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This year in Hawke's Bay, we have already seen two severe frosts over the spring period. The first on Sunday 15 September was too early to affect apples, but did quite a lot of damage to early shoots on grapes and developing summerfruit crops.
The second on 26 September was more severe, with air temperatures in the region of -10C to -30C and ground temperatures as low as -50C in some localities.
This frost inflicted further damage to grapes and summerfruit, as well as some damage to pipfruit in the colder parts of the district.
Among apples, Braeburn appears to have suffered most injury simply because its flowering was more advanced than other varieties. Where injury is present in apples and pears, it appears that any flowers which were open at the time of the frost were killed, and depending on how cold it got, some damage has occurred in the less mature buds.
Critical temperatures for a 10% kill from frost by growth stage are as follows for apples.
- Green tip minus 7.50C
- Tight cluster minus 3.90C
- First pink minus 2.80C
- Full pink minus 2.70C
- First bloom minus 2.30C
- Full bloom minus 2.90C
- Post bloom minus 2.30C
The increase in frost tolerance for full bloom compared to first bloom is due to the sheltering effects of the attached petals giving protection to the fruitlet.
As the table shows, buds which were still at tight cluster at the time of the frost will tolerate considerably more frost than those which were much more advanced.
Historical information on frost injury in Hawke's Bay indicates that pre-bloom frosts generally have little effect on crop for pipfruit. The reason for this is that there is a lot of late flower likely to follow up on the main flowering, and with much of the earlier set fruit lost, the inter-fruit competition is reduced, and this late flower will set strongly.
The effect of this frost is going to impact on the thinning management for the crop.
Where damage is severe and a high proportion of the developing flowers and fruitlets are showing brown staining applying an aggressive blossom thinner programme could give excessive thinning, so in these situations target a late Ethrel at the weaker one year wood later flower bud in the tops of the tree to remove some of it, then use a secondary post-blossom thinner such as Carbaryl (if the lower tree fruitset looks excessive).
Damaged fruit is likely to show little increase in size once the petals have fallen, and wil then drop out shortly afterwards. Unaffected fruit will grow away rapidly from the others.
Where damage is only slight, the fruit may not drop, but there could be some shape problems and most likely an increase in fruit russet.
With the likelihood of more frosts, precautions need to be taken to minimise further losses. In apples, wind machines and helicopters appear to have worked well for the 26th September frost. Under tree sprinklers have reduced injury, but may not have fully protected vulnerable summerfruit in colder locations, however they are definitely better than nothing.
Maintaining good weed control and tightly mown grass through the frost risk periods will also help.
There is also anecdotal evidence that application of foliar sprays of low biuret Urea at 500 g/100 l at 1000 l per hectare immediately prior to frost events will also reduce risk of injury.
In a few cases, the chemical thinning programme will need to be modified due to the frost, but in most places the hand thinning bill will be huge if the chemical thinning programme is neglected because of possible frost injury.
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