Managing changing climateResearch & Extension
Orchards worldwide are dealing with the impacts of changing climate. AgFirst’s John Wilton outlines what current research reveals on how to manage the impacts of milder winters and warmer springs.
In the last article, I reviewed what growers could expect at orchard level from Climate Change.
In this article my intention is to focus on managing the problems milder winters and higher spring temperatures are likely to bring. There are a number of locations around the world where winter chilling is marginal for pome fruit production. In the southern hemisphere marginal winter chilling occurs for pome fruit in southern Africa, and the southern highlands of south eastern Brazil as well as the mild winter parts of Australia such as Stanthorpe and Western Australia.
In the Northern hemisphere Israel, and lower altitude Northern Africa such as the Nile Delta area of Egypt are examples of apple and pear production in marginal locations. There is a lot of good published scientific information available on managing the effects mild winters on pome fruit dormancy behaviour.
Assessing low winter chill
Symptoms of inadequate winter chilling include slow and delayed budbreak, blind wood, particularly where annual shoot growth is excessively strong. Tree nutrition, particularly nitrogen deficiency will also increase delayed budbreak symptoms and also impact on flower quality and fruit set.
Summer heat and moisture stress can induce dormancy mid growing season, leading to an autumn growth flush and flowering prior to leaf fall. The effect of this is to reduce potential flowering sites for the next crop. The presence of late flower fruitlets through winter into spring can also harbour diseases, such as scab which in spring provides a ready source of Conidia to infect new tissue in the spring.
Managing the dormancy and budbreak problem therefore needs to be a whole growing season task, rather than just a leaf fall/dormancy problem and a spring dormancy breaker application. If at all possible, excessive mid-growing season stress needs to be avoided to minimise the autumn growth flush and flowering problem.
High quality apple production is dependent on careful nitrogen fertilizer management. Running into harvest, the tree needs to be on the verge of nitrogen deficiency for coloured apple varieties to ensure early high colour development ahead of optimum maturity for good storage.
Then, once harvest is over, nitrogen levels must be built up in the period between harvest and leaf fall. Foliar nitrogen, such as urea at 2-3 per cent concentration applied several times as leaf fall approaches is an efficient way to build up levels of nitrogen in the buds.
In locations with a mild winter, soil temperatures are high enough for active root growth to continue into the post-leaf fall period, so post-harvest soil applications of fertiliser will be readily available to the tree through the roots at this time. Avoid applying fertiliser nitrogen until growth has ceased and the leaves are beginning to fall because we do not want to prolong any growth flush that is underway.
Incidentally, trees with deep green leaves and poor autumn leaf colours already have too much nitrogen so do not need any more.
Weakly growing trees which terminate their extension shoot growth early perform better than vigorous trees in locations with marginal winter chilling. Dwarfing rootstocks perform better than more vigorous rootstocks in low chill situations. Some years ago I was shown a rootstock trial in Cacadu, Santa Catarina, Brazil, where they were looking at the influence of different rootstocks on bud break. I was there in the winter, so the trees were dormant, however it was easy to see the impact of rootstock on bud break by looking at the amount of blind wood in the trees. Incidentally, M26 and M9 were only ordinary in their regularity of bud break compared to some of the other rootstocks in the trial.
There is also some evidence from pears that low chill rootstock cultivars will influence scion bud break behaviour.
In low winter chill situations, strong terminal buds have shallower dormancy levels than lateral leaf buds so when growth commences these terminal buds are the first to move, along with the stronger flower buds. Rapid growth from these strong terminals, suppresses lateral leaf bud so in the low chill situations there can be very strong apical dominance, some flowers, and insufficient leaf and side shoot growth to support the developing fruitlets.
Where strong apical dominance from terminal shoot growth is a problem, delayed pruning to a couple of weeks after growth begins will remove these problem high vigour shoots, reduce their growth inhibition effect on the weaker leaf buds and improve lateral bud break. This is a very good technique for minimising blind wood.
Training branches into a slightly pendant position will improve bud break and reduce the amount of excessively strong vertical shoot growth. Even with branch training there will still be a tendency for vertical water shoots to occur particularly if bud break is sparse. Removal of these upright shoots in the spring may help increase lateral bud break.
Where excess vigour is a problem, vigour control techniques such as root pruning, or girdling, will bring vigour under control.
Trees go into dormancy to avoid hazardous growing conditions. Leaf drop and dormancy in deciduous trees is usually a response to water stress, or the onset of tissue-damaging cold temperatures.
In the case of pome fruit growing in cool temperature climates, the onset of winter preceded by autumn frosts and falling temperatures triggers leaf fall and dormancy. With climate change leading to warmer temperatures, these cold signals will become weaker, so dormancy will be delayed giving the tree a shorter dormancy period in which to accumulate sufficient chill units to overcome endo-dormancy.
