High temperatures and extreme weather this season have focused attention on the need to understand the impact of – and how to adapt orchards to – climate change. AgFirst’s John Wilton reviews the latest research on what can be expected as temperatures rise.
The events of this summer with its extreme heat during January leave little doubt that climate change is occurring. The trend is towards higher summer temperatures and milder winters. Hort Innovation has recently posted the final report of Heidi Parkes, “Understanding apple and pear production systems in a changing climate”.
This is a very comprehensive report covering the way these climate changes will impact on the behaviour of apple and pear trees between now and 2050.
Areas with marginal winter chilling, such as Western Australia and Stanthorpe in Queensland, will become more marginal leading to more erratic bud break and flowering, whereas areas with adequate chilling at present may start to become marginal and similar to what Western Australia already experiences. These climate changes will undoubtedly lead to a shift in cultivar selection towards cultivars better suited to lower chilling.
In the short term, chemical dormancy breakers can be used to improve and compress bud break and blossom periods where winter chill levels are marginal. In the longer term, adoption of improved varieties and maybe even rootstocks tolerant to lower chilling requirement, maybe the way forward.
Mild winter problems pale into insignificance when the problems caused by increasing summer temperatures are considered. Sunburn injury to fruit is the obvious problem which now occurs in most apple growing districts to some extent. In hot districts, it is already a serious problem and forecast to markedly increase.
In the absence of any protection measures, injury levels up to 20 to 30 per cent are forecast for some regions. This level of injury will clearly make continued production of the crop unsustainable. In addition to this very obvious injury effect, there are also a number of subtle effects likely to occur with higher growing season temperatures.
Most plants and cultivars within a species have optimum temperature ranges for performance. In the case of many apple varieties photosynthesis, the process which drives tree performance tends to fall away as temperatures approach and rise above 28°C. When this happens, fruit growth rates stall leading to lowered fruit size potential. Hot weather is usually accompanied by elevated night temperatures too, therefore respiration rates rise as well increasing the impact of lower leaf photosynthesis.
Depending on the growth stage when these high temperature periods occur, flower development for the following season will be adversely affected.
Analysis of apple production in British Columbia, by JM Caprio and HA Quamme, over a 72-year period found that high temperatures early in the season, during the flower initiation period, and later in the growing season when flower bud development occurs, depressed yields in the following season. Threshold temperatures for suppression of flower initiation were 30°C and for flower development, only 26°C.
This effect on flower bud development increases biennial bearing tendencies in varieties already prone to the problem, and will induce more regularly cropping cultivars such as the Gala group to become biennial bearing. In the case of the Gala group, which was bred in New Zealand, a moderately cool summer climate, it crops very regularly here in New Zealand, but in hotter climates such as Chile, it is prone to biennial bearing.
As summer temperatures rise, there will have to be more emphasis placed on managing the biennial bearing problem.
Water and soil temperatures
As temperatures rise evapotranspiration is also likely to increase. This means the trees will require more rainfall or supplementary irrigation to satisfy their water requirements. It is probable that rainfall patterns will change too, often towards less rainfall. This will lead to increasing problems with water stress and its availability.
Soil temperatures tend to track average ambient air temperatures, but usually lag behind air temperatures. Site aspect will impact on soil temperature and therefore soil moisture behaviour. Sites sloping towards the north and west will be hotter and drier than southerly or south easterly sloping sites. You often observe these effects in native plant populations. Plants that demand higher soil moisture for satisfactory growth are usually confined to cooler, south facing slopes.
It is probable that among apple rootstocks there will be different levels of soil temperature tolerance. It is generally recognised that shallow rooting stocks such as Malling 9, are more sensitive to high soil temperatures than deeper rooting, higher vigour rootstocks.
Climate change will influence insect life cycles and their distribution. It has already been reported that codling moth numbers may increase in a warmer climate because with higher temperatures the usual two generations per season may expand into a third generation.
Fruit fly ranges may extend southwards as has been happening in recent years with Queensland fruit fly. Red and two spotted mites may become more numerous. Woolly apple aphid is generally more of a problem in warmer locations. On the positive side, the wasp predator, Aphelinas mali will be more plentiful in warmer spring temperatures.
In locations where climate change increase summer rainfall and humidity, summer rots including internal core rots will become more plentiful. Alternaria leaf spots are also likely to increase too. Warmer drier climates tend to favour increased problems with powdery mildew.
On the other hand, apple scab may become less of a problem in a warmer, drier climate. Increased environmental stress will tend to increase incidence of bacterial blast which for instance is a problem in South Africa.
Deficiencies and physiological disorders
Calcium related disorders such as bitter pit and lenticel blotch increase markedly in higher summer temperatures. This is due to calcium being withdrawn from the fruit in the moisture movement out the fruit which occurs when the tree comes under water stress.
Lenticel injury problems are widely reported when Royal Gala types are grown in very hot climates such as Washington and Chile. Boron deficiency is more likely to express when trees come under moisture stress. Superficial scald is an example of a post-harvest disorder where incidence rises when hot sunny days precede harvest.
Climate change is clearly happening. Orchard husbandry practices will need to evolve to mitigate the adverse effects of climate change on the crop.
Written by John Wilton, Horticultural Consultant, AgFirst, New Zealand |t: +64 6872 7080
JM Caprio, HA Quamme (1998) Weather conditions associated with apple production in the Okanagan Valley of British Columbia, Canadian Journal of Plant Science 79, pp 129-137.
H Parkes (2017) “Understanding apple and pear production systems in a changing climate”, final report. Hort Innovation project number