Urea has been tested and used as a blossom thinner for apples in Germany for many years.
- It is recommended at 3% to 4% high volume sprays and is especially useful with varieties prone to biennial bearing, such as Elstar (Graf, 1997).
- The product seems to bring about its thinning action by scorching the spur leaves.
- Where no spur leaf damage is noted, thinning is often poor. One of the concerns with this urea strategy to thinning is that early loss of spur leaves may well reduce calcium uptake and increase subsequent problems with bitter pit (Volz et al., 1994).
- Growers using urea as a blossom thinner should also be aware that it may lead to poorer fruit colour at harvest, fruit russeting and reduced flowering in the subsequent season (Wertheim, 1997).
- Urea at concentrations of 3-4% has given good, albeit inconsistent, effects as a flower thinner on apples in German and Danish trials.
- However urea causes considerable damage to the spur leaves and for this reason appears to have little potential as a thinner in UK conditions.
- Urea is not approved currently for use as a flower thinner in UK orchards.