage rots is that they don't appear to be increased by the zero-residue regime. In 2008, however, about four per cent rots were recorded on February-stored Braeburn from the zero-residue plots - compared with negligible levels in the conventional plot fruit. In the same year, when the summer was very wet, Bramleys from both regimes, stored until late June, had a high incidence of rots.
The zero regime's pest control "appears to be satisfactory", she maintained, but it could be more expensive than the conventional system's due partly to its greater monitoring requirements and the high cost of the Granulosis virus for codling moth control.
The fruit was analysed at harvest. No residues were found on the zero-residue samples but they were on the conventional treated fruit depending on what pesticides were applied in the run up to harvest. However, all were below the maximum reside level.