Critical input: chilling hours (part 2)

Written by   Bas van den Ende

When you can see symptoms of delayed foliation it is too late and the damage has been done.

Minimise the risk (Continued from last month)
If it appeared that trees were not exposed to sufficient chilling and that delayed foliation would most likely occur, the trees could be sprayed with a rest–breaking agent so that bud break would be normal.
There are other things that you can do to minimise the risk of delayed foliation:
•Before you decide what to plant, get as much information on the chilling requirements of the variety.
•Take care of your trees after harvest—make sure that your trees go into winter with ample reserves of nutrients and carbohydrates.
•The nitrogen status of your trees must be optimum—trees with low reserves of nitrogen require more hours of chilling.
•Keep zinc in your trees at optimum level as indicated by leaf analysis in summer.
•Do not excessively irrigate, fertilise and prune your trees during late summer. These practices will prevent hardening of the fruiting wood, and buds on this wood will not have enough time to mature and accumulate sufficient hours of chilling.
•Buds formed in shade require more chilling than buds formed in an open canopy.
•Buds on horizontal branches have lower chilling requirements than do buds on vertical branches.
•There is a relationship between tree vigour and bud break. Long vigorous shoots have less bud break than do short weak shoots.
•Rootstocks can have an effect on the number of hours of chilling trees need.
Tree response to chilling
If a variety gets too much winter chilling it may flower too early and flowers can be damaged by frost.
If the chilling is insufficient, flowering occurs over a long period, leafing is sporadic and sparse, crops are light and ripening is delayed.
Varieties that receive the right amount of chilling have normal flowering and leafing, good fruit set and ripening is normal.
Battling the elements is not new for fruit growers. Wind, rain, frost, hail and heat waves are all a reality and can have devastating effects.
Many people believe that climate change is altering our weather and is shaping up as the world’s greatest environmental challenge. As a result, fruit growers in some temperate districts of Australia need to be aware of insufficient chilling.
Before delayed foliation occurs though, you have an opportunity to take appropriate action to minimise the effects.
South African experience
In most areas where apples and pears are growing in South Africa, the trees do not get enough winter chill to break dormancy satisfactorily. This results in delayed foliation, a protracted bloom period and poor yields.
(cont. next month)

See this article in Tree Fruit June 2018

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