Managing climate risks
Climate risks to pomefruit crops come in many forms. The main ones are frost, hail, wind, flooding, drought and high temperatures. These are the obvious ones.
There are other, more subtle effects too which can be put down to weather and climate patterns. For instance, prolonged wet or cloudy periods at critical phenological stages in the crop. Excessively warm nights near or over the harvest period impact on fruit colouring and maturity behaviour.
Pest and disease outbreaks are often linked to particular weather conditions. For instance, apple scab requires significant periods of leaf wetness when fruit and leaves are immature in the spring. Cloudy, rainy weather over the immediate post blossom period from about 10 to 14 days after full bloom (DAFB) through to about 30 DAFB induces russet in russet prone varieties. This cloudy weather may also induce high post bloom fruit drop. Periods of dry weather as harvest approaches increase bird damage in crops as they look for sources of water.
In colder climates, such as Tasmania, very cool weather over harvest may suppress fruit colour development in later varieties. There are also the mild winter effects on dormancy and bud break which I covered recently in this column.
The one thing we know for sure is that every growing season will be different in regard to these risks. When dealing with risk, control what you can and proactively manage what you can’t.
Australian fruit growers have had lots of experience with hail, so have developed effective ways to protect their crops from this hazard. Covering their orchards with hail netting is now widely adopted among Australian fruit growers.
As well as giving hail protection, netting orchards also brings numerous other benefits and contributes to managing other climatic risks such as wind and high temperatures. Water use in netted orchards is also lower due to less wind and lower evapotranspiration rates. Netting also deals well with the bird and flying fox predation problem. On the negative side, hail netting adds extra capital expenses to orchard development budgets and has significant maintenance costs.
In locations where winter snow is likely, hail nets have to be folded back until the snow risk has passed. Pollination and fruitset may also benefit through having hail nets open over the blossom period. In this situation, it is important to close hail nets quickly after blossom to give protection before fruitlets become vulnerable to hail injury. Incidentally, fruit crops have been lost from hail when closing of hail nets have been delayed.
It is more difficult to obtain satisfactory cross pollination under hail net. Pollinator layout needs to be more intensive, preferably in each row at 20 to 30m intervals because bees tend to travel along, rather than across rows.
I also suspect that hail net design impacts on bee behaviour under hail nets. We have observed that bees may prefer to work under some types of hail net, rather than in unprotected orchards. These observations have been made where the hail nets cover single rows with a gap between each row net and the hives are located just outside of the netted area. We think the bee preference for working under net is because there is less wind there and less wind chill to the bee, so they feel warmer.
Full hail net cover, including side netting as well, certainly creates bee management and pollination issues.
The netted orchard environment is more favourable to tree growth so excess tree vigour can be a problem under hail nets. More growth increases shading and canopy density often leading to fruit quality issues.
There are reports of more bitter pit under net. This is probably a tree vigour effect because increased shoot growth pulls calcium which travels in the xylem away from the fruit into the growing shoot tips. Shading where canopies are dense can reduce fruitset as well as suppress fruit colour development towards harvest.
Chemical thinner response is more aggressive in netted orchards so thinning strategies need to be adjusted to avoid over thinning. Netted orchards may need more attention to vigour management than un-netted orchards. Reflective mulches are often necessary under net to maintain good lower canopy fruit colour.
Pest and disease
Woolly apple aphid (WAA) have been reported to be more numerous under hail nets than in orchard blocks outside of nets. This applies to mites also, so more pest monitoring may be required under nets.
On the other hand, where the net has reasonably fine mesh, pests such as codling moth may be less of a problem provided the pupae are not over wintering within the net.
Humidity is higher in netted orchards, drying is slower. These conditions favour fungus diseases notably scab and possibly Alternaria and summer rots. On the positive side, there is lower wind inside netted orchards, so this gives more spraying opportunities.
Drought stress and high temperature injury
These are major climatic risks in Australia and forecast to increase with climate change. Netting orchards is an effective way to reduce this risk. Water requirements in netted orchards are significantly lower than in orchards outside of net. Sunburn injury risk is also significantly lower under net than in the open. These benefits are often sufficient to justify the cost of netting.
As water supply for irrigation is a major limitation in much of Australia, reducing the farmed area to match irrigation water supply, then intensifying production on that area, rather than orcharding a larger area is the best long term option for managing drought stress. Around 70 per cent of orchard operating costs are fixed so your best way to lower unit production costs and maximise revenue is to increase yields and fruit quality.
Irrigation and nutrition management
Once your water supply is secure, it becomes possible to adopt very water efficient irrigation methods such as pulsed drip irrigation or partial root zone drying (PRD) techniques. These systems need a secure water supply because tree root systems become confined to their wetted soil area so the trees will quickly succumb to drought stress if the water supply is interrupted.
PRD is a recently introduced technique in which only half of the plant’s root system is irrigated at a time, then once the un-irrigated half has dried out, irrigation is switched across to that side. Relative to full irrigation this technique has potential to halve water use, while maintaining fruit yield and size, along with higher dry matter and sugar levels. There is also some evidence that nitrogen and phosphorus uptake is enhanced.
