Late bloom removal in apples & pears using Thin-It (ATS)

Temperature conditions in late October and into November are frequently conducive to fire blight bacterial build up and infection of late blossoms. Late blossom infections play a significant part in establishing fire blight in orchards. Even a few overwintering cankers can become significant inoculum sources for fire blight epidemics in subsequent seasons. Newly planted blocks usually flower later than established trees, so are especially at risk to fire blight.

Removal of late blossom is relatively easy and should become an established part of an integrated fire blight management programme.

The most proven late bloom removal tool is the blossom thinner Thin-It (Ammonium Thiosulfate, ATS).

There is an extensive body of experience in using Thin-It in NZ for late bloom removal in apples and pears and the following bullet points summarise this experience.


  • Late bloom is relatively easy to remove. Apply Thin-It at label rates of 1.5% (1.5L per 100 litres of spray mix) on Apples and 1.25% (1.25L per 100 litres of spray mix) on Pears.
  • The addition of Gro-Wet wetting agent at a rate of 0.1% (100ml per 100 litres of spray mix) is highly recommended and is expected to reduce the potential for excessive wetting and crop and leaf damage.
  • Note that the Gro-Wet label contains a specific recommendation for use with Thin-It, but the current Thin-It label recommends no adjuvant additions. Do not apply Thin-It at full dilute water volumes when using Gro- Wet. Thin-It rate, maintain the rate per 100L of water.
  • Thin-It spray application volumes should not over-wet tree foliage and visible runoff is not desirable. An ideal spray volume for late bloom removal is approximately half the runoff volume for the target canopy. Aim to apply between 500 (smaller dwarf trees) to 1,000 (larger traditional centre leader trees) litres per hectare when using in combination with Gro-Wet.
  • DO NOT apply Thin-It if significant rain, or prolonged canopy wetness is expected within the next 48 hours or any rain within 8-12 hours. Apply under good drying conditions. Rewetting will reactivate the Thin-It and the risk of fruit and leaf damage can be significant.
  • Thin-It is only expected to work on recently opened flowers that have yet to be pollinated – which will also be susceptible to fire blight. The Thin-It effect is to burn out the flower stigmas, which will also make them unreceptive to fire blight bacteria multiplication and infection.
  • You should only need to apply Thin-It as a late bloom removal treatment when a fire blight infection period is forecast. Application should ideally be made 48-96 hours before an infection period.
  • Consider applications of Thin-It on approximately seven-day intervals during fire blight risk conditions while late blooms are still emerging.
  • Application using normal orchard sprayers using rates and volumes described above is expected to faster and safer than spot applications to blossom clusters by hand using a knapsack sprayer. Knapsack spraying is approximately 5-10X faster than hand removal, but there is a risk of over-wetting blossom clusters when using hand application methods, with increased risks of local leaf and fruitlet damage.
  • In pears late blooms typically appear throughout the tree and full tree spraying would be recommended. In apples late blooms are more usually associated with one year old wood and the spray output can be biased to the tops of trees – with a proportional reduction in spray volumes (eg if just spraying the top half of dwarf apple trees spray volumes could be reduced to 250-300 L/ha).
  • Always treat a small test area to gain confidence in the required application rates and volumes before committing to treating a large area.

Read and follow the Thin-It and Grow-Wet label recommendations and Grochem technical information for use of Thin-It prior to use.


DISCLAIMER: The information contained in this publication is of a general nature and should not be relied upon as a substitute for professional advice in specific cases. As no control can be exercised over the manner in which or the conditions under which this product is used, no responsibility will be accepted for any damage or injury whatsoever arising from the storage, handling, application or use of this product.