Where apple production is carried out in the sub-tropical climates with insufficient cold to induce dormancy, it’s necessary to adopt husbandry practices which induce dormancy. It is possible to trick the tree into dormancy by causing defoliation.
In dry climates it may be possible to manipulate of soil moisture levels to stress the roots to hasten leaf fall and dormancy, or at least to shut down active vegetative growth. For good bud break response in the spring, buds need to be fully developed at the time of leaf fall, so active shoot growth needs to be shut down well before leaf fall. There are a number of husbandry practices which could be used to hasten dormancy. These include:
- Trunk girdling
- Root pruning
- Chemical defoliant
- Growth regulators
In the nursery industry, chemical defoliants are widely used to defoliate trees prior to lifting so there is quite a lot already known about the use of defoliants on pome fruit. The tricky point is their timing relative to bud development stage. The objective of a dormancy induction program is to try and compress leaf fall and dormancy period so that in the spring, bud break and flowering will be compressed too.
Where pome fruit production is carried out in locations with marginal conditions for satisfying dormancy requirements, the use of chemical dormancy breakers is standard practice. Even in locations where there is adequate winter chill, dormancy breakers are often used to compress blossom. Crop management, particularly in regard to thinning and harvest, is much easier if most of the fruit on the tree is at a similar phenological stage.
There are now a wide range of dormancy breakers available. The trick is understanding their timing relative to anticipated bud break, suitable weather conditions for application and required water rates necessary for satisfactory coverage. Some products have a fairly slim margin between efficiency and phytotoxicity so good application technique is critical for a satisfactory result.
The biggest problem we see with dormancy breakers is premature application timing leading to irregular bud break response. Dormant buds need to have come out of endo-dormancy and have already progressed to an early bud swell stage to become responsive. When dormancy breakers have been applied too soon only the most advanced buds respond. These are usually the terminal buds, and strongest floral buds. In this situation, blossom periods are extended rather than compressed. There is very little response from axillary buds on one-year old laterals when dormancy breakers are applied too soon.
Spray coverage and speed to drying will also affect response so thorough spray coverage is necessary. Poor spray coverage often shows up as weak response in the upper tree. In some locations, weak upper tree responses may also be an indication of premature timing if that upper tree bud development is less advanced than in the lower tree. Calm, moderately slow drying conditions at the time of application tend to give the best results for hydrogen cyanamide dormancy breakers.
The surfactant plus nitrogen fertiliser-based products appear to require a period of relatively high ambient temperatures following application, as was necessary with the old DNOC plus oil dormancy breaker sprays which can no longer be used. For these products part of the response mechanism appears to require increased respiration within the bud following application. These products possibly behave in a similar manner to oil, by sealing the bud to allow a build-up of CO2 levels within the bud leading to bud break.
Some of these products also have a slim margin of safety in which case there needs to be careful spray nozzle calibration to minimise any zones of over spray in the canopy. Dormancy breaker response for some products may be poor if they were applied in windy fast-drying conditions.
It is sometimes difficult to determine if poor upper tree response is due to timing or spray coverage. If it’s a timing problem, axillary buds on one-year laterals in the lower tree, where spray coverage was good, will also show indications of poor responses.
With good climate data using one of the better chill unit models, such as daily positive chill units, to determine when about 75 to 85 per cent of the chill unit requirement has been met, followed by some heat unit accumulation, it should be possible to model the application timing window.
Buds usually enter dormancy before leaf fall occurs, so as long as active shoot growth has ceased, many buds on the tree will already have become dormant. This being the case, it’s not necessary to wait for leaf fall before starting the chill unit model.
Another approach used by some growers is to observe the behaviour of other deciduous species with earlier bud break behaviours. Once the dormancy breaker application dates have been benchmarked against bud break in these earlier bud movement species, sprays are applied when the benchmark species bud break reaches the appropriate stage.
Future orchard planning
Property elevation and aspect will affect micro-climate. High elevations generally experience more chilling than low elevation. Cloud and fog levels over the dormant period will lift chilling hours by reducing the level of solar radiation reaching the buds. Fog is a very important contributor to chilling hours in the central valleys of California and Chile as well as some high-altitude pome fruit growing areas in the tropics.
South and southeast-facing slopes accumulate more chill than those facing the north and west. Plant alternative crops such as citrus or avocados, which need warmer winter conditions than pome fruit, on the warmer slopes and plant the latter on the colder, south-facing slopes.
In the long term, plant breeding will probably solve some of the crop adaptability problems associated with climate change by developing rootstocks and cultivars with less chilling requirement.