PRD has been widely adopted in the wine grape industry, particularly in Australia where water savings of 50 per cent have been reported along with less vine vigour, improved fruit quality and yields maintained.
Where PRD has been compared with regulated deficit irrigation (RDI), early indications are that apple fruit size and yields may be more likely to be maintained under PRD than under RDI.
Pulsed irrigation, applying water through a drip irrigation system will give water savings and allow fertigation to be taken care of. Tree nutrition to maintain tree growth and foliage during hot weather maintains production and minimises fruit sunburn problems without the need for netting the crop. This may be a viable approach where the expense of netting cannot be fully justified for other risks such as hail. However, in the longer term summer temperatures in many Australian fruit growing locations are forecast to rise so more robust sunburn protection will be required.
Managing both irrigation and fertiliser inputs well is an essential part of making these water saving irrigation techniques successful. This requires regular crop monitoring for water stress and nutrition, then making the appropriate adjustments to irrigation and fertigation as required.
Mulching along the row with straw or similar materials, together with an irrigation system – such as drip – supplying irrigation water under the mulch, will conserve soil moisture and minimise irrigation water use.
Mulching works by providing an insulating barrier over the soil which lowers soil temperatures and reduces evaporation from the soil surface. Studies on the effects of mulching have shown that over time soil water holding capacity will increase.
In areas where soil moisture holding capacity is less than the surrounding soils, mulching those areas will extend irrigation intervals required to maintain soil moisture and even up water requirement over the block, leading to more even tree growth and cropping.
Thinning and crop loading
Crop loads drive fruit size. Sunburn problems are also positively linked to increasing crop loads. Setting crop loads conservatively and as early as practical after flowering will capture the full crop potential. There are now some very good thinning tools available to achieve this objective.
Ideally fruit should be brought down to singles so that if it is necessary to drop crop load further later in the growing season once high temperatures have arrived, this second thinning can be achieved with minimal sunburn risk.
Apple trees are reasonably resilient when it comes to drought stress. If trees begin to come under drought stress, marketable fruit sizes can be maintained by dropping crop loads down to around 60 per cent of normal. This thinning should selectively remove those fruits at high risk of sunburn.
Pit and blotch
Drought stress and high temperatures increase problems with calcium related disorders such as bitter pit and lenticel blotch and breakdown. This is because during periods of stress, moisture is withdrawn from the fruit to maintain leaf condition. This moisture travels in the xylem and with it calcium leaks from the fruit too. Photosynthates from the leaves travel to the fruit in the phloem, but calcium does not, so calcium which is withdrawn from fruit to support the leaves cannot readily move back into the fruit.
It is important that a robust calcium spray programme is maintained through the season to minimise bitter pit and blotch problems.
Cultivars and rootstocks
Looking into the longer term, changing varieties and rootstocks from the heat sensitive ones we rely on at present, such as Royal Gala and Fuji, towards cultivars developed for hot climates such as Cripps Pink, will improve the heat tolerance
of the crop.
There are now a number of breeding programmes around the world focusing on developing cultivars suited to higher temperatures. In general, pears are more tolerant to higher temperatures than apples.
Draining and rooting depth
High soil temperatures increase drought stress effects so if deeper rooting can be encouraged, trees will be more resilient towards heat stress. Deeper rooting will also increase rooting zone volumes which means more available water can be stored in the rooting zone.
Relative to many pome fruit growing areas around the work, spring frost injury is less frequent in Australia.
Climate change may advance budbreak in some locations bringing flowering and fruitset into a period when frosts are more likely to occur. It is also possible that the weather will become more erratic, increasing the risk of out of season frosts occurring. The higher altitude inland areas such as Batlow, Orange and Stanthorpe are prone to this type of frost event.
Hail netting may give protection against marginal frost events but is certainly inadequate protection for damaging frost events where temperatures are going to drop well into the injury zone.
Occasional frosts, because of their erratic nature and relatively low long term impact on the orchard business, probably do not justify the capital expense required to install a frost protection system. As most orchards have irrigation, using sprinklers in particularly frost-prone parts of the orchards may be the most cost-effective way to manage frost, provided there is an adequate secure water supply available. Where frosts are a regular occurrence, frost fans are usually the most cost effective solution.
If frosts occur, proactively manage the injured crop to salvage its full potential.
Passive frost control measures such as blocking cold air entry into the orchard and making provision for cold air to drain away by opening up gaps in shelterbelts in valley floors, will lower frost severity. Keeping weeds down, and the orchard sward well-mown, during the frost season will also minimise frost effects.
Where under-tree sprinkling is used for frost protection, the sward should be left long to provide a large surface area for the sprinkled water to freeze on. It is the freezing of the water that releases the heat required to combat the frost. Under-tree sprinkling will lift minimum temperatures by 1 to 1.5°C which is often sufficient to avoid crop loss.